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Title: The Steam Engine Familiarly Explained and Illustrated
With an historical sketch of its invention and progressive
improvement; etc.
Author: Dionysius Lardner
Contributor: James Renwick
Release Date: August 26, 2017 [EBook #55428]
Language: English
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[Illustration: Pl. XIII. AMERICAN HIGH-PRESSURE ENGINE]
THE STEAM ENGINE
FAMILIARLY EXPLAINED AND ILLUSTRATED;
WITH
AN HISTORICAL SKETCH OF ITS INVENTION AND
PROGRESSIVE IMPROVEMENT;
ITS APPLICATIONS TO
NAVIGATION AND RAILWAYS;
WITH
PLAIN MAXIMS FOR RAILWAY SPECULATORS.
BY THE
REV. DIONYSIUS LARDNER, LL. D., F. R. S.,
FELLOW OF THE ROYAL SOCIETY OF EDINBURGH; OF THE ROYAL IRISH ACADEMY;
OF THE ROYAL ASTRONOMICAL SOCIETY; OF THE CAMBRIDGE
PHILOSOPHICAL SOCIETY; OF THE STATISTICAL SOCIETY OF PARIS; OF THE
LINNÆAN AND ZOOLOGICAL SOCIETIES; OF THE SOCIETY FOR
PROMOTING USEFUL ARTS IN SCOTLAND, ETC.
WITH ADDITIONS AND NOTES,
BY JAMES RENWICK, LL. D.,
PROFESSOR OF NATURAL EXPERIMENTAL PHILOSOPHY AND CHEMISTRY
IN COLUMBIA COLLEGE, NEW YORK.
ILLUSTRATED BY ENGRAVINGS AND WOODCUTS.
SECOND AMERICAN, FROM THE FIFTH LONDON, EDITION,
CONSIDERABLY ENLARGED.
PHILADELPHIA:
E. L. CAREY & A. HART.
1836
Entered, according to the Act of Congress, in the year 1836, by E. L.
CAREY & A. HART, in the Clerk's office of the District Court for the
Eastern District of Pennsylvania.
PREFACE
OF
THE AMERICAN EDITOR.
Several of the additions, which were made by the Editor to the first
American edition, have been superseded by the great extension, which
the original has from time to time received from its author. This is
more particularly the case, with the sections which had reference to
the character of steam at temperatures other than that of boiling
water, to the use of steam in navigation, and to its application to
locomotion. These sections have of course been omitted. A few new
sections, and several notes have been added, illustrative of such
points as may be most interesting to the American reader.
COLUMBIA COLLEGE,
_New York, March, 1836_.
PREFACE
TO
THE FIFTH EDITION.
This volume should more properly be called a new work than a new
edition of the former one. In fact the book has been almost rewritten.
The change which has taken place, even in the short period which has
elapsed since the publication of the first edition, in the relation of
the steam engine to the useful arts, has been so considerable as to
render this inevitable.
The great extension of railroads, and the increasing number of
projects which have been brought forward for new lines connecting
various points of the kingdom, as well as the extension of steam
navigation, not only through the seas and channels surrounding and
intersecting these islands, and throughout other parts of Europe, but
through the larger waters which are interposed between our dominions
in the East and the countries of Egypt and Syria, have conferred so
much interest on the application of steam to transport, that I have
thought it adviseable to extend the limits of the present edition
considerably beyond those of the last. The chapter on railroads has
been enlarged and improved. Three chapters have been added. The
twelfth chapter contains a view of steam navigation; the thirteenth
contains several important points connected with the economy of steam
power, which, when this work was first published, would not have
offered sufficient interest to justify their admission into a popular
treatise; and the fourteenth chapter contains a series of compendious
maxims, for the instruction and guidance of persons desirous of making
investments or speculating in railway property.
_London, December, 1835._
PREFACE
TO
THE FIRST EDITION.
There are two classes of persons whose attention may be attracted by a
treatise on such a subject as the Steam Engine. One consists of those
who, by trade or profession, are interested in mechanical science, and
who therefore seek information on the subject of which it treats, as a
matter of necessity, and a wish to acquire it in a manner and to an
extent which may be practically available in their avocations. The
other and more numerous class is that part of the public in general,
who, impelled by choice rather than necessity, think the interest of
the subject itself, and the pleasure derivable from the instances of
ingenuity which it unfolds, motives sufficiently strong to induce them
to undertake the study of it. Without leaving the former class
altogether out of view, it is for the use of the latter principally
that the following lectures are designed.
To this class of readers the Steam Engine is a subject which, if
properly treated of, must present strong and peculiar attractions.
Whether we consider the history of its invention as to time and place,
the effects which it has produced, or the means by which it has caused
these effects, we find everything to gratify our national pride,
stimulate our curiosity, excite our wonder, and command our
admiration. The invention and progressive improvement of this
extraordinary machine, is the work of our own time and our own
country; it has been produced and brought to perfection almost within
the last century, and is the exclusive offspring of British genius
fostered and supported by British capital. To enumerate the effects of
this invention, would be to count every comfort and luxury of life. It
has increased the sum of human happiness, not only by calling new
pleasures into existence, but by so cheapening former enjoyments as to
render them attainable by those who before never could have hoped to
share them. Nor are its effects confined to England alone: they extend
over the whole civilized world; and the savage tribes of America,
Asia, and Africa, must ere long feel the benefits, remote or
immediate, of this all-powerful agent.
If the effect which this machine has had on commerce and the wealth of
nations raise our astonishment, the means by which this effect has
been produced will not less excite our admiration. The history of the
Steam Engine presents a series of contrivances, which, for exquisite
and refined ingenuity, stand without a parallel in the annals of human
invention. These admirable contrivances, unlike other results of
scientific investigation, have also this peculiarity, that to
understand and appreciate their excellence requires little previous
or subsidiary knowledge. A simple and clear explanation, divested as
far as possible of technicalities, and assisted by well selected
diagrams, is all that is necessary to render the principles of the
construction and operation of the Steam Engine intelligible to a
person of plain understanding and moderate information.
The purpose for which this volume is designed, as already explained,
has rendered necessary the omission of many particulars which, however
interesting and instructive to the practical mechanic or professional
engineer, would have little attraction for the general reader. Our
readers require to be informed of the general principles of the
construction and operation of Steam Engines, rather than of their
practical details. For the same reasons we have confined ourselves to
the more striking and important circumstances in the history of the
invention and progressive improvement of this machine, excluding many
petty disputes which arose from time to time respecting the rights of
invention, the interest of which is buried in the graves of their
respective claimants.
In the descriptive parts of the work we have been governed by the same
considerations. The application of the force of steam to mechanical
purposes has been proposed on various occasions, in various countries,
and under a great variety of forms. The list of British patents alone
would furnish an author of common industry and application with matter
to swell his book to many times the bulk of this volume. By far the
greater number of these projects have, however, proved abortive.
Descriptions of such unsuccessful, though frequently ingenious
machines, we have thought it adviseable to exclude from our pages, as
not possessing sufficient interest for the readers to whose use this
volume is dedicated. We have therefore strictly confined our
descriptions either to those Steam Engines which have come into
general use, or to those which form an important link in the chain of
invention.
_December 26, 1827._
CONTENTS.
CHAPTER I.
PRELIMINARY MATTER.
Motion the Agent in Manufactures. -- Animal Power. -- Power
depending on physical Phenomena. -- Purpose of a Machine. --
Prime Mover. -- Mechanical qualities of the Atmosphere. -- Its
Weight. -- The Barometer. -- Fluid Pressure. -- Pressure of
rarefied Air. -- Elasticity of Air. -- Bellows. -- Effects of
Heat. -- Thermometer. -- Method of making one. -- Freezing and
Boiling Points. -- Degrees. -- Dilatation of bodies. --
Liquefaction and Solidification. -- Vaporisation and
Condensation. -- Latent Heat of Steam. -- Expansion of Water in
Evaporating. -- Effects of Repulsion and Cohesion. -- Effect of
Pressure upon Boiling Point. -- Formation of a Vacuum by
Condensation. Page 17
CHAPTER II.
FIRST STEPS IN THE INVENTION.
Futility of early Claims. -- Watt the real Inventor. -- Hero of
Alexandria. -- Blasco Garay. -- Solomon De Caus. -- Giovanni
Branca. -- Marquis of Worcester. -- Sir Samuel Morland. -- Denis
Papin. -- Thomas Savery. 38
CHAPTER III.
ENGINES OF SAVERY AND NEWCOMEN.
Savery's Engine. -- Boilers and their Appendages. -- Working
Apparatus. -- Mode of Operation. -- Defects of the Engine. --
Newcomen and Cawley. -- Atmospheric Engine. -- Accidental
discovery of Condensation by Jet. -- Potter's discovery of the
Method of working the Valves. 51
CHAPTER IV.
ENGINE OF JAMES WATT.
Advantages of the Atmospheric Engine over that of Captain Savery.
-- It contained no new Principle. -- Papin's Engine. -- James
Watt. -- Particulars of his Life. -- His first conceptions of the
Means of economising Heat. -- Principle of his projected
Improvements. 69
CHAPTER V.
WATT'S SINGLE-ACTING STEAM ENGINE.
Expansive Principle applied. -- Failure of Roebuck, and
partnership with Bolton. -- Patent extended to 1800. -- Counter.
-- Difficulties in getting the Engines into Use. 80
CHAPTER VI.
DOUBLE-ACTING STEAM ENGINE.
The Single-acting Engine unfit to impel Machinery. -- Various
Contrivances to adapt it to this Purpose. -- Double-Cylinder. --
Double-acting Cylinder. -- Various modes of connecting the Piston
with the Beam. -- Rack and Sector. -- Double Chain. -- Parallel
Motion. -- Crank. -- Sun and Planet Motion. -- Fly Wheel. --
Governor. 91
CHAPTER VII.
DOUBLE-ACTING STEAM ENGINE, _continued_.
On the Valves of the Double-acting Steam Engine. -- Original
Valves. -- Spindle Valves. -- Sliding Valve. -- D Valve. --
Four-Way Cock. 108
CHAPTER VIII.
BOILER AND ITS APPENDAGES.
Level Gauges. -- Feeding apparatus. -- Steam Gauge. -- Barometer
Gauge. -- Safety Valves. -- Self-regulating Damper. --
Edelcrantz's Valve. -- Furnace. -- Smoke-consuming Furnace. --
Brunton's Self-regulating Furnace. -- Oldham's Modification. 117
CHAPTER IX.
DOUBLE-CYLINDER ENGINES.
Hornblower's Engine. -- Woolf's Engine. -- Cartwright's
Engine. 134
CHAPTER X.
LOCOMOTIVE ENGINES ON RAILWAYS.
High-pressure Engines. -- Leupold's Engine. -- Trevithick and
Vivian. -- Effects of Improvement in Locomotion. -- Historical
Account of the Locomotive Engine. -- Blenkinsop's Patent. --
Chapman's Improvement. -- Walking Engine. -- Stephenson's First
Engines. -- His Improvements. -- Liverpool and Manchester Railway
Company. -- Their Preliminary Proceedings. -- The Great
Competition of 1829. -- The Rocket. -- The Sanspareil. -- The
Novelty. -- Qualities of the Rocket. -- Successive Improvements.
-- Experiments. -- Defects of the Present Engines. -- Inclined
Planes. -- Methods of surmounting them. -- Circumstances of the
Manchester Railway Company. -- Probable Improvements in
Locomotives. -- Their capabilities with respect to speed. --
Probable Effects of the Projected Railroads. -- Steam Power
compared with Horse Power. -- Railroads compared with Canals. 145
CHAPTER XI.
LOCOMOTIVE ENGINES ON TURNPIKE ROADS.
Railway and Turnpike Roads compared. -- Mr. Gurney's inventions.
-- His Locomotive Steam Engine. -- Its performances. --
Prejudices and errors. -- Committee of the House of Commons. --
Convenience and safety of Steam Carriages. -- Hancock's Steam
Carriage. -- Mr. N. Ogle. -- Trevithick's invention. --
Proceedings against Steam Carriages. -- Turnpike Bills. -- Steam
Carriage between Gloucester and Cheltenham. -- Its
discontinuance. -- Report of the Committee of the Commons. --
Present State and Prospects of Steam Carriages. 213
CHAPTER XII.
STEAM NAVIGATION.
Propulsion by paddle-wheels. -- Manner of driving them. -- Marine
Engine. -- Its form and arrangement. -- Proportion of its
cylinder. -- Injury to boilers by deposites and incrustation. --
Not effectually removed by blowing out. -- Mr. Samuel Hall's
condenser. -- Its advantages. -- Originally suggested by Watt. --
Hall's _steam saver_. -- Howard's vapour engine. -- Morgan's
paddle-wheels. -- Limits of steam navigation. -- Proportion of
tonnage to power. -- Average speed. -- Consumption of fuel. --
Iron Steamers. -- American steam raft. -- Steam navigation to
India. -- By Egypt and the Red Sea to Bombay. -- By same route to
Calcutta. -- By Syria and the Euphrates to Bombay. -- Steam
communication with the United States from the west coast of
Ireland to St. Johns, Halifax, and New York. 241
CHAPTER XIII.
GENERAL ECONOMY OF STEAM POWER.
Mechanical efficacy of steam -- proportional to the quantity of
water evaporated, and to the fuel consumed. -- Independent of the
pressure. -- Its mechanical efficacy by condensation alone. -- By
condensation and expansion combined -- by direct pressure and
expansion -- by direct pressure and condensation -- by direct
pressure, condensation, and expansion. -- The power of engines.
-- The duty of engines. -- Meaning of horse power. -- To compute
the power of an engine. -- Of the power of boilers. -- The
structure of the grate-bars. -- Quantity of water and steam room.
-- Fire surface and flue surface. -- Dimensions of steam pipes.
-- Velocity of piston. -- Economy of fuel. -- Cornish duty
reports. 277
CHAPTER XIV.
Plain Rules for Railway Speculators. 307
THE STEAM ENGINE
EXPLAINED AND ILLUSTRATED.
CHAPTER I.
PRELIMINARY MATTER.
Motion the Agent in Manufactures. -- Animal Power. -- Power
depending on Physical Phenomena. -- Purpose of a Machine. --
Prime Mover. -- Mechanical qualities of the Atmosphere. -- Its
Weight. -- The Barometer. -- Fluid Pressure. -- Pressure of
Rarefied Air. -- Elasticity of Air. -- Bellows. -- Effects of
Heat. -- Thermometer. -- Method of making one. -- Freezing and
Boiling Points. -- Degrees. -- Dilatation of Bodies. --
Liquefaction and Solidification. -- Vaporisation and
Condensation. -- Latent heat of Steam. -- Expansion of Water in
Evaporating. -- Effects of Repulsion and Cohesion. -- Effect of
Pressure upon Boiling-Point. -- Formation of a Vacuum by
Condensation.
(1.) Of the various productions designed by nature to supply the wants
of man, there are few which are suited to his necessities in the state
in which the earth spontaneously offers them: if we except atmospheric
air, we shall scarcely find another instance: even water, in most
cases, requires to be transported from its streams or reservoirs; and
food itself, in almost every form, requires culture and preparation.
But if, from the mere necessities of physical existence in a primitive
state, we rise to the demands of civil and social life,--to say
nothing of luxuries and refinements,--we shall find that everything
which contributes to our convenience, or ministers to our pleasure,
requires a previous and extensive expenditure of labour. In most
cases, the objects of our enjoyment derive all their excellences, not
from any qualities originally inherent in the natural substances out
of which they are formed, but from those qualities which have been
bestowed upon them by the application of human labour and human skill.
In all those changes to which the raw productions of the earth are
submitted in order to adapt them to our wants, one of the principal
agents is _motion_. Thus, for example, in the preparation of clothing
for our bodies, the various processes necessary for the culture of the
cotton require the application of moving power, first to the soil, and
subsequently to the plant from which the raw material is obtained: the
wool must afterwards be picked and cleansed, twisted into threads, and
woven into cloth. In all these processes motion is the agent: to
cleanse the wool and arrange the fibres of the cotton, the wool must
be beaten, teased, carded, and submitted to other processes, by which
all the foreign and coarser matter may be separated, and the fibres or
threads arranged evenly, side by side. The threads must then receive a
rotatory motion, by which they may be twisted into the required form;
and finally peculiar motions must be given to them in order to produce
among them that arrangement which characterises the cloth which it is
our final purpose to produce.
In a rude state of society, the motions required in the infant
manufactures are communicated by the immediate application of the
hand. Observation and reflection, however, soon suggest more easy and
effectual means of attaining these ends: the strength of animals is
first resorted to for the relief of human labour. Further reflection
and inquiry suggest still better expedients. When we look around us in
the natural world, we perceive inanimate matter undergoing various
effects in which motion plays a conspicuous part: we see the falls of
cataracts, the currents of rivers, the elevation and depression of the
waters of the ocean, the currents of the atmosphere; and the question
instantly arises, whether, without sharing our own means of
subsistence with the animals whose force we use, we may not equally,
or more effectually, derive the powers required from these various
phenomena of nature? A difficulty, however, immediately presents
itself: we require motion of a particular kind; but wind will not
blow, nor water fall as we please, nor as suits our peculiar wants,
but according to the fixed laws of nature. We want an _upward_ motion;
water falls _downwards_: we want a _circular_ motion; wind blows in a
_straight_ line. The motions, therefore, which are in actual existence
must be modified to suit our purposes: the means whereby these
modifications are produced, are called _machines_. A machine,
therefore, is an instrument interposed between some natural force or
motion, and the object to which force or motion is desired to be
transmitted. The construction of the machine is such as to modify the
natural motion which is impressed upon it, so that it may transmit to
the object to be moved that peculiar species of motion which it is
required to have. To give a very obvious example, let us suppose that
a circular or rotatory motion is required to be produced, and that the
only natural source of motion at our command is a perpendicular fall
of water: a wheel is provided, placed upon the axle destined to
receive the rotatory motion; this wheel is furnished with cavities in
its rim; the water is conducted into the cavities near the top of the
wheel on one side; and being caught by these, its weight bears down
that side of the wheel, the cavities on the opposite side being empty
and in an inverted position. As the wheel turns, the cavities on the
descending side discharge their contents as they arrive near the
lowest point, and ascend empty on the other side. Thus a load of water
is continually pressing down one side of the wheel, from which the
other side is free, and a continued motion of rotation is produced.
In every machine, therefore, there are three objects demanding
attention:--first, The power which imparts motion to it, this is
called the _prime mover_; secondly, The nature of the _machine_
itself; and thirdly, The object to which the motion is to be conveyed.
In the steam engine the first mover arises from certain phenomena
which are exhibited when heat is applied to liquids; but in the
details of the machine and in its application there are several
physical effects brought into play, which it is necessary perfectly to
understand before the nature of the machine or its mode of operation
can be rendered intelligible. We propose therefore to devote the
present chapter to the explanation and illustration of these
phenomena.
(2.) The physical effects most intimately connected with the
operations of steam engines are some of the mechanical properties of
atmospheric air. The atmosphere is the thin transparent fluid in which
we live and move, and which, by respiration, supports animal life.
This fluid is apparently so light and attenuated, that it might be at
first doubted whether it be really a body at all. It may therefore
excite some surprise when we assert, not only that it is a body, but
also that it is one of considerable _weight_. We shall be able to
prove that it presses on every _square inch_[1] of surface with a
weight of about 15lb. avoirdupois.
[Footnote 1: As we shall have frequent occasion to mention this
magnitude, it would be well that the reader should be familiar with
it. It is a _square_, each side of which is an inch. Such as A B C D,
Fig. 1.]
(3.) Take a glass tube A B (fig. 2.) more than 32 inches long, open at
one end A, and closed at the other end B, and let it be filled with
mercury (quicksilver.) Let a glass vessel or cistern C, containing a
quantity of mercury, be also provided. Applying the finger at A so as
to prevent the mercury in the tube from falling out, let the tube be
inverted, and the end, stopped by the finger, plunged into the mercury
in _C_. When the end of the tube is below the surface of the mercury
in C (fig. 3.) let the finger be removed. It will be found that the
mercury in the tube will not, as might be expected, fall to the level
of the mercury in the cistern C, which it would do were the end B open
so as to admit the air into the upper part of the tube. On the other
hand, the level D of the mercury in the tube will be about 30 inches
above the level C of the mercury in the cistern.
(4.) The cause of this effect is, that the weight of the atmosphere
rests on the surface C of the mercury in the cistern, and tends
thereby to press it up, or rather to resist its fall in the tube; and
as the fall is not assisted by the weight of the atmosphere on the
surface D (since B is closed), it follows, that as much mercury
remains suspended in the tube above the level C as the weight of the
atmosphere is able to support.
If we suppose the section of the tube to be equal to the magnitude of
a square inch, the weight of the column of mercury in the tube above
the level C will be exactly equal to the weight of the atmosphere on
each square inch of the surface C. The height of the level D above C
being about 30 inches, and a column of mercury two inches in height,
and having a base of a square inch, weighing about one pound
avoirdupois, it follows that the weight with which the atmosphere
presses on each square inch of a level surface is about 15lb.
avoirdupois.
An apparatus thus constructed, and furnished with a scale to indicate
the height of the level D above the level C, is the _common
barometer_. The difference of these levels is subject to a small
variation, which indicates a corresponding change in the atmospheric
pressure. But we take 30 inches as a standard or average.
(5.) It is an established property of fluids that they press equally
in all directions; and air, like every other fluid, participates in
this quality. Hence it follows, that since the downward pressure or
weight of the atmosphere is about 15lb. on the square inch, the
lateral, upward, and oblique pressures are of the same amount. But,
independently of the general principle, it may be satisfactory to give
experimental proof of this.
Let four glass tubes A, B, C, D, (fig. 4.) be constructed of
sufficient length, closed at one end A, B, C, D, and open at the
other. Let the open ends of three of them be bent, as represented in
the tubes B, C, D. Being previously filled with mercury, let them all
be gently inverted so as to have their closed ends up as here
represented. It will be found that the mercury will be sustained in
all,[2] and that the difference of the levels in all will be the same.
Thus the mercury is sustained in A by the upward pressure of the
atmosphere, in B by its horizontal or lateral pressure, in C by its
downward pressure, and in D by its oblique pressure; and as the
difference of the levels is the same in all, these pressures are
exactly equal.
[Footnote 2: This experiment with the tube A requires to be very
carefully executed, and the tube should be one of small bore.]
(6.) In the experiment described in (3.) the space B D (fig. 3.) at
the top of the tube from which the mercury has fallen is perfectly
void and empty, containing neither air nor any other fluid: it is
called therefore a _vacuum_. If, however, a small quantity of air be
introduced into that space, it will immediately begin to exert a
pressure on D, which will cause the surface D to descend, and it will
continue to descend until the column of mercury C D is so far
diminished that the weight of the atmosphere is sufficient to sustain
it, as well as the pressure exerted upon it by the air in the space B
D.
The quantity of mercury which falls from the tube in this case is
necessarily an equivalent for the pressure of the air introduced, so
that the pressure of this air may be exactly ascertained by allowing
about one pound per square inch for every two inches of mercury which
has fallen from the tube. The pressure of the air or any other fluid
above the mercury in the tube, may at once be ascertained by comparing
the height of the mercury in the tube with the height of the
barometer; the difference of the heights will always determine the
pressure on the surface of the mercury in the tube. This principle
will be found of some importance in considering the action of the
modern steam engines.
The air which we have supposed to be introduced into the upper part of
the tube, presses on the surface of the mercury with a force much
greater than its weight. For example, if the space B D (fig. 3.) were
filled with atmospheric air in its ordinary state, it would exert a
pressure on the surface D equal to the whole pressure of the
atmosphere, although its weight might not amount to a single grain.
The property in virtue of which the air exerts this pressure is its
_elasticity_, and this force is diminished in precisely the proportion
in which the space which the air occupies is increased.
Thus it is known that atmospheric air in its ordinary state exerts a
pressure on the surface of any vessel in which it is confined,
amounting to about 15lb. on every square inch. If the capacity of the
vessel which contains it be doubled, it immediately expands and fills
the double space, but in doing so it loses half its elastic force, and
presses only with the force of 7-1/2lb. on every square inch. If the
capacity of the vessel had been enlarged five times, the air would
still have expanded so as to fill it, but would exert only a fifth
part of its first pressure, or 3lb. on every square inch.
This property of losing its elastic force as its volume or bulk is
increased, is not peculiar to air. It is common to all elastic fluids,
and we accordingly find it in steam; and it is absolutely necessary to
take account of it in estimating the effects of that agent.
(7.) There are numerous instances of the effects of these properties
of atmospheric air which continually fall under our observation. If
the nozzle and valve-hole of a pair of bellows be stopped, it will
require a very considerable force to separate the boards. This effect
is produced by the diminished elastic force of the air remaining
between the boards upon the least increase of the space within the
bellows, while the atmosphere presses, with undiminished force, on the
external surfaces of the boards. If the boards be separated so as to
double the space within, the elastic force of the included air will be
about 7-1/2lb. on every square inch, while the pressure on the
external surfaces will be 15lb. on every square inch; consequently, it
will require as great a force to sustain the boards in such a
position, as it would to separate them if each board were forced
against the other, with a pressure of 7-1/2lb. per square inch on
their external surfaces.
When boys apply a piece of moistened leather to a stone, so as to
exclude the air from between them, the stone, though it be of
considerable weight, may be lifted by a string attached to the
leather: the cause of which is the atmospheric pressure, which keeps
the leather and the stone in close contact.
(8.) The next class of physical effects which it is necessary to
explain, are those which are produced when heat is imparted or
abstracted from bodies.
In general, when heat is imparted to a body, an enlargement of bulk
will be the immediate consequence, and at the same time the body will
become warmer to the touch. These two effects of expansion and
increase of warmth going on always together, the one has been taken as
a measure of the other; and upon this principle the common thermometer
is constructed. That instrument consists of a tube of glass,
terminated in a bulb, the magnitude of which is considerable, compared
with the bore of the tube. The bulb and part of the tube are filled
with mercury, or some other liquid. When the bulb is exposed to any
source of heat, the mercury contained in it, being warmed or increased
in temperature, is at the same time increased in bulk, or expanded or
dilated, as it is called. The bulb not having sufficient capacity to
contain the increased bulk of mercury, the liquid is forced up in the
tube, and the quantity of expansion is determined by observing the
ascent of the column in the tube.
An instrument of this kind, exposed to heat or cold, will fluctuate
accordingly, the mercury rising as the heat to which it is exposed is
increased, and falling by exposure to cold. In order, however, to
render it an accurate measure of temperature, it is necessary to
connect with it a scale by which the elevation or depression of the
mercury in the tube may be measured. Such a scale is constructed for
thermometers in this country in the following manner:--Let us suppose
the instrument immersed in a vessel of melting ice: the column of
mercury in the tube will be observed to fall to a certain point, and
there maintain its position unaltered: let that point be marked upon
the tube. Let the instrument be now transferred to a vessel of boiling
water at a time when the barometer stands at the altitude of 30
inches: the mercury in the tube will be observed to rise until it
attain a certain elevation, and will there maintain its position. It
will be found, that though the water continue to be exposed to the
action of the fire, and continue to boil, the mercury in the tube will
not continue to rise, but will maintain a fixed position: let the
point to which the mercury has risen, in this case, be likewise marked
upon the tube.
The two points, thus determined, are called the _freezing_ and the
_boiling_ points. If the distance upon the tube between these two
points be divided into 180 equal parts, each of these parts is called
a _degree_; and if this division be continued, by taking equal
divisions below the freezing point, until 32 divisions be taken, the
last division is called the _zero_, or _nought_ of the thermometer. It
is the point to which the mercury would fall, if the thermometer were
immersed in a certain mixture of snow and salt. When thermometers were
first invented, this point was taken as the zero point, from an
erroneous supposition that the temperature of such a mixture was the
lowest possible temperature.
The degrees upon the instrument thus divided are counted upwards from
the zero, and are expressed, like the degrees of a circle, by placing
a small ° over the number. Thus it will be perceived that the freezing
point is 32° of our thermometer, and the boiling-point will be found
by adding 180° to 32°; it is therefore 212°.
The temperature of a body is that elevation to which the thermometer
would rise when the mercury enclosed in it would acquire the same
temperature. Thus, if we should immerse the thermometer, and should
find that the mercury would rise to the division marked 100°, we
should then affirm that the temperature of the water was 100°.
(9.) The dilatation which attends an increase of temperature is one of
the most universal effects of heat. It varies, however, in different
bodies: it is least in solid bodies; greater in liquids; and greatest
of all in bodies in the aeriform state. Again, different solids are
differently susceptible of this expansion. Metals are the most
susceptible of it; but metals of different kinds are differently
expansible.
As an increase of temperature causes an increase of bulk, so a
diminution of temperature causes a corresponding diminution of bulk,
and the same body always has the same bulk at the same temperature.
A flaccid bladder, containing a small quantity of air, will, when
heated, become quite distended; but it will again resume its flaccid
appearance when cold. A corked bottle of fermented liquor, placed
before the fire, will burst by the effort of the air contained in it
to expand when heated.
Let the tube A B (fig. 5.) open at both ends, have one end inserted in
the neck of a vessel C D, containing a coloured liquid, with common
air above it; and let the tube be fixed so as to be air-tight in the
neck: upon heating the vessel, the warm air inclosed in the vessel C D
above the liquid will begin to expand, and will press upon the surface
of the liquid, so as to force it up in the tube A B.
In bridges and other structures, formed of iron, mechanical
provisions are introduced to prevent the fracture or strain which
would take place by the expansion and contraction which the metal must
undergo by the changes of temperature at different seasons of the
year, and even at different hours of the day.
Thus all nature, animate and inanimate, organized and unorganized, may
be considered to be incessantly breathing heat; at one moment drawing
in that principle through all its dimensions, and at another moment
dismissing it.
(10.) Change of bulk, however, is not the only nor the most striking
effect which attends the increase or diminution of the quantity of
heat in a body. In some cases, a total change of form and of
mechanical qualities is effected by it. If heat be imparted in
sufficient quantity to a solid body, that body, after a certain time,
will be converted into a liquid. And again, if heat be imparted in
sufficient quantity to this liquid, it will cease to exist in the
liquid state, and pass into the form of vapour.
By the abstraction of heat, a series of changes will be produced in
the opposite order. If from the vapour produced in this case, a
sufficient quantity of heat be taken, it will return to the liquid
state; and if again from this liquid heat be further abstracted, it
will at length resume its original solid state.
The transmission of a body from the solid to the liquid state, by the
application of heat, is called _fusion_ or _liquefaction_, and the
body is said to be fused, _liquefied_, or melted.
The reciprocal transmission from the liquid to the solid state, is
called _congelation_, or _solidification_; and the liquid is said to
be _congealed_ or _solidified_.
The transmission of a body from the liquid to the vaporous or aeriform
state, is called _vaporization_, and the liquid is said to be
_vaporized_ or _evaporated_.
The reciprocal transmission of vapour to the liquid state is called
_condensation_, and the vapour is said to be _condensed_.
We shall now examine more minutely the circumstances which attend
these remarkable and important changes in the state of body.
(11.) Let us suppose that a thermometer is imbedded in any solid body;
for example, in a mass of sulphur; and that it stands at the ordinary
temperature of 60 degrees: let the sulphur be placed in a vessel, and
exposed to the action of fire. The thermometer will now be observed
gradually to rise, and it will continue to rise until it exhibit the
temperature of 218°. Here, however, notwithstanding the continued
action of the fire upon the sulphur, the thermometer will become
stationary; proving, that notwithstanding the supply of heat received
from the fire, the sulphur has ceased to become hotter. At the moment
that the thermometer attains this stationary point, it will be
observed that the sulphur has commenced the process of fusion; and
this process will be continued, the thermometer being stationary,
until the whole mass has been liquefied. The moment the liquefaction
is complete, the thermometer will be observed again to rise, and it
will continue to rise until it attain the elevation of 570°. Here,
however, it will once more become stationary; and notwithstanding the
heat supplied to the sulphur by the fire, the liquid will cease to
become hotter: when this happens, the sulphur will boil; and if it
continue to be exposed to the fire a sufficient length of time, it
will be found that its quantity will gradually diminish, until at
length it will all disappear from the vessel which contained it. The
sulphur will, in fact, be converted into vapour.
From this process we infer, that all the heat supplied during the
processes of liquefaction and vaporization is consumed in effecting
these changes in the state of the body; and that under such
circumstances, it does not increase the temperature of the body on
which the change is produced.
These effects are general: all solid bodies would pass into the
liquid state by a sufficient application of heat; and all liquid
bodies would pass into the vaporous state by the same means. In all
cases the thermometer would be stationary during these changes, and
consequently the temperature of the body, in those periods, would be
maintained unaltered.
(12.) Solids differ from one another in the temperatures at which they
become liquid. These temperatures are called their _melting points_.
Thus the melting point of ice is 32°; that of lead 612°; that of gold
5237°.[3] The heat which is supplied to a body during the processes of
fusion or vaporization, and which does not affect the thermometer, or
increase the temperature of the body fused or vaporized, is said to
become _latent_. It can be proved to exist in the body fused or
vaporized, and may even be taken from that body. In parting with it
the body does not fall in temperature, and consequently the loss of
this heat is not indicated by the thermometer any more than its
reception. The term latent heat is merely intended to express this
fact, of the thermometer being insensible to the presence or absence
of this portion of heat, and is not intended to express any
theoretical notions concerning it.
[Footnote 3: Temperatures above 650° cannot be measured by the
mercurial thermometer. They can be inferred only with probability by
pyrometers.]
(13.) In explaining the construction and operation of the steam
engine, although it is necessary occasionally to refer to the effects
of heat upon bodies in general, yet the body, which is by far the most
important to be attended to, so far as the effects of heat upon it are
concerned, is water. This body is observed to exist in the three
different states, the solid, the liquid, and the vaporous, according
to the varying temperature to which it is exposed. All the
circumstances which have been explained in reference to metals, and
the substance sulphur in particular, will, _mutatis mutandis_, be
applicable to water. But in order perfectly to comprehend the
properties of the steam engine, it is necessary to render a more
rigorous and exact account of these phenomena, so far as they apply to
the changes produced upon water by the effects of heat.
Let us suppose a mass of ice immersed in the mixture of snow and salt
which determines the zero point of the thermometer: this mass, if
allowed to continue a sufficient length of time submerged in the
mixture, will necessarily acquire its temperature, and the thermometer
immersed in it will stand at zero. Let the ice be now withdrawn from
the mixture, still keeping the thermometer immersed in it, and let it
be exposed to the atmosphere at the ordinary temperature, say 60°. At
first the thermometer will be observed gradually and continuously to
rise until it attain the elevation of 32°; it will then become
stationary, and the ice will begin to melt: the thermometer will
continue standing at 32° until the ice shall be completely liquefied.
The liquid ice and the thermometer being contained in the same vessel,
it will be found, when the liquefaction is completed, that the
thermometer will again begin to rise, and will continue to rise until
it attain the temperature of the atmosphere, viz. 60°. Hitherto the
ice or water has received a supply of heat from the surrounding air;
but now an equilibrium of temperature having been established, no
further supply of heat can be received; and if we would investigate
the further effects of increased heat, it will be necessary to expose
the liquid to fire, or some other source of heat. But previous to
this, let us observe the time which the thermometer remains stationary
during the liquefaction of the ice: if noted by a chronometer, it
would be found to be a hundred and forty times the time during which
the water in the liquid state was elevated one degree; the inference
from which is, that in order to convert the solid ice into liquid
water, it was necessary to receive from the surrounding atmosphere one
hundred and forty times as much heat as would elevate the liquid water
one degree in temperature; or, in other words, that to liquefy a
given weight of ice requires as much heat as would raise the same
weight of water 140° in temperature: or from 32° to 172°.
The latent heat of water acquired in liquefaction is therefore 140°.
(14.) Let us now suppose that, a spirit lamp being applied to the
water already raised to 60°, the effects of a further supply of heat
be observed: the thermometer will continue to rise until it attain the
elevation of 212°, the barometer being supposed to stand at 30 inches.
The thermometer having attained this elevation will cease to rise; the
water will therefore cease to become hotter, and at the same time
bubbles of steam will be observed to be formed at the bottom of the
vessel containing the water, near the flame of the spirit lamp. These
bubbles will rise through the water, and escape at the surface,
exhibiting the phenomena of ebullition, and the water will undergo the
process of _boiling_.
During this process, the thermometer will constantly be maintained at
the same elevation of 212°; but if the time be noted, it will be found
that the water will be altogether evaporated, if the same source of
heat be continued to be applied to it six and a half times as long as
was necessary to raise it from the freezing to the boiling-point.
Thus, if the application of the lamp to water at 32°, be capable of
raising that water to 212° in one hour, the same lamp will require to
be applied to the boiling water for six hours and a half, in order to
convert the whole of it into steam. Now if the steam into which it is
thus converted were carefully preserved in a receiver, maintained at
the temperature of 212°, this steam would be found to have that
temperature, and not a greater one; but it would be found to fill a
space about 1700 times greater than the space it occupied in the
liquid state, and it would possess an elastic force equal to the
pressure of the atmosphere under which it was boiled; that is to say,
it would press the sides of the vessel which contained it with a
pressure equivalent to that of a column of mercury of 30 inches in
height; or what is the same thing, at the rate of about 15lb. on every
square inch of surface.
(15.) As the quantity of heat expended in raising the water from 32°
to 212°, is 180°; and as the quantity of heat necessary to convert the
same water into steam is six and a half times this quantity, it
follows that the quantity of heat requisite for converting a given
weight of water into steam, will be found by multiplying 180° by
5-1/2. The product of these numbers being 990°, it follows, that, to
convert a given weight of water at 212° into steam of the same
temperature, under the pressure of the atmosphere, when the barometer
stands at 30 inches, requires as much heat as would be necessary to
raise the same water 990° higher in temperature. The heat, not being
sensible to the thermometer, is latent heat; and accordingly it may be
stated, that the latent heat, necessary to convert water into steam
under this pressure is, in round numbers, 1000°.
(16.) All the effects of heat which we have just described may be
satisfactorily accounted for, by supposing that the principle of heat
imparts to the constituent atoms of bodies a force, by virtue of which
they acquire a tendency to repel each other. But in conjunction with
this, it is necessary to notice another force, which is known to exist
in nature: there is observable among the corpuscles of bodies a force,
in virtue of which they have a tendency to cohere, and collect
themselves together in solid concrete masses: this force is called the
attraction of cohesion. These two forces--the natural cohesion of the
particles, and the repulsive energy introduced by heat--are directly
opposed to one another, and the state of the body will be decided by
the predominance of the one or the other, or their mutual equilibrium.
If the natural cohesion of the constituent particles of the body
considerably predominate over the repulsive energy introduced by the
heat, then the cohesion will take effect; the particles of the body
will coalesce, the mass will become rigid and solid, and the
particles will hold together in one invariable mass, so that they
cannot drop asunder by the mere effect of their weight. In such cases,
a more or less considerable force must be applied, in order to break
the body, or to tear its parts asunder. Such is the quality which
characterises the state, which in mechanics is called the state of
solidity.
If the repulsive energy introduced by the application of heat be
equal, or nearly equal, to the natural cohesion with which the
particles of the body are endued, then the predominance of the
cohesive force may be insufficient to resist the tendency which the
particles may have to drop asunder by their weight. In such a case,
the constituent particles of the body cannot cohere in a solid mass,
but will separate by their weight, fall asunder, and drop into the
various corners, and adapt themselves to the shape of any vessel in
which the body may be contained. In fact, the body will take the
liquid form. In this state, however, it does not follow that the
cohesive principle will be altogether inoperative: it may, and does,
in some cases, exist in a perceptible degree, though insufficient to
resist the separate gravitation of the particles. The tendency which
particles of liquids have, in some cases, to collect in globules,
plainly indicates the predominance of the cohesive principle: drops of
water collected upon the window pane; drops of rain condensed in the
atmosphere; the tear which trickles on the cheek; drops of mercury,
which glide over any flat surface, and which it is difficult to
subdivide or scatter into smaller parts; are all obvious indications
of the predominance of the cohesive principle in liquids.
By the due application of heat, even this small degree of cohesion may
be conquered, and a preponderance of the opposite principle of
repulsion may be created. But another physical influence here
interposes its aid, and conspires with cohesion in resisting the
transmission of the body from the liquid to the vaporous state: this
force is no other than the pressure of the atmosphere, already
explained. This pressure has an obvious tendency to restrain the
particles of the liquid, to press them together, and to resist their
separation. The repulsive principle of the heat introduced must
therefore not only neutralize the cohesion, but must also impart to
the atoms of the liquid a sufficient elasticity or repulsive energy to
enable them to fly asunder, and assume the vaporous form in spite of
this atmospheric resistance.
Now it is clear, that if this atmospheric resistance be subject to any
variation in its intensity, from causes whether natural or artificial,
the repulsive energy necessary to be introduced by the heat, will vary
proportionally: if the atmospheric pressure be diminished, then less
heat will be necessary to vaporize the liquid. If, on the other hand,
this pressure be increased, a greater quantity of heat will be
required to impart the necessary elasticity.
(17.) From this reasoning we must expect that any cause, whether
natural or artificial, which diminishes the atmospheric pressure upon
the surface of a liquid, will cause that liquid to boil at a lower
temperature: and on the other hand, any cause which may increase the
atmospheric pressure upon the liquid, will render it necessary to
raise it to a higher temperature before it can boil.
These inferences we accordingly find supported by experience. Under a
pressure of 15lb. on the square inch, _i. e._ when the barometer is at
30 inches, water boils at the temperature of 212° of the common
thermometer. But if water at a lower temperature, suppose 180°, be
placed under the receiver of an air-pump, and, by the process of
exhaustion the atmospheric pressure be removed, or very much
diminished, the water will boil, although its temperature still remain
at 180°, as may be indicated by a thermometer placed in it.
On the other hand, if a thermometer be inserted air-tight in the lid
of a close digester containing water with common atmospheric air above
it, when the vessel is heated the air acquires an increased
elasticity; and being confined by the cover, presses, with increased
force, on the surface of the water. By observing the thermometer while
the vessel is exposed to the action of heat, it will be seen to rise
considerably above 212°, suppose to 230°, and would continue so to
rise until the strength of the vessel could no longer resist the
pressure within it.
The temperature at which water boils is commonly said to be 212°,
which is called _the boiling-point_ of the thermometer; but, strictly
speaking, this is only true when the barometer stands at 30 inches. If
it be lower, water will boil at a lower temperature, because the
atmospheric pressure is less; and if it be higher, as at 31, water
will not boil until it receives a higher temperature, the pressure
being greater.
According as the cohesive forces of the particles of liquids are more
or less active, they boil at greater or less temperatures. In general
the lighter liquids, such as _alcohol_ and _ether_, boil at lower
temperatures. These fluids, in fact, would boil by merely removing the
atmospheric pressure, as may be proved by placing them under the
receiver of an air-pump, and withdrawing the air. From this we may
conclude that these and similar substances would never exist in the
liquid state at all, but for the atmospheric pressure.
(18.) The elasticity of vapour raised from a boiling liquid, is equal
to the pressure under which it is produced. Thus, steam raised from
water at 212°, and, therefore, under a pressure of 15lb. on the square
inch, is endued with an elastic force which would exert a pressure on
the sides of any vessel which confines it, also equal to 15lb. on the
square inch. Since an increased pressure infers an increased
temperature in boiling, it follows, that where steam of a higher
pressure than the atmosphere is required, it is necessary that the
water should be boiled at a higher temperature.
(19.) We have already stated that there is a certain point at which
the temperature of a liquid will cease to rise, and that all the heat
communicated to it beyond this is consumed in the formation of vapour.
It has been ascertained, that when water boils at 212°, under a
pressure equal to 30 inches of mercury, a cubic inch of water forms a
cubic foot[4] of steam, the elastic force of which is equal to the
atmospheric pressure, and the temperature of which is 212°. Since
there are 1728 cubic inches in a cubic foot, it follows, that when
water at this temperature passes from the liquid to the vaporous
state, it is dilated into 1728 times its bulk.
[Footnote 4: The terms _cubic inch_ and _cubic foot_ are easily
explained. A common die, used in games of chance, has the figure which
is called a cube. It is a solid having twelve straight edges equal to
one another. It has six sides, each of which is square, and which are
also equal to one another. If its edges be each one inch in length, it
is called a _cubic inch_, if one foot, a _cubic foot_, if one yard, a
_cubic yard_, &c. This figure is represented in perspective, in fig.
6.]
(20.) We have seen that about 1000 degrees of heat must be
communicated to any given quantity of water at 212°, in order to
convert it into steam of the same temperature, and possessing a
pressure amounting to about 15 pounds on the square inch, and that
such steam will occupy above 1700 times the bulk of the water from
which it was raised. Now we might anticipate, that by abstracting the
heat thus employed in converting the liquid into vapour, a series of
changes would be produced exactly the reverse of those already
described; and such is found to be actually the case. Let us suppose a
vessel, the capacity of which is 1728 cubic inches, to be filled with
steam, of the temperature of 212°, and exerting a pressure of 15
pounds on the square inch; let 5-1/2 cubic inches of water, at the
temperature of 32°, be injected into this vessel, immediately the
steam will impart the heat, which it has absorbed in the process of
vaporisation to the water thus injected, and will itself resume the
liquid form. It will shrink into its primitive dimensions of one cubic
inch, and the heat which it will dismiss will be sufficient to raise
the 5-1/2 cubic inches of injected water to the temperature of 212°.
The contents of the vessel will thus be 6-1/2 cubic inches of water at
the temperature of 212°. One of these cubic inches is in fact the
steam which previously filled the vessel reconverted into water, the
other 5-1/2 are the injected water which has been raised from the
temperature of 32° to 212° by the heat which has been dismissed by the
steam in resuming the liquid state. It will be observed that in this
transmission no temperature is lost, since the cubic inch of water
into which the steam is converted has the same temperature as the
steam had before the cold water was injected.
These consequences are in perfect accordance with the results already
obtained from observing the time necessary to convert a given quantity
of water into steam by the application of heat. From the present
result it follows, that in the reduction of a given quantity of steam
to water it parts with as much heat as is sufficient to raise 5-1/2
cubic inches from 32° to 212°, that is, 5-1/2 times 180° or 990°.
(21.) There is an effect produced in this process to which it is
material that we should attend. The steam which filled the space of
1728 cubic inches shrinks when reconverted into water into the
dimensions of 1 cubic inch. It therefore leaves 1727 cubic inches of
the vessel it contains unoccupied. By this property steam is rendered
instrumental in the formation of a vacuum.
By allowing steam to circulate through a vessel, the air may be
expelled from it, and its place filled by steam. If the vessel be then
closed and cooled the steam will be reduced to water, and, falling in
drops on the bottom and sides of the vessel, the space which it filled
will become a _vacuum_. This may be easily established by experiment.
Let a long glass tube be provided with a hollow ball at one end, and
having the other end open.[5] Let a small quantity of spirits be
poured in at the open end, and placing the glass ball over the flame
of a lamp, let the spirits be boiled. After some time the steam will
be observed to issue copiously from the open end of the tube which is
presented upwards. When this takes place, let the tube be inverted,
and its open end plunged in a basin of cold water. The heat being thus
removed, the cool air will reconvert the steam in the tube into
liquid, and a vacuum will be produced, into which the pressure of the
atmosphere on the surface of the water in the basin will force the
water through the tube, and it will rush up with considerable force,
and fill the glass ball.
[Footnote 5: A common glass flask with a long neck will answer the
purpose.]
In this experiment it is better to use spirits than water, because
they boil at a lower heat, and expose the glass to less liability to
break, and also the tube may more easily be handled.
CHAPTER II.
FIRST STEPS IN THE INVENTION.
Futility of early claims. -- Watt, the real Inventor. -- Hero of
Alexandria. -- Blasco Garay. -- Solomon de Caus. -- Giovanni
Branca. -- Marquis of Worcester. -- Sir Samuel Morland. -- Denis
Papin. -- Thomas Savery.
(22.) In the history of the progress of the useful arts and
manufactures, there is perhaps no example of any invention the credit
of which has been so keenly contested as that of the steam engine.
Claims to it have been advanced by different nations, and by different
individuals of the same nation. The partisans of the competitors for
this honour have argued their pretensions, and pressed their claims,
with a zeal which has occasionally outstripped the bounds of
discretion; and the contest has not unfrequently been tinged with
prejudices, both national and personal, and marked with a degree of
asperity quite unworthy of so noble a cause, and altogether beneath
the dignity of science.
The efficacy of the steam engine considered as a mechanical agent
depends, first, on the several physical properties from which it
derives its operation, and, secondly, on the various pieces of
mechanism and details of mechanical arrangement by which these
properties are rendered practically available. If the merit of the
invention must be ascribed to the discoverer and contriver of these,
then the contest will be easily decided, because it will be obvious
that the prize is not due to any one individual, but must be
distributed in different proportions among several. If, however, he is
best entitled to the credit of the invention, who has by the powers of
his mechanical genius imparted to the machine that form and those
qualities from which it has received its present extensive utility,
and by which it has become an agent of transcendent power, which has
spread its beneficial effects throughout every part of the civilized
globe, then the universal consent of mankind will, as it were by
acclamation, award the prize to one individual, whose pre-eminent
genius places him far above all other competitors, and from the
application of whose mental energies to this machine may be dated
those grand effects which have rendered it a topic of interest to
every individual for whom the progress of human civilization has any
attractions. Before the era marked by the discoveries of JAMES WATT,
the steam engine, which has since become an object of such universal
interest, was a machine of extremely limited power, greatly inferior
in importance to most other mechanical contrivances used as prime
movers. But from that time it is scarcely necessary here to state that
it became a subject not of British interest only, but one with which
the progress of the human race became intimately mixed up.
Since, however, the question of the progressive developement of those
physical principles on which the steam engine depends, and of their
mechanical application, has of late years received some importance, as
well from the interest which the public manifest towards them as from
the rank of the writers who have investigated them, we have thought it
expedient to state briefly, but we trust with candour and fairness,
the successive steps which appear to have led to this invention.
The engine as it exists at present is not, strictly speaking, the
exclusive invention of any one individual: it is the result of a
series of discoveries and inventions which have for the last two
centuries been accumulating. When we attempt to trace back its
history, and to determine its first inventor, we experience the same
difficulty as is felt in tracing the head of a great river: as we
ascend its course, we are embarrassed by the variety of its tributary
streams, and find it impossible to decide which of those channels into
which it ramifies ought to be regarded as the principal stream; and it
terminates at length in a number of threads of water, each in itself
so insignificant as to be unworthy of being regarded as the source of
the majestic object which has excited the inquiry.
From a very early period the effects of heat upon liquids, and more
especially the production of steam or vapour, was regarded as a
probable source of mechanical power, and numerous speculators directed
their attention to it, and exerted their inventive faculties to derive
from it an effective mover. It was not, however, until the
commencement of the eighteenth century that any invention was produced
which was practically applied, even unsuccessfully. All the attempts
previous to that time were either suggestions which were limited to
paper or experiments confined to models; or, if they exceeded this,
they never outlived a single trial on a larger scale. Nevertheless
many of these suggestions and experiments being recorded and
accessible to future inquirers doubtless offered useful hints and some
practical aid to those more successful investigators who subsequently
contrived engines in such forms as to be practically available on a
large scale for mechanical purposes. It is right and just,
therefore--mere suggestions and abortive experiments though they may
have been--to record them, that each inventor and discoverer may
receive the just credit due to his share in this splendid mechanical
invention. We shall then in the present chapter briefly enumerate, in
chronological order, the successive steps so far as they have come to
our knowledge.
HERO OF ALEXANDRIA, 120 B. C.
[Illustration: Fig. 1.]
(23.) In a work entitled _Spiritalia seu Pneumatica_, one of the
numerous works of this philosopher which has remained to us, is
contained a description of a machine moved by vapour of water. A
hollow sphere, of which A B represents a section, is supported on two
pivots at A and B, which are the extremities of tubes A C D and B E F,
which pass into a boiler where steam is generated. This steam flows
through small apertures at the extremities A and B, and fills the
hollow sphere. One or more horizontal arms K G, I H, project from this
sphere, and are likewise filled with steam, but are closed at their
extremities. Conceive a small hole made near the extremity G, but at
one side of one of the tubes; the steam confined in the tube and globe
would immediately rush from the hole with a force proportionate to
its pressure within the globe. On the common principle of mechanics a
re-action would be produced, and the tube would recoil in the same
manner as a gun when discharged. The tubular arm K G being thus
pressed in a direction opposed to that in which the steam issues, the
sphere would revolve accordingly, and would continue to revolve so
long as the steam would continue to flow from the aperture. The force
of recoil would be increased by making a similar aperture in two or
more arms, care being taken that all the apertures should be placed so
as to cause the sphere to revolve in the same direction.
This motion being once produced might be transmitted by ordinary
mechanical contrivance to any machinery which its power might be
adequate to move.
This method of using steam is not adopted in any part or any form of
the modern steam engine.
BLASCO DE GARAY, A. D. 1543.
(24.) In the year 1826 there appeared in Zach's Correspondence a
communication from Thomas Gonsalez, Director of the Royal Archives of
Simancas, giving an account of an experiment reported to have been
made in the year 1543 by order of Charles V. in the port of Barcelona.
Blasco de Garay, a sea captain, had contrived a machine by which he
proposed to propel vessels without oars or sails. Garay concealed
altogether the nature of the machine which he used: all that was seen
during the experiment was that it consisted of a great boiler for
water, and that wheels were kept in revolution at each side of the
vessel. The experiment was made upon a vessel called the Trinity, of
200 tons burden, and was witnessed by several official personages,
whose presence on the occasion was commanded by the king. One of the
witnesses reported that it was capable of moving the vessel at the
rate of two leagues in three hours, that the machine was too
complicated and expensive, and was exposed to the danger of explosion.
The other witnesses, however, reported more favourably. The result of
the experiment was thought to be favourable: the inventor was
promoted, and received a pecuniary reward, besides having all his
expenses defrayed.
From the circumstance of the nature of the impelling power having been
concealed by the inventor it is impossible to say in what this machine
consisted, or even whether steam exerted any agency whatever in it,
or, if it did, whether it might not have been, as was most probably
the case, a reproduction of Hero's contrivance. It is rather
unfavourable to the claims advanced by the advocates of the Spaniard,
that although it is admitted that he was rewarded and promoted in
consequence of the experiment, yet it does not appear that it was
again tried, much less brought into practical use.
SOLOMON DE CAUS, 1615.
[Illustration: Fig. 2.]
(25.) A work entitled "Les Raisons des Forces Mouvantes, avec diverses
Machines tant utiles que plaisantes," published at Frankfort in 1615,
by Solomon de Caus, a native of France, contains the following
theorem:--
"_Water will mount by the help of fire higher than its level_," which
is explained and proved in the following terms:--
"The third method of raising water is by the aid of fire. On this
principle may be constructed various machines: I shall here describe
one. Let a ball of copper marked A, well soldered in every part, to
which is attached a tube and stopcock marked D, by which water may be
introduced; and also another tube marked B C, which will be soldered
into the top of the ball, and the lower end C of which shall descend
nearly to the bottom of the ball without touching it. Let the said
ball be filled with water through the tube D, then shutting the
stopcock D, and opening the stopcock in the vertical tube B C, let the
ball be placed upon a fire, the heat acting upon the said ball will
cause the water to rise in the tube B C."
Such is the description of the apparatus of De Caus as given by
himself; and on this has been founded a claim to the invention of the
steam engine. It will be observed, that neither in the original
theorem nor in the description of the machine which accompanies it, is
the word _steam_ anywhere used. Now it was well known, by all
conversant in physics, long before the date of the publication
containing this description, that atmospheric air when heated acquires
an increased elastic force. As the experiment is described, the other
part of the ball A is filled with atmospheric air; the heat of the
fire acting upon the air through the external surface of the ball, and
likewise transmitted through the water, would of course raise the
temperature of the air contained in the vessel, would thereby increase
its elasticity, and would cause the water to rise in the tube B C,
upon a physical principle altogether independent of the qualities of
steam. The effect produced, therefore, is just what might have been
expected by any one acquainted with the common properties of air,
though entirely ignorant of those of steam; and, in point of fact, the
pressure of the air is as much concerned in this case in raising the
water as the pressure of the steam.
This objection, however, is combated by another theorem contained in
the same work, in which De Caus speaks of "the strength of the vapour
produced by the action of the fire, which causes water to mount; which
vapour will issue from the stopcock with great violence after the
water has been expelled."
If De Caus be admitted to have understood the elastic property of the
vapour of water, and to have attributed the ascent of the water in the
tube C B to the pressure of that vapour upon the surface of the water
confined in the copper ball, it must be admitted that he suggested
one of the ways of using the power of steam as a mechanical agent. In
the modern steam engine this pressure is not now used against a liquid
surface, but against the solid surface of a piston. This, however,
should not take from De Caus whatever credit be due to the suggestion
of the physical property in question.
GIOVANNI BRANCA, 1629.
(26.) In a work published at Rome in 1629, entitled "Le Machine del G.
Branca," is contained a description of a machine for propelling a
wheel by a blast of steam. This contrivance consists of a wheel
furnished with flat vanes upon its rim, like the boards of a
paddle-wheel. The steam is produced in a close vessel, and made to
issue with violence from the extremity of a pipe. Being directed
against the vanes, it causes the wheel to revolve, and this motion may
be imparted by the usual mechanical contrivances to any machinery
which it was intended to move.
This contrivance has no analogy whatever to any part of the modern
steam engines in any of their various forms.
EDWARD SOMERSET, MARQUIS OF WORCESTER, 1663.
(27.) Of all the individuals to whom the invention of the steam engine
has been ascribed the most celebrated was the Marquis of Worcester,
the author of a work entitled "The Scantling of One Hundred
Inventions," but which is more commonly known by the title "A Century
of Inventions." It is to him that by far the greater number of writers
and inquirers on this subject ascribe the merit of the discovery of
the invention. This contrivance is described in the following terms in
the sixty-eighth invention in the work above named:--
"I have invented an admirable and forcible way to drive up water by
fire; not by drawing or sucking it upwards, for that must be, as the
philosopher terms it, _infra spæram activitatis_, which is but at such
a distance. But this way hath no bounder if the vessels be strong
enough. For I have taken a piece of whole cannon whereof the end was
burst, and filled it three quarters full of water, stopping and
screwing up the broken end, as also the touch-hole and making a
constant fire under it; within twenty-four hours, it burst and made a
great crack. So that, having a way to make my vessels so that they are
strengthened by the force within them, and the one to fill after the
other, I have seen the water run like a constant fountain stream forty
feet high. One vessel of water rarefied by fire driveth up forty of
cold water, and a man that tends the work has but to turn two cocks;
that one vessel of water being consumed, another begins to force and
refill with cold water, and so successively; the fire being tended and
kept constant, which the self-same person may likewise abundantly
perform in the interim between the necessity of turning the said
cocks."
These experiments must have been made before the year 1663, in which
the "Century of Inventions" was published. The description of the
machine here given, like other descriptions in the same work, was only
intended to express the effects produced, and the physical principle
on which their production depends. It is, however, sufficiently
explicit to enable any one conversant with the subsequent contrivance
of Savery, to perceive that Lord Worcester must have contrived a
machine containing all that part of Savery's engine in which the
direct force of steam is employed. As in the above description, the
separate boiler or generator of steam is distinctly mentioned; that
the steam from this is conducted into another vessel containing the
cold water to be raised; that this water is raised by the pressure of
steam acting upon its surface; that when one vessel of water has thus
been discharged, the steam acts upon the water contained in another
vessel, while the first is being replenished; and that a continued
upward current of water is maintained by causing the steam to act
alternately upon two vessels, employing the interval to fill one while
the water is discharged from the other.
On comparing this with the contrivance previously suggested by De
Caus, it will be observed, that even if De Caus knew the physical
agent by which the water was driven upwards in the apparatus contrived
by him, still it was only a means of causing a vessel of boiling water
to empty itself; and before a repetition of the process could be
obtained, the vessel should be refilled, and again boiled. In the
contrivance of Lord Worcester, on the other hand, the agency of the
steam was employed in the same manner as it is in the steam engines of
the present day, being generated in one vessel, and used for
mechanical purposes in another. Nor must this distinction be regarded
as trifling and insignificant, because on it depends the whole
practicability of using steam as a mechanical agent. Had its action
been confined to the vessel in which it was produced, it never could
have been employed for any useful purpose.
SIR SAMUEL MORLAND, 1683.
(28.) It appears, by a MS. in the Harleian Collection in the British
Museum, that a mode of applying steam to raise water was proposed to
Louis XIV. by Sir Samuel Morland. It contains, however, nothing more
than might have been collected from Lord Worcester's description, and
is only curious, because of the knowledge the writer appears to have
had of the expansion which water undergoes in passing into steam. The
following is extracted from the MS.:
"The principles of the new force of fire invented by Chevalier Morland
in 1682, and presented to his Most Christian Majesty in 1683:--'Water
being converted into vapour by the force of fire, these vapours
shortly require a greater space (about 2000 times) than the water
before occupied, and sooner than be constantly confined would split a
piece of cannon. But being duly regulated according to the rules of
statics, and by science reduced to measure, weight, and balance, then
they bear their load peaceably (like good horses,) and thus become of
great use to mankind, particularly for raising water, according to the
following table, which shows the number of pounds that may be raised
1800 times per hour to a height of six inches by cylinders half filled
with water, as well as the different diameters and depths of the said
cylinders.'"
DENIS PAPIN, 1695.
(29.) Denis Papin, a native of Blois in France, and professor of
mathematics at Marbourg, had been engaged about this period in the
contrivance of a machine in which the atmospheric pressure should be
made available as a mechanical agent by creating a partial vacuum in a
cylinder under a piston. His first attempts were directed to the
production of this vacuum by mechanical means, having proposed to
apply a water-wheel to work an air-pump, and so maintain the degree of
rarefaction required. This, however, would eventually have amounted to
nothing more than a mode of transmitting the power of the water-wheel
to another engine, since the vacuum produced in this way could only
give back the power exerted by the water-wheel diminished by the
friction of the pumps; still this would attain the end first proposed
by Papin, which was merely to transmit the force of the stream of a
river, or a fall of water, to a distant point, by partially exhausted
pipes or tubes. He next, however, attempted to produce a partial
vacuum by the explosion of gunpowder; but this was found to be
insufficient, since so much air remained in the cylinder under the
piston, that at least half the power due to a vacuum would have been
lost. "I have, therefore," proceeds Papin, "attempted to attain this
end by another method. Since water being converted into steam by heat
acquires the property of elasticity like air, and may afterwards be
recondensed so perfectly by cold that there will no longer remain the
appearance of elasticity in it, I have thought that it would not be
difficult to construct machines in which, by means of a moderate heat,
and at a small expense, water would produce that perfect vacuum which
has been vainly sought by means of gunpowder."
Papin accordingly constructed the model of a machine, consisting of a
small pump, in which was placed a solid piston, and in the bottom of
the cylinder under the piston was contained a small quantity of water.
The piston being in immediate contact with this water, so as to
exclude the atmospheric air, on applying fire to the bottom of the
cylinder steam was produced, the elastic force of which raised the
piston to the top of the cylinder: the fire being then removed, and
the cylinder being cooled by the surrounding air, the steam was
condensed and reconverted into water, leaving a vacuum in the cylinder
into which the piston was pressed by the force of the atmosphere. The
fire being applied and subsequently removed, another ascent and
descent were accomplished; and in the same manner the alternate motion
of the piston might be continued. Papin described no other form of
machine by which this property could be rendered available in
practice; but he states generally that the same end may be attained by
various forms of machines easy to be imagined.[6]
[Footnote 6: Recueil de diverses pièces touchant quelques nouvelles
machines, p. 38.]
THOMAS SAVERY, 1698.
(30.) The discovery of the method of producing a vacuum by the
condensation of steam was reproduced before 1688, by Captain Thomas
Savery, to whom a patent was granted in that year for a steam engine
to be applied to the raising of water, &c. Savery proposed to combine
the machine described by the Marquis of Worcester, with an apparatus
for raising water by suction into a vacuum produced by the
condensation of steam.
Savery appears to have been ignorant of the publication of Papin, in
1695, and states that his discovery of the condensing principle arises
from the following circumstance:--
Having drunk a flask of Florence at a tavern and flung the empty flask
on the fire, he called for a basin of water to wash his hands. A small
quantity which remained in the flask began to boil and steam issued
from its mouth. It occurred to him to try what effect would be
produced by inverting the flask and plunging its mouth in the cold
water. Putting on a thick glove to defend his hand from the heat, he
seized the flask, and the moment he plunged its mouth in the water,
the liquid immediately rushed up into the flask and filled it. (21.)
Savery stated that this circumstance immediately suggested to him the
possibility of giving effect to the atmospheric pressure by creating a
vacuum in this manner. He thought that if, instead of exhausting the
barrel of a pump by the usual laborious method of a piston and sucker,
it was exhausted by first filling it with steam and then condensing
the same steam, the atmospheric pressure would force the water from
the well into the pump-barrel and into any vessel connected with it,
provided that vessel were not more than about 34 feet above the
elevation of the water in the well. He perceived, also, that, having
lifted the water to this height, he might use the elastic force of
steam in the manner described by the Marquis of Worcester to raise the
same water to a still greater elevation, and that the same steam which
accomplished this mechanical effect would serve by its subsequent
condensation to repeat the vacuum and draw up more water. It was on
this principle that Savery constructed the first engine in which steam
was ever brought into practical operation.
CHAPTER III.
ENGINES OF SAVERY AND NEWCOMEN.
Savery's Engine. -- Boilers and their appendages. -- Working
apparatus. -- Mode of Operation. -- Defects of the Engine. --
Newcomen and Cawley. -- Atmospheric Engine. -- Accidental
Discovery of Condensation by Jet. -- Potter's Discovery of the
Method of Working the Valves.
(31.) The steam engine contrived by Savery, like every other which has
since been constructed, consists of two parts essentially distinct.
The first is that which is employed to generate the steam, which is
called the boiler, and the second, that in which the steam is applied
as a moving power.
The former apparatus in Savery's engine consists of two strong
boilers, sections of which are represented at D and E in fig. 7.; D
the greater boiler, and E the less. The tubes T and T´ communicate
with the working apparatus which we shall presently describe. A thin
plate of metal R is applied closely to the top of the greater boiler D
turning on a centre C, so that by moving a lever applied to the axis C
on the outside of the top, the sliding plate R can be brought from the
mouth of the one tube to the mouth of the other alternately. This
sliding valve is called the _regulator_, since it is by it that the
communications between the boiler and two steam vessels (hereafter
described,) are alternately opened and closed, the lever which effects
this being constantly wrought by the hand of the attendant.
Two _gauge-pipes_ are represented at G, G´, the use of which is to
determine the depth of water in the boiler. One G has its lower
aperture a little above the proper depth, and the other G´ a little
below it. Cocks are attached to the upper ends G, G´, which can be
opened or closed at pleasure. The steam collected in the top of the
boiler pressing on the surface of the water forces it up in the tubes
G, G´, if their lower ends be immersed. Upon opening the cocks G, G´,
if water be forced from them, there is too much water in the boiler,
since the mouth of G is _below_ its level. If steam issue from both
there is too little water in the boiler, since the mouth of G´ is
_above_ its level. But if steam issue from G and water from G´ the
water in the boiler is at its proper level. This ingenious contrivance
for determining the level of the water in the boiler is the invention
of Savery, and is used in many instances at the present day.
The mouth of G should be at a level of a little less than one-third of
the whole depth, and the mouth of G´ at a level a little lower than
one-third; for it is requisite that about two-thirds of the boiler
should be kept filled with water. The tube I forms a communication
between the greater boiler D and the lesser or feeding boiler E,
descending nearly to the bottom of it. This communication can be
opened and closed at pleasure by the cock K. A gauge pipe is inserted
similar to G, G´, but extending nearly to the bottom. From this boiler
a tube F extends which is continued to a cistern C (fig. 8.) and a
cock is placed at M which, when opened, allows the water from the
cistern to flow into the feeding boiler E, and which is closed when
that boiler is filled. The manner in which this cistern is supplied
will be described hereafter.
Let us now suppose that the principal boiler is filled to the level
between the gauge-pipes, and that the subsidiary boiler is nearly full
of water, the cock K and the gauge cocks G, G´ being all closed. The
fire being lighted beneath D and the water boiled, steam is produced
and is transmitted through one or other of the tubes T T´, to the
working apparatus. When evaporation has reduced the water in D below
the level of G´ it will be necessary to replenish the boiler D. This
is effected thus. A fire being lighted beneath the feeding boiler E,
steam is produced in it above the surface of the water, which having
no escape presses on the surface so as to force it up in the pipe I.
The cock K being then opened, the boiling water is forced into the
principal boiler D, into which it is allowed to flow until water
issues from the gauge cock G´. When this takes place, the cock K is
closed, and the fire removed from E until the great boiler again wants
replenishing. When the feeding boiler E has been exhausted, it is
replenished from the cistern C (fig. 8.) through the pipe F by opening
the cock M.
(32.) We shall now describe the working apparatus in which the steam
is used as a moving power.
Let V V´ (fig. 8.) be two steam vessels communicating by the tubes T
T´ (marked by the same letters in fig. 7.) with the greater boiler D.
Let S be a pipe, called the _suction pipe_, descending into the well
or reservoir from which the water is to be raised, and communicating
with each of the steam vessels through tubes D D´ by valves A A´ which
open upwards. Let F be a pipe continued from the level of the engine
of whatever higher level it is intended to elevate the water. The
steam vessels V V´ communicate with the _force-pipe_ F by valves B B´
which open upward, through the tubes E E´. Over the steam vessels and
on the force-pipe is placed a small cistern C already mentioned, which
is kept filled with cold water from the force-pipe, and from the
bottom of which proceeds a pipe terminated with a cock G. This is
called the _condensing pipe_, and can be brought alternately over each
steam vessel. From this cistern another pipe communicates with the
feeding boiler (fig. 7.) by the cock M.[7]
[Footnote 7: This pipe in fig. 9. is represented as proceeding from
the force-pipe above the cistern C.]
The communication of the pipes T T´ with the boiler can be opened and
closed, alternately, by the regulator R, (fig. 7.) already described.
Now suppose the steam vessels and tubes to be all filled with common
atmospheric air, and that the regulator be placed so that the
communication between the tube T and the boiler be opened, the
communication between the other tube T´ and the boiler being closed,
steam will flow into V through T. At first, while the vessel V is
cold, the steam will be condensed and will fall in drops of water on
the bottom and sides of the vessel. The continued supply of steam from
the boiler will at length impart such a degree of heat to the vessel V
that it will cease to condense it. Mixed with the heated air contained
in the vessel V, it will have an elastic force greater than the
atmospheric pressure, and will therefore force open the valve B,
through which a mixture of air and steam will be driven until all the
air in the vessel V will have passed out, and it will contain nothing
but the pure vapour of water.
When this has taken place, suppose the regulator be moved so as to
close the communication between the tube T and the boiler, and to stop
the further supply of steam to the vessel V; and at the same time let
the condensing pipe G be brought over the vessel V and the cock opened
so as to let a stream of cold water flow upon it. This will cool the
vessel V, and the steam with which it is filled will be condensed and
fall in a few drops of water, leaving the interior of the vessel a
vacuum. The valve B will be kept closed by the atmospheric pressure.
But the elastic force of the air between the valve A and the surface
of the water in the well or reservoir, will open A, so that a part of
this air will rush in (6.) and occupy the vessel V. The air in the
suction pipe S, being thus allowed an increased space, will be
proportionably diminished in its elastic force (6.), and its pressure
will no longer balance that of the atmosphere acting on the external
surface L[8] of the water in the reservoir. This pressure will,
therefore, force water up in the tube S until its weight, together
with the elastic force of the air above it, balances the atmospheric
pressure on L (7.). When this has taken place, the water will cease to
ascend.
[Footnote 8: Not in the diagram.]
Let us now suppose that, by shifting the regulator, the communication
is opened between T and the boiler, so that steam flows again into V.
The condensing cock G being removed, the vessel will be again heated
as before, the air expelled, and its place filled by the steam. The
condensing pipe being again allowed to play upon the vessel V, and the
further supply of steam being stopped, a vacuum will be produced in V,
and the atmospheric pressure on L will force the water through the
valve A into the vessel V, which it will nearly fill, a small quantity
of air, however, remaining above it.
Thus far the mechanical agency employed in elevating the water is the
atmospheric pressure; and the power of steam is no further employed
than in the production of a vacuum. But, in order to continue the
elevation of the water through the force-pipe F, above the level of
the steam vessel, it will be necessary to use the elastic pressure of
the steam. The vessel V is now nearly filled by the water which has
been forced into it by the atmosphere. Let us suppose that, the
regulator being shifted again, the communication between the tube T
and the boiler is opened, the condensing cock removed, and that steam
flows into V. At first coming in contact with the cold surface of the
water and that of the vessel, it is condensed; but the vessel is soon
heated, and the water formed by the condensed steam collects in a
sheet or film on the surface of the water in V, so as to form a
surface as hot as boiling water.[9] The steam then being no longer
condensed, presses on the surface of the water with its elastic force;
and when that pressure becomes greater than the atmospheric pressure,
the valve B is forced open and the water, issuing through it, passes
through E into the force-pipe F; and this is continued until the steam
has forced all the water from V, and occupies its place.
[Footnote 9: Hot water being lighter than cold, it floats on the
surface.]
The further admission of steam through T is once more stopped by
moving the regulator; and the condensing pipe being again allowed to
play on V, so as to condense the steam which fills it, produces a
vacuum. Into this vacuum, as before, the atmospheric pressure on L
will force the water, and fill the vessel V. The condensing pipe being
then closed and steam admitted through T, the water in V will be
forced by its pressure through the valve B and tube E into F, and so
the process is continued.
We have not yet noticed the other steam vessel V´, which as far as we
have described, would have remained filled with common atmospheric
air, the pressure of which, on the valve A´, would have prevented the
water raised in the suction pipe S from passing through it. However,
this is not the case; for, during the entire process which has been
described in V, similar effects have been produced in V´, which we
have only omitted to notice, to avoid the confusion which the two
processes might produce. It will be remembered, that after the steam,
in the first instance, having flowed from the boiler through T, has
blown the air out of V through B, the communication between T and the
boiler is closed. Now the same motion of the regulator which closes
this opens the communication between T´ and the boiler; for the
sliding plate R (fig. 7.) is moved from the one tube to the other, and
at the same time, as we have already stated, the condensing pipe is
brought to play on V. While, therefore, a vacuum is being formed in V
by condensation, the steam, flowing through T´, blows out the air
through B´, as already described in the other vessel V; and, while the
air in S is rushing up through A into V followed by the water raised
in S by the atmospheric pressure on L, the vessel V´ is being filled
with steam, and the air is completely expelled from it.
The communication between T and the boiler is now again opened, and
the communication between T´ and the boiler closed by moving the
regulator R (fig. 7.) from the tube T to T´; at the same time the
condensing pipe is removed from over V and brought to play upon V´.
While the steam once more expels the air from V through B, a vacuum is
formed by condensation in V´, into which the water in S rushes through
the valve A´. In the mean time V is again filled with steam. The
communication between T and the boiler is now closed, and that between
T´ and the boiler is opened, and the condensing pipe removed from V´
and brought to play on V. While the steam from the boiler forces the
water in V´ through B´ into the force-pipe F, a vacuum is being
produced in V into which water is raised by the atmospheric pressure
at L.
Thus each of the vessels V V´ is alternately filled from S and the
water thence forced into F. The same steam which forces the water from
the vessels into F, having done its duty, is condensed, and brings up
the water from S by giving effect to the atmospheric pressure.
During this process, two alternate motions or adjustments must be
constantly made; the communication between T and the boiler must be
opened, and that between T´ and the boiler closed, which is done by
one motion of the regulator. The condensing pipe at the same time must
be brought from V to play on V´ which is done by the lever placed upon
it. Again the communication between T´ and the boiler is to be opened,
and that between T and the boiler closed; this is done by moving back
the regulator. The condensing pipe is brought from V´ to V by moving
back the other lever, and so on alternately.
For the clearness and convenience of description, some slight and
otherwise unimportant changes have been made in the position of the
parts.[10] A perspective view of this engine is presented in fig. 9.
The different parts already described will easily be recognised, being
marked with the same letters as in figs. 6, 7.
[Footnote 10: In the diagrams used for explaining the principles and
operations of machines, I have found it contribute much to the
clearness of the description to adopt an arrangement of parts somewhat
different from that of the real machine. When once the nature and
principles on which the machine acts are well understood, the reader
will find no difficulty in transferring every part to its proper
place, which is represented in the perspective drawings.]
(33.) In order duly to appreciate the value of improvements, it is
necessary first to perceive the defects which these improvements are
designed to remove. Savery's steam engine, considering how little was
known of the value and properties of steam, and how low the general
standard of mechanical knowledge was in his day, is certainly highly
creditable to his genius. Nevertheless it had very considerable
defects, and was finally found to be inefficient for the most
important purposes to which he proposed applying it.
At the time of this invention, the mines in England had greatly
increased in depth, and the process of draining them had become both
expensive and difficult; so much so, that it was found in many
instances that their produce did not cover the cost of working them.
The drainage of these mines was the most important purpose to which
Savery proposed to apply his steam engine.
It has been already stated, that the pressure of the atmosphere
amounts to about 15 lbs. (3.) on every square inch. Now, a column of
water, whose base is one square inch, and whose height is 34 feet,
weighs about 15lbs. If we suppose that a perfect vacuum were produced
in the steam-vessels V V´ (fig. 8.) by condensation, the atmospheric
pressure on L would fail to force up the water, if the height of the
top of these vessels exceeded 34 feet. It is plain, therefore, that
the engine cannot be more than 34 feet above the water which it is
intended to elevate. But in fact it cannot be so much; for the vacuum
produced in the steam-vessels V V´ is never perfect. Water, when not
submitted to the pressure of the atmosphere, will vaporise at a very
low temperature (17.); and it was found that a vapour possessing a
considerable elasticity would, notwithstanding the condensation,
remain in the vessels V V´ and the pipe S, and would oppose the ascent
of the water. In consequence of this, it was found that the engine
could never be placed with practical advantage at a greater height
than 26 feet above the level of the water to be raised.
(34.) When the water is elevated to the engine, and the steam-vessels
filled, if steam be introduced above the water in V, it must first
balance the atmospheric pressure, before it can force the water
through the valve B. Here, then, is a mechanical pressure of 15lbs.
per square inch expended, without any water being raised by it. If
steam of twice that elastic force be used, it will elevate a column in
F of 34 feet in height; and if steam of triple the force be used, it
will raise a column of 68 feet high, which, added to 26 feet raised by
the atmosphere, gives a total lift of 94 feet.
In effecting this, steam of a pressure equal to three times that of
the atmosphere acts on the inner surface of the vessels V V´. One
third of this bursting of the pressure is balanced by the pressure of
the atmosphere on the external surface of the vessels; but an
effective pressure of 30lbs. per square inch still remains, tending to
burst the vessels. It was found, that the apparatus could not be
constructed to bear more than this with safety; and, therefore, in
practice the lift of such an engine was limited to about 90
perpendicular feet. In order to raise the water from the bottom of the
mine by these engines, therefore, it was necessary to place one at
every 90 feet of the depth; so that the water raised by one through
the first 90 feet should be received in a reservoir, from which it was
to be elevated the next 90 feet by another, and so on.
Besides this, it was found that sufficient strength could not be
given to those engines, if constructed upon a large scale. They were,
therefore, necessarily very limited in their dimensions, and were
incapable of raising the water with sufficient speed. Hence arose a
necessity for several engines at each level, which greatly enhanced
the expense.
(35.) These, however, were not the only defects of Savery's engines.
The consumption of fuel was enormous, the proportion of heat wasted
being much more than what was used in either forcing up the water, or
producing a _vacuum_. This will be very easily understood by attending
to the process of working the engine already described.
When the steam is first introduced from the boiler into the
steam-vessels V V´, preparatory to the formation of a vacuum, it is
necessary that it should heat these vessels up to the temperature of
the steam itself; for until then the steam will be condensed the
moment it enters the vessel by the cool surface. All this heat,
therefore, spent in raising the temperature of the steam vessels is
wasted. Again, when the water has ascended and filled the vessels V
V´, and steam is introduced to force this water through B B´ into F,
it is immediately condensed by the cold surface in V V´, and does not
begin to act until a quantity of hot water, formed by condensed steam,
is collected on the surface of the cold water which fills the vessel V
V´. Hence another source of the waste of heat arises.
When the steam begins to act upon the surface of the water in V V´,
and to force it down, the cold surface of the vessel is gradually
exposed to the steam, and must be heated while the steam continues its
action; and when the water has been forced out of the vessel, the
vessel itself has been heated to the temperature of the steam which
fills it, all which heat is dissipated by the subsequent process of
condensation. It must thus be evident that the steam used in forcing
up the water in F, and in producing a vacuum, bears a very small
proportion indeed to what is consumed in heating the apparatus after
condensation.
[Illustration: Pl. I.]
(36.) There is also another circumstance which increases the
consumption of fuel. The water must be forced through b, not only
against the atmospheric pressure, but also against a column of 68 feet
of water. Steam is therefore required of a pressure of 45lbs. on the
square inch. Consequently the water in the boiler must be boiled under
this pressure. That this should take place, it is necessary that the
water should be raised to a temperature considerably above 212° (17.),
even so high as 267°; and thus an increased heat must be given to the
boiler. Independently of the other defects, this intense heat weakened
and gradually destroyed the apparatus.
Besides the drainage of mines, Savery proposed to apply his steam
engine to a variety of other purposes; such as supplying cities with
water, forming ornamental waterworks in pleasure grounds, turning
mills, &c.
Savery was the first who suggested the method of expressing the power
of an engine with reference to that of horses. In this comparison,
however, he supposed each horse to work but 8 hours a day, while the
engine works for 24 hours. This method of expressing the power of
steam engines will be explained hereafter.
(37.) The failure of the engines proposed by Captain Savery in the
great work of drainage, from the causes which have been just mentioned
and the increasing necessity for effecting this object arising from
the circumstance of the large property in mines, which became every
year unproductive by it, stimulated the ingenuity of mechanics to
contrive some means of rendering those powers of steam exhibited in
Savery's engine practically available. Among others, Thomas Newcomen,
a blacksmith of Dartmouth, and John Cawley, a plumber of the same
place, turned their attention to this inquiry.
Newcomen appears to have resumed the old method of raising the water
from the mines by ordinary pumps, but conceived the idea of working
these pumps by some moving power less expensive than that of horses.
The means whereby he proposed effecting this was by connecting the end
of a pump rod D (fig. 10.), by a chain, with the arch-head A of a
working beam A B, playing on an axis C. The other arch-head B of this
beam was connected by a chain with the rod E of a solid piston P,
which moved air-tight in a cylinder F. If a vacuum be created beneath
the piston P, the atmospheric pressure acting upon it will press it
down with a force of 15 lbs. per square inch; and the end A of the
beam being thus raised, the pump-rod D will be drawn up. If a pressure
equivalent to the atmosphere be then introduced below the piston, so
as to neutralize the downward pressure, the piston will be in a state
of indifference as to rising or falling; and if in this case the rod D
be made heavier than the piston and its rod, so as to overcome the
friction, &c. it will descend, and elevate the piston again to the top
of the cylinder. The vacuum being again produced, another descent of
the piston, and consequent elevation of the pump-rod, will take place;
and so the process may be continued.
[Illustration: Pl. II. SAVERY'S STEAM ENGINE.]
Such was Newcomen's first conception of the _atmospheric engine_; and
the contrivance had much, even at the first view, to recommend it. The
power of such a machine would depend entirely on the magnitude of the
piston; and being independent of a highly elastic steam, would not
expose the materials to the destructive heat which was necessary for
working Savery's engine. Supposing a perfect vacuum to be produced
under the piston in the cylinder, an effective downward pressure would
be obtained, amounting to 15 times as many pounds as there are square
inches in the section of the piston.[11] Thus, if the base of the
piston were 100 square inches, a pressure equal to 1500lbs. would be
obtained.
[Footnote 11: As the calculation of the power of an engine depends on
the number of square inches in the section of the piston, it may be
useful to give a rule for computing the number of square inches in a
circle. The following rule will always give the dimensions with
sufficient accuracy:--_Multiply the number of inches in the diameter
by itself; divide the product by 14, and multiply the quotient thus
obtained by 11, and the result will be the number of square inches in
the circle._ Thus if there be 12 inches in the diameter, this
multiplied by itself gives 144, which divided by 14 gives 10-4/14,
which multiplied by 11 gives 115, neglecting fractions. There are,
therefore, 115 square inches in a circle whose diameter is 12 inches.]
(38.) In order to accomplish this design, two things were necessary:
1. To make a speedy and effectual vacuum below the piston in the
descent; and 2. To contrive a counterpoise for the atmosphere in the
ascent.
The condensation of steam immediately presented itself as the most
effectual means of accomplishing the former; and the elastic force of
the same steam previous to condensation an obvious method of effecting
the latter. Nothing now remained to carry the design into execution,
but the contrivance of means for the alternate introduction and
condensation of the steam; and Newcomen and Cawley were accordingly
granted a patent in 1707, in which Savery was united, in consequence
of the principle of condensation for which he had previously received
a patent being necessary to the projected machine. We shall now
describe the _atmospheric engine_, as first constructed by Newcomen:--
The boiler K is placed over a furnace I, the flue of which winds round
it, so as to communicate heat to every part of the bottom of it. In
the top, which is hemispherical, two gauge-pipes G G´ are placed, as
in Savery's engine, and a _puppet valve_ V, which opens upward, and is
loaded at one pound per square inch; so that when the steam produced
in the boiler exceeds the pressure of the atmosphere by more than one
pound on the square inch, the valve V is lifted, and the steam escapes
through it, and continues to escape until its pressure is sufficiently
diminished, when the valve V again falls into its seat.
The great steam-tube is represented at S, which conducts steam from
the boiler to the cylinder; and a feeding pipe T furnished with a
cock, which is opened and closed at pleasure, proceeds from a cistern
L to the boiler. By this pipe the boiler may be replenished from the
cistern, when the gauge cock G´ indicates that the level has fallen
below it. The cistern L is supplied with hot water by means which we
shall presently explain.
(39.) To understand the mechanism necessary to work the piston, let us
consider how the supply and condensation of steam must be regulated.
When the piston has been forced to the bottom of the cylinder by the
atmospheric pressure acting against a vacuum, in order to balance that
pressure, and enable it to be drawn up by the weight of the pump-rod,
it is necessary to introduce steam from the boiler. This is
accomplished by opening the cock R in the steam-pipe S. The steam
being thus introduced from the boiler, its pressure balances the
action of the atmosphere upon the piston, which is immediately drawn
to the top of the cylinder by the weight of the pump-rod D. It then
becomes necessary to condense this steam, in order to produce a
vacuum. To accomplish this the further supply of steam must be cut
off, which is done by closing the cock R. The supply of steam from the
boiler being thus suspended, the diffusion of cold water on the
external surface of the cylinder becomes necessary to condense the
steam within it. This was done by enclosing the cylinder within
another, leaving a space between them.[12] Into this space cold water
is allowed to flow from a cock M placed over it, which is supplied by
a pipe from the cistern N. This cistern is supplied with water by a
pump O, which is worked by the engine itself, from the beam above it.
[Footnote 12: The external cylinder is not represented in the
diagram.]
The cold water supplied from M, having filled the space between the
two cylinders, abstracts the heat from the inner one; and condensing
the steam, produces a vacuum, into which the piston is immediately
forced by the atmospheric pressure. Preparatory to the next descent,
the water which thus fills the space between the cylinders, and which
is warmed by the heat it has abstracted from the steam, must be
discharged, in order to give room for a fresh supply of cold water
from M. An aperture, furnished with a cock, is accordingly provided in
the bottom of the cylinder, through which the water is discharged into
the cistern L; and being warm, is adapted for the supply of the boiler
through T, as already mentioned.
The cock R being now again opened, steam is admitted below the piston,
which, as before, ascends, and the descent is again accomplished by
opening the cock M, admitting cold water between the cylinders, and
thereby condensing the steam below the piston.
The condensed steam, thus reduced to water, will collect in the bottom
of the cylinder, and resist the descent of the piston. It is,
therefore, necessary to provide an exit for it, which is done by a
valve opening _outwards_ into a tube which leads to the feeding
cistern L, into which the condensed steam is driven.
That the piston should continue to be air-tight, it was necessary to
keep a constant supply of water over it; this was done by a cock
similar to M, which allowed water to flow from the pipe M on the
piston.
(40.) Soon after the first construction of these engines, an
accidental circumstance suggested to Newcomen a much better method of
condensation than the effusion of cold water on the external surface
of the cylinder. An engine was observed to work several strokes with
unusual rapidity, and without the regular supply of the condensing
water. Upon examining the piston, a hole was found in it, through
which the water, which was poured on to keep it air-tight, flowed, and
instantly condensed the steam under it.
On this suggestion Newcomen abandoned the external cylinder, and
introduced a pipe H furnished with a cock Q into the bottom of the
cylinder, so that on turning the cock the pressure of the water in the
pipe H, from the level of the water in the cistern N, would force the
water to rise as a jet into the cylinder, and would instantly condense
the steam. This method of condensing by a jet formed a very important
improvement in the engine, and is the method still used.
(41.) Having taken a general view of the parts of the atmospheric
engine, let us now consider more particularly its operation.
When the engine is not working the weight of the pump-rod D draws down
the beam A, and draws the piston to the top of the cylinder, where it
rests. Let us suppose all the cocks and valves closed, and the boiler
filled to the proper depth. The fire being lighted beneath it, the
water is boiled until the steam acquires sufficient force to lift the
valve V. When this takes place, the engine may be started. For this
purpose the regulating valve R is opened. The steam rushes in and is
first condensed by the cold cylinder. After a short time the cylinder
acquires the temperature of the steam, which then ceases to be
condensed, and mixes with the air which filled the cylinder. The steam
and heated air, having a greater force than the atmospheric pressure,
will open a valve placed at the end X of a small tube in the bottom of
the cylinder, and which opens outwards. From this (which is called the
_blowing valve_[13]) the steam and air rush in a constant stream until
all the air has been expelled, and the cylinder is filled with the
pure vapour of water. This process is called _blowing_ the engine
preparatory to starting it.
[Footnote 13: Also called the _snifting_ valve, from the peculiar
noise made by the air and steam escaping from it.]
When it is about to be started, the engine-man closes the regulator R,
and thereby suspends the supply of steam from the boiler. At the same
time he opens the _condensing valve_ H,[14] and thereby throws up a
jet of cold water into the cylinder. This immediately condenses the
steam contained in the cylinder, and produces the vacuum. (The
atmosphere cannot enter the _blowing_ valve, because it opens
_outwards_, so that no air can enter to vitiate the vacuum.) The
atmospheric pressure above the piston now takes effect, and forces it
down in the cylinder. The descent being completed, the engine-man
closes the condensing valve H, and opens the regulator R. By this
means he stops the play of the jet within the cylinder, and admits the
steam from the boiler. The first effect of the steam is to expel the
condensing water and condensed steam which are collected in the bottom
of the cylinder through the tube Y, containing a valve which opens
_outwards_, (called the _eduction valve_,) which leads to the hot
cistern L, into which this water is therefore discharged.
[Footnote 14: Also called the _injection valve_.]
When the steam admitted through R ceases to be condensed, it balances
the atmospheric pressure above the piston, and thus permits it to be
drawn to the top of the cylinder by the weight of the rod D. This
ascent of the piston is also assisted by the circumstance of the steam
being somewhat stronger than the atmosphere.
When the piston has reached the top, the regulating valve r is closed,
and the condensing valve H opened, and another descent produced as
before, and so the process is continued.
The manipulation necessary in working this engine was, therefore, the
alternate opening and closing of two valves; the regulating and
condensing valves. When the piston reached the top of the cylinder,
the former was to be closed, and the latter opened; and, on reaching
the bottom, the former was to be opened, and the latter closed.
(42.) From the imperfect attention which even an assiduous attendant
could give to the management of these valves, the performance of the
engines was very irregular, and the waste of fuel very great, until a
boy named _Humphrey Potter_ contrived means of making the engine work
its own valves. This contrivance, although made with no other design
than the indulgence of an idle disposition, nevertheless constituted a
most important step in the progressive improvement of the
steam-engine; for by its means, not only the irregularity arising from
the negligence of attendants was avoided, but the speed of the engine
was doubled.
Potter attached strings to the levers which worked the valves, and
carrying these strings to the working beam, fastened them upon it in
such a manner that as the beam ascended and descended, it pulled the
strings so as to open and close the proper valves with the most
perfect regularity and certainty. This contrivance was afterwards much
improved by an engineer named _Beighton_, who attached to the working
beam a straight beam called a _plug frame_, carrying pins which, in
the ascent and descent of the beam, struck the levers attached to the
valves, and opened and closed them exactly at the proper moment.
The engine thus improved required no other attendance except to feed
the boiler occasionally by the cock T, and to attend the furnace.
CHAP. IV.
ENGINE OF JAMES WATT.
Advantages of the Atmospheric Engine over that of Captain Savery.
-- It contained no new Principle. -- Papin's Engine. -- James
Watt. -- Particulars of his Life. -- His first conceptions of the
means of Economising Heat. -- Principle of his projected
Improvements.
(43.) Considered practically, the engine described in the last chapter
possessed considerable advantages over that of Savery; and even at the
present day this machine is not unfrequently used in districts where
fuel is very abundant and cheap, the first cost being considerably
less than that of a modern engine. The low pressure of the steam
necessary to work it rendered the use of the atmospheric engine
perfectly safe; there being only a bursting pressure of about 1lb. per
inch, while in Savery's there was a bursting pressure amounting to
30lbs. The temperature of the steam not exceeding 216°, did not weaken
or destroy the materials; while Savery's engines required steam raised
from water at 267°, which in a short time rendered the engine unable
to sustain the pressure.
The power of Savery's engines was also very limited, both as to the
quantity of water raised, and the height to which it was elevated
(34.). On the other hand, the atmospheric engine had no other limit
than the dimensions of the piston. In estimating the power of these
engines, however, we cannot allow the full atmospheric pressure as an
effective force. The condensing water being mixed with the condensed
steam, forms a quantity of hot water in the bottom of the cylinder,
which, not being submitted to the atmospheric pressure (17.), produces
a vapour which resists the descent of the piston. In practice we find
that an allowance of at least 3lbs. per square inch should be made for
the resistance of this vapour, and 1lb. per square inch for friction,
&c.; so that the effective force will be found by subtracting these
4lbs. per square inch from the atmospheric pressure; which, if
estimated at 15lbs., leaves an effective working power of about 11lbs.
per square inch. This, however, is rather above what is commonly
obtained.
Another advantage which this engine has over those of Savery, is the
facility with which it might be applied to drive machinery by means of
the working beam.
The merit of this engine as an invention, must be ascribed principally
to its mechanism and combinations. We find in it no new principle; the
agency of atmospheric pressure acting against a vacuum, or a partial
vacuum, was long known. The formation of a vacuum by the condensation
of steam had been suggested by Papin and Savery, and carried into
practical effect by the latter. The mechanical power derivable from
the direct pressure of the elastic force of steam was distinctly
pointed out by Lord Worcester, and even prior to his time; the boiler,
gauge-pipes, and regulator of the atmospheric engine, were evidently
borrowed from Savery's engine. The idea of working a piston in a
cylinder by the atmospheric pressure against a vacuum below, was
suggested by Otto Guericke, an ingenious German philosopher, the
inventor of the air-pump, and subsequently by Papin; and the use of a
working beam could not have been unknown. Nevertheless, considerable
credit must be acknowledged to be due to Newcomen for the judicious
combination of those scattered principles. "The mechanism contrived by
him," says Tredgold, "produces all the difference between an efficient
and inefficient engine, and should be more highly valued than the
fortuitous discovery of a new principle." The rapid condensation of
steam by the injection of water, the method of clearing the cylinder
of air and water after the stroke, are two contrivances not before in
use, and which are quite essential to the effective operation of the
engine: these are wholly due to Newcomen and his associates.
(44.) The patent of Newcomen was granted in 1705; and in 1707, Papin
published a work, entitled "A New Method of raising Water by Fire," in
which a steam engine is described, which would scarcely merit notice
here but for the contests which have arisen upon the claims of
different nations for a share in the invention of the steam engine.
The publication of this work of Papin was nine years after Savery's
patent, with which he acknowledges himself acquainted, and two years
after Newcomen's. The following is a description of Papin's steam
engine:--
An oval boiler, A (fig. 11.), is filled to about two thirds of its
entire capacity with water, through a valve B in the top, which opens
upwards, and is kept down by a lever carrying a sliding weight. The
pressure on the valve is regulated by moving the weight to or from B,
like the common steelyard. This boiler communicates with a cylinder,
C, by a syphon tube furnished with a stopcock at D. The cylinder C has
a valve F in the top, closed by a lever and weight similar to B, and a
tube with a stopcock G opening into the atmosphere. In this cylinder
is placed a hollow copper piston H, which moves freely in it, and
floats upon the water. Another tube forms a communication between the
bottom of this cylinder and the bottom of a close cylindrical vessel
I, called the _air-vessel_. In this tube is a valve at K, opening
_upwards_; also a pipe terminated in a funnel, and furnished with a
valve L, which opens _downwards_. From the lower part of the
air-vessel a tube proceeds, furnished also with a stopcock M, which is
continued to whatever height the water is to be raised.
Water being poured into the funnel, passes through the valve L, which
opens downwards; and filling the tube, ascends into the cylinder C,
carrying the floating piston H on its surface, and maintains the same
level in C which it has in the funnel. In this manner the cylinder C
may be filled to the level of the top of the funnel. In this process
the cock G should be left open, to allow the air in the cylinder to
escape as the water rises.
Let us now suppose that, a fire being placed beneath the boiler, steam
is being produced. On opening the cock D, and closing G, the steam,
flowing through the syphon tube into the top of the cylinder, presses
down the floating piston, and forces the water into the lower tube.
The passage at L being stopped, since L opens _downwards_, the water
forces open the valve K, and passes into the air-vessel I. When the
piston H has been forced to the bottom of the cylinder, the cock D is
closed, and G is opened, and the steam allowed to escape into the
atmosphere. The cylinder is then replenished from the funnel as
before; and the cock G being closed, and D opened, the process is
repeated, and more water forced into the air-vessel I.
By continuing this process, water is forced into the air-vessel, and
the air which originally filled that vessel is compressed into the
space above the water; and its elastic force increases exactly in the
same proportion as its bulk is diminished. (6.) Now, suppose that half
of the vessel I has been filled by the water which is forced in, the
air above the water being reduced to half its bulk has acquired twice
the elastic force, and therefore presses on the surface of the water
with twice the pressure of the atmosphere. Again, if two thirds of the
air-vessel be filled with water, the air is compressed into one third
of its bulk, and presses on the surface of the water with three times
the pressure of the atmosphere, and so on.
[Illustration: Pl. III.]
Now if the cock M be opened, the pressure of the condensed air will
force the water up in the tube N, and it will continue to rise until
the column balances the pressure of the condensed air. If, when the
water is suspended in the tube, and the cock M open, the vessel I is
half filled, the height of the column in N will be 34 feet, because 34
feet of water has a pressure equal to the atmosphere; and this, added
to the atmospheric pressure on it, gives a total pressure equal to
twice that of the atmosphere, which balances the pressure of the air
in I reduced to half its bulk. If two thirds of I be filled with
water, a column of 68 feet will be supported in N; for such a column,
united with the atmospheric pressure on it, gives a total pressure
equal to three times that of the atmosphere, which balances the air in
I compressed into one third of its original bulk.
By omitting the principle of condensation, this machine loses 26 feet
in the perpendicular lift. But, indeed, in every point of view, it is
inferior to the engines of Savery and Newcomen.
(45.) From the construction of the atmospheric engine by Newcomen, in
1705, for about half a century, no very important step had been made
in the improvement of the steam engine. During this time the
celebrated Smeaton had given much attention to the details of the
atmospheric engine, and brought that machine to as high a state of
perfection as its principle seemed to admit, and as it has ever since
reached.
In the year 1763, JAMES WATT, a name illustrious in the history of
mechanical science, commenced his experiments on steam. He was born at
Greenock, in the year 1736; and at the age of 16 was apprenticed to a
mathematical instrument-maker, with whom he spent four years. At the
age of 20 he removed to London, where he still pursued the same trade
under a mathematical instrument-maker in that city. After a short
time, however, finding his health declining, he returned to Scotland,
and commenced business on his own account at Glasgow. In 1757 he was
appointed mathematical instrument-maker to the university of Glasgow,
where he resided and carried on business.
This circumstance produced an acquaintance between him and the
celebrated Dr. Robison, then a student in Glasgow, who directed Watt's
attention to the steam engine. In his first experiments he used steam
of a high-pressure; but found it attended with so much danger of
bursting the boiler, and difficulty of keeping the joints tight, and
other objections, that he relinquished the inquiry at that time.
(46.) In the winter of 1763, Watt was employed to repair the model of
an atmospheric engine, belonging to the natural philosophy class in
the university--a circumstance which again turned his attention to the
subject of the steam engine. He found the consumption of steam in
working this model so great, that he inferred that the quantity
wasted, must have had a very large proportion to that used in working
the piston. His first conclusion was, that the material of the
cylinder (brass) was too good a conductor of heat, and that much was
thereby lost. He made some experiments, accordingly, with wooden
cylinders, soaked in linseed oil, which, however, he soon laid aside.
Further consideration convinced him that a prodigious waste of steam
was essential to the very principle of the atmospheric engine. This
will be easily understood.
When the steam has filled the cylinder so as to balance the
atmospheric pressure on the piston, the cylinder must have the same
temperature as the steam itself. Now, on introducing the condensing
jet, the steam mixed with this water forms a mass of hot water in the
bottom of the cylinder. This water, not being under the atmospheric
pressure, boils at very low temperatures, and produces a vapour which
resists the descent of the piston.
The heat of the cylinder itself assists this process; so that in order
to produce a tolerably perfect vacuum, it was found necessary to
introduce a quantity of condensing water, sufficient to reduce the
temperature of the water in the cylinder lower than 100°, and
consequently to cool the cylinder itself to that temperature. Under
these circumstances, the descent of the piston was found to suffer
very little resistance from any vapour within the cylinder: but then
on the subsequent ascent, an immense waste of steam ensued; for the
steam, on being admitted under the piston, was immediately condensed
by the cold cylinder and water of condensation, and this continued
until the cylinder became again heated up to 212°, to which point the
whole cylinder should be heated before the ascent could be completed.
Here, then, was an obvious and an extensive cause of the waste of
heat. At every descent of the piston, the cylinder should be cooled
below 100°; and at every ascent it should be again heated to 212°. It,
therefore, became a question whether the force gained by the increased
perfection of the vacuum was adequate to the waste of fuel in
producing the vacuum; and it was found, on the whole, more profitable
not to cool the cylinder to so low a temperature, and consequently to
work with a very imperfect vacuum, and a diminished power.
Watt, therefore, found the engine involved in this dilemma: either
much or little condensation-water must be used. If much were used, the
vacuum would be perfect; but then the cylinder would be cooled, and
would entail an extensive waste of fuel in heating it. If little were
used, a vapour would remain, which would resist the descent of the
piston, and rob the atmosphere of a part of its power. The great
problem then pressed itself on his attention, _to condense the steam
without cooling the cylinder_.
From the small quantity of water in the form of steam which filled the
cylinder, and the large quantity of injected water to which this
communicated heat, Watt was led to inquire what proportion the bulk of
water in the liquid state bore to its bulk in the vaporous state; and
also what proportion subsisted between the heat which it contained in
these two states. He found by experiment that a cubic inch of water
formed about a cubic foot of steam; and that the cubic foot of steam
contained as much heat as would raise a cubic inch of water to about
1000°. (15.) This gave him some surprise, as the thermometer indicated
the same temperature, 212°, for both the steam and the water from
which it was raised. What then became of all the additional heat which
was contained in the steam, and not indicated by the thermometer? Watt
concluded that this heat must be in some way engaged in maintaining
the water in its new form.
Struck with the singularity of this circumstance, he communicated it
to Dr. Black, who then explained to Watt his doctrine of _latent
heat_, which he had been teaching for a short time before that, but of
which Watt had not previously heard; and thus, says Watt, "I stumbled
upon one of the material facts on which that theory is founded."
(47.) Watt now gave his whole mind to the discovery of a method of
"condensing the steam without cooling the cylinder." The idea occurred
to him of providing a vessel separate from the cylinder, in which a
constant vacuum might be sustained. If a communication could be opened
between the cylinder and this vessel, the steam, by its expansive
property, would rush from the cylinder to this vessel, where, being
exposed to cold, it would be immediately condensed, the cylinder
meanwhile being sustained at the temperature of 212°.
This happy conception formed the first step of that brilliant career
which has immortalized the name of Watt, and which has spread his fame
to the very skirts of civilization. He states, that the moment the
notion of "separate condensation" struck him, all the other details of
his improved engine followed in rapid and immediate succession, so
that in the course of a day his invention was so complete that he
proceeded to submit it to experiment.
His first notion was, as we have stated, to provide a separate vessel,
called a _condenser_, having a pipe or tube communicating with the
cylinder. This condenser he proposed to keep cold by being immersed in
a cistern of cold water, and by providing a jet of cold water to play
within it. When the communication with the cylinder is opened, the
steam, rushing into the condenser, is immediately condensed by the jet
and the cold surface. But here a difficulty presented itself, viz. how
to dispose of the condensing water, and condensed steam, which would
collect in the bottom of the condenser. But besides this, a quantity
of air or permanent uncondensible gas would collect from various
sources. Water in its ordinary state always holds more or less air in
combination with it: the air thus combined with the water in the
boiler passes through the tubes and cylinder with the steam, and would
collect in the condenser. Air also would enter in combination with the
condensing water, which would be set free by the heat it would receive
from admixture with the steam. The air proceeding from these sources
would, as Watt foresaw, accumulate in the condenser, even though the
water might be withdrawn from it, and would at length resist the
descent of the piston. To remedy this he proposed _to form a
communication between the bottom of the condenser and a pump which he
called the_ AIR PUMP, _so that the water and air which might be
collected in the condenser would be drawn off_; and it was easy to see
how this pump could be worked by the machine itself. This constituted
the second great step in the invention.
To make it air-tight in the cylinder, it had been found necessary to
keep a quantity of water supplied above the piston. In the present
case, any of this water which might escape through the piston, or
between it and the cylinder, would boil, the cylinder being kept at
212°; and would thus, by the steam it would produce, vitiate the
vacuum. To avoid this inconvenience, Watt proposed to lubricate the
piston, and keep it air-tight, by employing melted wax and tallow.
Another inconvenience was still to be removed. On the descent of the
piston, the air which must then enter the cylinder would lower its
temperature; so that upon the next ascent, some of the steam which
would enter it would be condensed, and hence would arise a source of
waste. To remove this difficulty, Watt proposed to close the top of
the cylinder altogether by an air-tight and steam-tight cover,
allowing the piston-rod to play through a hole furnished with a
stuffing-box, and _to press down the piston by steam instead of the
atmosphere_.
This was the third step in this great invention, and one which
totally changed the character of the machine. It now became really a
_steam engine_ in every sense; for the pressure above the piston was
the elastic force of steam, and the vacuum below it was produced by
the condensation of steam; so that steam was used both directly and
indirectly as a moving power; whereas, in the atmospheric engine, the
indirect force of steam only was used, being adopted merely as an easy
method of producing a vacuum.
The last difficulty respecting the economy of heat which remained to
be removed, was the circumstance of the cylinder being liable to be
cooled on the external surface by the atmosphere. To obviate this, he
first proposed casing the cylinder in wood, that being a substance
which conducted heat slowly. He subsequently, however, adopted a
different method, and inclosed one cylinder within another, leaving a
space between them, which he kept constantly supplied with steam. Thus
the inner cylinder was kept continually at the temperature of the
steam which surrounded it. The outer cylinder was called the
_jacket_.[15]
[Footnote 15: It is a remarkable circumstance, that Watt used the same
means for keeping the cylinder hot as Newcomen used in his earlier
engines to cool it. (38.)]
(48.) Watt computed that in the atmospheric engine three times as much
heat was wasted in heating the cylinder, &c. as was spent in useful
effect. And, as by the improvements proposed by him nearly all this
waste was removed, he contemplated, and afterwards actually effected,
a saving of three fourths of the fuel.
The honour due to Watt for his discoveries is enhanced by the
difficulties under which he laboured from contracted circumstances at
the time he made them. He relates, that when he was endeavouring to
determine the heat consumed in the production of steam, his means did
not permit him to use an efficient and proper apparatus, which would
have been attended with expense; and it was by experiments made with
apothecaries' phials, that he discovered the property already
mentioned, which was one of the facts on which the doctrine of latent
heat was founded.
A large share of the merit of Watt's discoveries has, by some writers,
been attributed to Dr. Black, to whose instructions on the subject of
latent heat it is said that Watt owed the knowledge of those facts
which led to his improvements. Such, however, was not the case; and
the mistake arose chiefly from some passages respecting Watt in the
works of Dr. Robison, in one of which he states that Watt had been a
_pupil_ and intimate friend of Dr. Black; and that he attended two
courses of his lectures at college in Glasgow. Such, however, was not
the case; for "Unfortunately for me," says Watt in a letter to Dr.
Brewster, "the necessary avocations of my business prevented me from
attending his or any other lectures at college. In further noticing
Dr. Black's opinion, that his fortunate observation of what happens in
the formation and condensation of elastic vapour 'has contributed in
no inconsiderable degree to the public good, by suggesting to my
friend Mr. Watt of Birmingham, then of Glasgow, his improvements on
the steam-engine,' it is very painful for me to controvert any opinion
or assertion of my revered friend; yet, in the present case, I find it
necessary to say, that he appears to me to have fallen into an error.
These improvements proceeded upon the established fact, that steam was
condensed by the contact of cold bodies, and the later known one, that
water boiled at heats below 100°, and consequently that a vacuum could
not be obtained unless the cylinder and its contents were cooled every
stroke below the heat."
CHAPTER V.
WATT'S SINGLE-ACTING STEAM ENGINE.
Expansive Principle applied. -- Failure of Roebuck, and
Partnership with Bolton. -- Patent extended to 1800. -- Counter.
-- Difficulties in getting the Engines into use.
(49.) The first machine in which Watt realised the conceptions which
we mentioned in the last chapter, is that which was afterwards called
his _Single-acting Steam Engine_. We shall now describe the working
apparatus in this machine.
The cylinder is represented at C (fig. 12.)--in which the piston P
moves steam-tight. It is closed at the top, and the piston-rod being
very accurately turned, runs in a steam-tight collar B furnished with
a stuffing-box, and constantly supplied with melted tallow or wax.
Through a funnel in the top of the cylinder, melted grease flows upon
the piston so as to maintain it steam-tight. Two boxes A A, containing
the valves for admitting and withdrawing the steam, connected by a
tube of communication T, are attached to the cylinder; the action of
these valves will be presently described. Below the cylinder, placed
in a cistern of cold water, is a close cylindrical vessel D, called
the condenser, communicating with the cylinder by a tube T´, leading
to the lower valve-box A. In the side of this condenser is inserted a
tube, the inner end of which is pierced with holes like the rose of a
watering-pot; and a cock E in the cold cistern is placed on the
outside, through which, when open, the water passing, rises in a jet
on the inside.
The tube S, which conducts steam from the boiler, enters the top of
the upper valve-box at F. Immediately under it is placed a valve G,
which is opened and closed by a lever or rod G´. This valve, when
open, admits steam to the top of the piston, and also to the tube T,
which communicates between the two valve-boxes, and when closed
suspends the admission of steam. There are two valves in the lower
box, one H in the top worked by the lever H´, and one I in the bottom
worked by the lever I´. The valve H, when open, admits steam to pass
from the cylinder _above_ the piston, by the tube T, to the cylinder
_below_ the piston, the valve I being supposed in this case to be
closed. This valve I, when open, (the valve H being closed,) admits
steam to pass from below the cylinder through T´ to the condenser.
This steam, entering the condenser, meets the jet, admitted to play by
the valve E, and is condensed.
The valve G is called the _upper steam valve_; H, _lower steam valve_;
I, the _exhausting-valve_; and E, the _condensing valve_. Let us now
consider how these valves must be worked in order to produce the
alternate ascent and descent of the piston.
It is in the first place necessary that all the air which fills the
cylinder, tubes, and condenser should be expelled. To accomplish this
it is only necessary to open at once the valves G, H, and I. The steam
then rushing from F through the valve G will pass into the upper part
of the cylinder, and through the tube T and the valve H into the lower
part, and also through the valve I into the condenser. After the steam
ceases to be condensed by the cold of the apparatus, it will rush out
mixed with air through the valve M, which opens outward; and this will
continue until all the air has been expelled, and the apparatus filled
with pure steam. Then suppose all the valves again closed. The
cylinder both above and below the piston is filled with steam; and the
steam which filled the condenser being cooled by the cold surface, a
vacuum has been produced in that vessel.
The apparatus being in this state, let the upper steam valve G, the
exhausting-valve I, and the condensing valve E be opened. Steam will
thus be admitted through G to press on the top of the piston; and this
steam will be prevented from circulating to the lower part of the
cylinder by the lower steam-valve H being closed. Also the steam which
filled the cylinder below the piston rushes through the open
exhausting-valve I to the condenser, where it meets the jet allowed to
play by the open condensing valve E. It is thus instantly condensed,
and a vacuum is left in the cylinder below the piston. Into this
vacuum the piston is pressed without resistance by the steam which is
admitted through G. When the piston has thus been forced to the bottom
of the cylinder, let the three valves G, I, and E, which were before
opened, be closed, and let the lower steam-valve H be opened. The
effects of this change are easily perceived. By closing the upper
steam-valve G, the further admission of steam to the apparatus is
stopped. By closing the exhausting-valve I, all transmission of steam
from the cylinder to the condenser is stopped. Thus the steam which is
_in_ the cylinder, valve-boxes, and tubes is shut up in them, and no
more admitted, nor any allowed to escape. By closing the condensing
valve E, the play of the jet in the condenser is suspended.
Previously to opening the valve H, the steam contained in the
apparatus was confined to the part of the cylinder _above_ the piston
and the tube T and the valve-box A. But on opening this valve, the
steam is allowed to circulate above and below the piston; and in fact
through every part included between the upper steam valve G, and the
exhausting-valve I. The same steam circulating on both sides, the
piston is thus equally pressed upward and downward.
In this case there is no force tending to retain the piston at the
bottom of the cylinder except its own weight. Its ascent is produced
in the same manner as the ascent of the piston in the atmospheric
engine. The piston-rod is connected by chains G to the arch-head of
the beam, and the weight of the pump-rod R, or any other counterpoise
acting on the chains suspended from the other arch-head, draws the
piston to the top of the cylinder.
When the piston has arrived at the top of the cylinder, suppose the
three valves G, I, and E to be again opened, and H closed. Steam
passes from the steam-pipe F through the upper steam-valve G to the
top of the piston, and at the same time the steam which filled the
cylinder below the piston is drawn off through the open
exhausting-valve I into the condenser, where it is condensed by the
jet allowed to play by the open condensing valve E. The pressure of
the steam above the piston then forces it without resistance into the
vacuum below it, and so the process is continued.
It should be remembered, that of the four valves necessary to work the
piston, three are to be opened the moment the piston reaches the top
of the cylinder, and the fourth is to be closed; and on the piston
arriving at the bottom of the cylinder, these three are to be closed
and the fourth opened. The three valves which are thus opened and
closed together are the upper steam-valve, the exhausting-valve, and
the condensing valve. The lower steam-valve is to be opened at the
same instant that these are closed, and _vice versâ_. The manner of
working these valves we shall describe hereafter.
The process which has just been described, if continued for any
considerable number of reciprocations of the piston, would be attended
with two very obvious effects which would obstruct and finally destroy
the action of the machine. First, the condensing water and condensed
steam would collect in the condenser D, and fill it; and secondly, the
water in the cistern in which the condenser is placed would gradually
become heated, until at last it would not be cold enough to condense
the steam when introduced in the jet. Besides this, it will be
recollected that water boils in a vacuum at a very low temperature
(17); and, therefore, the hot water collected in the bottom of the
condenser would produce steam which, rising into the cylinder through
the exhausting-valve, would resist the descent of the piston, and
counteract the effects of the steam above it. A further disadvantage
arises from the air or other permanently elastic fluid which enters in
combination with the water, both in the boiler and condensing jet, and
which is disengaged by its own elasticity.
To remove these difficulties, a pump is placed near the condenser
communicating with it by a valve M, which opens from the condenser
into the pump. In this pump is placed a piston which moves air-tight,
and in which there is a valve N, which opens upwards. Now suppose the
piston at the bottom of the pump. As it rises, since the valve in it
opens _upwards_, no air can pass _down_ through it, and consequently
it leaves a vacuum _below_ it. The water and any air which may be
collected in the condenser open the valve M, and pass into the lower
part of the pump from which they cannot return in consequence of the
valve M opening _outwards_. On the descent of the pump-piston, the
fluids which occupy the lower part of the pump, force open the
piston-valve N; and passing through it, get _above_ the piston, from
which their return is prevented by the valve N. In the next ascent,
the piston lifts these fluids to the top of the pump, whence they are
discharged through a conduit into a small cistern B by a valve K which
opens outwards. The water which is thus collected in B is heated by
the condensed steam, and is reserved in B, which is called the hot
well for feeding the boiler, which is effected by means which we shall
presently explain. The pump which draws off the hot water and air from
the condenser is called the _air-pump_.
(50.) We have not yet explained the manner in which the valves and the
air-pump piston are worked. The rod Q of the latter is connected with
the working beam, and the pump is therefore wrought by the engine
itself. It is not very material to which arm of the beam it is
attached. If it be on the same side of the centre of the beam with the
cylinder, it rises and falls with the steam-piston; but if it be on
the opposite side, the pump-piston rises when the steam-piston falls,
and _vice versâ_. In the single-engine there are some advantages in
the latter arrangement. As the steam-piston _descends_, the steam
rushes into the condenser, and the jet is playing; and this,
therefore, is the most favourable time for drawing out the water and
condensed steam from the condenser by the ascent of the pump-piston,
since by this means the descent of the steam-piston is assisted; an
effect which would not be produced if the steam-piston and pump-piston
descended together.
With respect to the method of opening and closing the valves, it is
evident that the three valves which are simultaneously opened and
closed may be so connected as to be worked by the same lever. This
lever may be struck by a pin fixed upon the rod Q of the air-pump, so
that when the pistons have arrived at the top of the cylinders the pin
strikes the lever and opens the three valves. A catch or detent is
provided for keeping them open during the descent of the piston, from
which they are disengaged in a similar manner on the arrival of the
piston at the bottom of the cylinder, and they close by their own
weight.
In exactly the same way the lower steam-valve is opened on the arrival
of the piston at the bottom of the cylinder, and closed on its arrival
at the top by the action of a pin placed on the piston-rod of the
air-pump.
(51.) Soon after the invention of these engines, Watt found that in
some instances inconvenience arose from the too rapid motion of the
steam-piston at the end of its stroke, owing to its being moved with
an _accelerated motion_. This was owing to the uniform action of the
steam-pressure upon it: for upon first putting it in motion at the top
of the cylinder, the motion was comparatively slow; but from the
continuance of the same pressure the velocity with which the piston
descended was continually increasing, until it reached the bottom of
the cylinder, where it acquired its greatest velocity. To prevent
this, and to render the descent as nearly as possible uniform, it was
proposed to cut off the steam before the descent was completed, so
that the remainder might be effected merely by the expansion of the
steam which was admitted to the cylinder. To accomplish this, he
contrived, by means of a pin on the rod of the air-pump, to close the
upper steam-valve when the steam-piston had completed one-third of its
entire descent, and to keep it closed during the remainder of the
descent, and until the piston again reached the top of the cylinder.
By this arrangement the steam pressed the piston with its full force
through one-third of the descent, and thus put it into motion; during
the other two thirds the steam thus admitted acted merely by its
expansive force, which became less in exactly the same proportion as
the space given to it by the descent of the piston increased. Thus,
during the last two thirds of the descent the piston is urged by a
gradually decreasing force, which in practice was found just
sufficient to sustain in the piston a uniform velocity.
(52.) We have already mentioned the difficulty arising from the water
in the cistern, in which the condenser and air-pump are placed,
becoming heated, and the condensation therefore being imperfect. To
prevent this, a waste-pipe is placed in this cistern, from which the
water is continually discharged, and a pump L (called the
_cold-water-pump_) is worked by the engine itself, which raises a
supply of cold water and sends it through a pipe in a constant stream
into the cold cistern. The waste-pipe, through which the water flows
from the cistern, is placed near the top of it, since the heated
water, being lighter than the cold, remains on the top. Thus the
heated water is continually flowing off, and a constant stream of cold
water supplied. The piston-rod of the cold-water-pump is attached to
the beam (by which it is worked), usually on the opposite side from
the cylinder.
Another pump O (called the _hot-water-pump_) enters the _hot well_ B;
and raising the water from it, forces it through a tube to the boiler
for the purpose of feeding it. The manner in which this is effected
will be more particularly described hereafter. A part of the heat
which would otherwise be lost, is thus restored to the boiler to
assist in the production of fresh steam. We may consider a portion of
the heat to be in this manner _circulating_ continually through the
machine. It proceeds from the boiler in steam, works the piston,
passes into the condenser, and is reconverted into hot water; thence
it is passed to the hot well, from whence it is pumped back into the
boiler, and is again converted into steam, and so proceeds in constant
circulation.
From what has been described, it appears that there are four pistons
attached to the great beam and worked by the piston of the
steam-cylinder. On the same side of the centre with the cylinder is
the piston-rod of the air-pump, and on the opposite side are the
piston-rods of the hot-water pump and the cold-water-pump; and lastly,
at the extremity of the beam opposite to that at which the
steam-piston works, is the piston of the pump to be wrought by the
engine.
(53.) The position of these piston-rods with respect to the centre of
the beam depends on the play necessary to be given to the piston. If
the play of the piston be short, its rod will be attached to the beam
near the centre; and if longer, more remote from the centre. The
cylinder of the air-pump is commonly half the length of the
steam-cylinder, and its piston-rod is attached to the beam at the
point exactly in the middle between the end of the beam and the
centre. The hot-water pump not being required to raise a considerable
quantity of water, its piston requires but little play, and is
therefore placed near the centre of the beam, the piston-rod of the
cold-water pump being farther from the centre.
(54.) It appears to have been about the year 1763, that Watt made
these improvements in the steam engine, and constructed a model which
fully realized his expectations. Either from want of influence or the
fear of prejudice and opposition, he did not make known his discovery
or attempt to secure it by a patent at that time. Having adopted the
profession of a land surveyor, his business brought him into
communication with Dr. Roebuck, at that time extensively engaged in
mining speculations, who possessed some command of capital, and was of
a very enterprising disposition. By Roebuck's assistance and
countenance, Watt erected an engine of the new construction at a coal
mine on the estate of the Duke of Hamilton, at Kinneil near
Burrowstoness. This engine being a kind of experimental one, was
improved from time to time as circumstances suggested, until it
reached considerable perfection. While it was being erected, Watt in
conjunction with Roebuck applied for and obtained a patent to secure
the property in the invention. This patent was enrolled in 1769, six
years after Watt invented the improved engine.
Watt was now preparing to manufacture the new engines on an extensive
scale, when his partner Roebuck suffered a considerable loss by the
failure of a mining speculation in which he had engaged, and became
involved in embarrassments, so as to be unable to make the pecuniary
advances necessary to carry Watt's designs into execution. Again
disappointed, and harassed by the difficulties which he had to
encounter, Watt was about to relinquish the further prosecution of his
plans, when Mr. Matthew Bolton, a gentleman who had established a
factory at Birmingham a short time before, made proposals to purchase
Dr. Roebuck's share in the patent, in which he succeeded; and in 1773,
Watt entered into partnership with Bolton.
His situation was now completely changed. Bolton was not only a man of
extensive capital, but also of considerable personal influence, and
had a disposition which led him, from taste, to undertakings which
were great and difficult, and which he prosecuted with the most
unremitting ardency and spirit. "Mr. Watt," says Playfair, "was
studious and reserved, keeping aloof from the world; while Mr. Bolton
was a man of address, delighting in society, active, and mixing with
people of all ranks with great freedom, and without ceremony. Had Mr.
Watt searched all Europe, he probably would not have found another
person so fitted to bring his invention before the public, in a manner
worthy of its merit and importance; and although of most opposite
habits, it fortunately so happened that no two men ever more cordially
agreed in their intercourse with each other."
The delay in the progress of the manufacture of engines occasioned by
the failure of Dr. Roebuck was such, that Watt found that the duration
of his patent would probably expire before he would even be reimbursed
the necessary expenses attending the various arrangements for the
manufacture of the engines. He therefore, with the advice and
influence of Bolton, Roebuck, and other friends, in 1775, applied to
parliament for an extension of the terms of his patent, which was
granted for 25 years from the date of his application, so that his
exclusive privilege should expire in 1800.
An engine was now erected at Soho (the name of Bolton's factory) as a
specimen for the examination of mining speculators, and the engines
were beginning to come into demand. The manner in which Watt chose to
receive remuneration from those who used his engines was as remarkable
for its ingenuity as for its fairness and liberality. He required that
one-third of the saving of coals effected by his engines, compared
with the atmospheric engines hitherto used, should be paid to him,
leaving the benefit of the other two-thirds to the public. Accurate
experiments were made to ascertain the saving of coals; and as the
amount of this saving in each engine depended on the length of time it
was worked, or rather on the number of descents of the piston, Watt
invented a very ingenious method of determining this. The vibrations
of the great working beam were made to communicate with a train of
wheelwork, in the same way as those of a pendulum communicate with the
work of a clock. Each vibration of the beam moved one tooth of a small
wheel, and the motion was communicated to a hand or index, which moved
on a kind of graduated plate like the dial plate of a clock. The
position of this hand marked the number of vibrations of the beam.
This apparatus, which was called the _counter_, was locked up and
secured by two different keys, one of which was kept by the
proprietor, and the other by Bolton and Watt, whose agents went round
periodically to examine the engines, when the counters were opened by
both parties and examined, and the number of vibrations of the beam
determined, and the value of the patent third found.[16]
[Footnote 16: The extent of the saving in fuel may be judged from
this: that for three engines erected at Chacewater mine in Cornwall,
it was agreed by the proprietors that they would compound for the
patent third at 2400_l._ per annum; so that the whole saving must have
exceeded 7200_l._ per annum.]
Notwithstanding the manifest superiority of these engines over the old
atmospheric engines; yet such were the influence of prejudice and the
dislike of what is new, that Watt found great difficulties in getting
them into general use. The comparative first cost also probably
operated against them; for it was necessary that all the parts should
be executed with great accuracy, which entailed proportionally
increased expense. In many instances they felt themselves obliged to
induce the proprietors of the old atmospheric engines to replace them
by the new ones, by allowing them in exchange an exorbitant price for
the old engines; and in some cases they were induced to erect engines
at their own expense, upon an agreement that they should only be paid
if the engines were found to fulfil the expectations, and brought the
advantages which they promised. It appeared since, that Bolton and
Watt had actually expended a sum of nearly 50,000_l._ on these engines
before they began to receive any return. When we contemplate the
immense advantages which the commercial interests of the country have
gained by the improvements in the steam engine, we cannot but look
back with disgust at the influence of that fatal prejudice which
opposes the progress of improvement under the pretence of resisting
innovation. It would be a problem of curious calculation to determine
what would have been lost to the resources of this country, if chance
had not united the genius of such a man as Watt with the spirit,
enterprise, and capital, of such a man as Bolton! The result would
reflect little credit on those who think novelty alone a sufficient
reason for opposition.
CHAPTER VI.
DOUBLE-ACTING STEAM ENGINE.
The Single-Acting Engine unfit to impel Machinery. -- Various
contrivances to adapt it to this purpose. -- Double-Cylinder. --
Double-Acting Cylinder. -- Various mode of connecting the Piston
with the Beam. -- Rack and Sector. -- Double Chain. -- Parallel
Motion. -- Crank. -- Sun and Planet Motion. -- Fly Wheel. --
Governor.
In the atmospheric engine of Newcomen, and in the improved steam
engine of Watt, described in the last chapter, the action of the
moving power is an intermitting one. While the piston descends, the
moving power is in action, but its action is suspended during the
ascent. Thus the opposite or working end of the beam can only be
applied in cases where a lifting power is required. This action is
quite suitable to the purposes of pumping, which was the chief or only
object to which the steam engine had hitherto been applied. In a more
extended application of the machine, this intermission of the moving
power and its action taking place only in one direction would be
inadmissible. To drive the machinery generally employed in
manufactures a constant and uniform force is required; and to render
the steam engine available for this purpose, it would be necessary
that the beam should be driven by the moving power as well in its
ascent as in its descent.
When Watt first conceived the notion of extending the application of
the engine to manufactures generally, he proposed to accomplish this
double action upon the beam by placing a steam cylinder under each end
of it, so that while each piston would be ascending, and not impelled
by the steam, the other would be descending, being urged downwards by
the steam above it acting against the vacuum below. Thus, the power
acting on each during the time when its action on the other would be
suspended, a constant force would be exerted upon the beam, and the
uniformity of the motion would be produced by making both cylinders
communicate with the same boiler, so that both pistons would be driven
by steam of the same pressure. One condenser might also be used for
both cylinders, so that a similar vacuum would be produced under each.
This arrangement, however, was soon laid aside for one much more
simple and obvious. This consisted in the production of exactly the
same effect by a single cylinder in which steam was introduced
alternately above and below the piston, being at the same time
withdrawn by the condenser at the opposite side. Thus the piston being
at the top of the cylinder, steam is introduced from the boiler above
it, while the steam in the cylinder below it is drawn off by the
condenser. The piston, therefore, is pressed from above into the
vacuum below, and descends to the bottom of the cylinder. Having
arrived there, the top of the cylinder is cut off from all
communication with the boiler; and, on the other hand, a communication
is opened between it and the condenser. The steam which has pressed
the piston down is therefore drawn off by the condenser, while a
communication is opened between the boiler and the bottom of the
cylinder, so that steam is admitted below the piston: the piston, thus
pressed from below into the vacuum above, ascends, and in the same way
the alternate motion is continued. Such is the principle of what is
called the _Double-acting Steam Engine_, in contradistinction to that
described in the last chapter, in which the steam acts only above the
piston while a vacuum is produced below it.
It is evident that, in the arrangement now described, the condenser
must be in constant action: while the piston is descending the
condenser must draw off the steam below it, and while it is ascending,
it must draw off the steam above it. As steam, therefore, must be
constantly drawn into the condenser, the jet of cold water which
condenses the steam must be kept constantly playing. This jet,
therefore, will not be worked by the valve alternately opening and
closing, as in the single engine, but will be worked by a cock, the
opening of which will be adjusted according to the quantity of cold
water necessary to condense the steam. When the steam is used at a low
pressure, and, therefore, in a less compressed state, less condensing
water would be necessary than when it is used at a higher pressure and
in a more compressed state. In the one case, therefore, the condensing
cock would be less open than in the other. Again, the quantity of
condensing water must vary with the speed of the engine, because the
greater the speed of the engine, the more rapidly will the steam flow
from the cylinder into the condenser; and, as the same quantity of
steam requires the same quantity of condensing water, the supply of
the condensing water must be proportional to the speed of the engine.
In the double-acting engine, then, the jet cock is regulated by a
lever or index which moves upon a graduated arch, and which is
regulated by the engineer according to the manner in which the engine
works.
This change in the action of the steam upon the piston rendered it
necessary to make a corresponding change in the mechanism by which the
piston-rod was connected with the beam. In the single acting engine,
the piston-rod pulled the end of the beam down during the descent, and
was pulled up by it in the ascent. The connection by which this action
was transmitted between the beam and piston was, as we have seen, a
flexible chain passing from the end of the piston and playing upon the
arch-head of the beam. Now, where the mechanical action to be
transmitted is a _pull_, and not a _push_, a flexible chain, or cord,
or strap, is always sufficient; but if a _push_ or _thrust_ is
required to be transmitted, then the flexibility of the medium of
mechanical communication afforded by a chain, renders it inapplicable.
In the double-acting engine, during the descent, the piston-rod still
pulls the beam down, and so far a chain connecting the piston-rod with
the beam would be sufficient to transmit the action of the one to the
other; but in the ascent the beam no longer pulls up the piston-rod,
but is pushed up by it. A chain from the piston-rod to the arch-head,
as described in the single acting engine, would fail to transmit this
force. If such a chain were used with the double engine, where there
is no counter weight on the opposite end of the beam, the consequence
would be, that in the ascent of the piston the chain would slacken,
and the beam would still remain depressed. It is therefore necessary
that some other mechanical connection be contrived between the
piston-rod and the beam, of such a nature that in the _descent_ the
piston-rod may _pull_ the beam down, and may _push_ it up in the
_ascent_.
Watt first proposed to effect this by attaching to the end of the
piston-rod a straight rack, faced with teeth, which should work in
corresponding teeth raised on the arch-head of the beam as represented
in fig. 13. If his improved steam engines required no further
precision of operation and construction than the atmospheric engines,
this might have been sufficient; but in these engines it was
indispensably necessary that the piston-rod should be guided with a
smooth and even motion through the stuffing-box in the top of the
cylinder, otherwise any shake or irregularity would cause it to work
loose in the stuffing-box, and either to admit the air, or to let the
steam escape. In fact, it was necessary to turn these piston-rods very
accurately in the lathe, so that they may work with sufficient
precision in the cylinder. Under these circumstances, the motion of
the rack and toothed arch-head were inadmissible, since it was
impossible by such means to impart to the piston-rod that smooth and
equable motion which was requisite. Another contrivance which occurred
to Watt was, to attach to the top of the piston-rod a bar which should
extend above the beam, and to use two chains or straps, one extending
from the top of the bar to the lower end of the arch-head, and the
other from the bottom of the bar to the upper end of the arch-head. By
such means the latter strap would pull the beam down when the piston
would descend, and the former would pull the beam up when the piston
would ascend. These contrivances, however, were superseded by the
celebrated mechanism, since called the _Parallel Motion_, one of the
most ingenious mechanical combinations connected with the history of
the steam engine.
It will be observed that the object was to connect by some inflexible
means the end of the piston-rod with the extremity of the beam, and so
to contrive the mechanism, that while the end of the beam would move
alternately up and down in a circle, the end of the piston-rod
connected with the beam should move exactly up and down in a straight
line. If the end of the piston-rod were fastened upon the end of the
beam by a pivot without any other connexion, it is evident that, being
moved up and down in the arch of a circle, it would be bent to the
left and the right alternately, and would consequently either be
broken, or would work loose in the stuffing box. Instead of connecting
the end of the rod immediately with the end of the beam by a pivot,
Watt proposed to connect them by certain moveable rods so arranged
that, as the end of the beam would move up and down in the circular
arch, the rods would so accommodate themselves to that motion, that
the end connected with the piston-rod should not be disturbed from its
rectilinear course.
To accomplish this, he conceived the notion of connecting three rods
in the following manner:--A B and C D (fig. 14.), are two rods or
levers turning on fixed pivots or centres at A and C. A third rod, B
D, is connected with them by pivots placed at their extremities, B and
D, and the lengths of the rods are so adjusted that when A B and C D
are horizontal, B D shall be perpendicular or vertical, and that A B
and C D shall be of equal lengths. Now, let a pencil be imagined to be
placed at P, exactly in the middle of the rod B D: if the rod A B be
caused to move up and down like the beam of the steam engine in the
arch represented in the figure, it is clear, from the mode of their
connexion, that the rod C D will be moved up and down in the other
arch. Now Watt conceived that, under such circumstances, the pencil P
would be moved up and down in a perpendicular straight line.
However difficult the first conception of this mechanism may have
been, it is easy to perceive why the desired effect will be produced
by it. When the rod A B rises to the upper extremity of the arch, the
point B departs a little to the right; at the same time, the point D
is moved a little to the left. Now the extremities of the rod B D
being thus at the same time, carried slightly in opposite directions,
the pencil in the middle of it will ascend directly upwards; the one
extremity of the rod having a tendency to draw it as much to the right
as the other has to draw it to the left. In the same manner, when the
rod A B moves to the lower extremity of the arch, the rod C D will be
likewise moved to the lower extremity of its arch. The point B is thus
transferred a little to the right, and the point D to the left; and,
for the same reason as before, the point P in the middle will move
neither to the right nor to the left, but straight downwards.[17]
[Footnote 17: In a strict mathematical sense, the path of the point P
is a curve of a high order, but in the play which is given to it in
the application used in the steam engine, it describes only a part of
its entire locus; and this part extending equally on each side of a
point of inflection, its radius of curvature is infinite, so that, in
practice, the deviation from a straight line, when proper proportions
are observed in the rods, and too great a play not given to them, is
insignificant.]
Now Watt conceived that his object would be attained if he could
contrive to make the beam perform the part of A B in fig. 14., and to
connect with it other two rods, C D and D B, attaching the end of the
piston to the middle of the rod D B. The practical application of this
principle required some modification, but is as elegant as the notion
itself is ingenious.
The apparatus adopted for carrying it into effect is represented on
the arm which works the piston in fig. 15. The beam, moving on its
axis C, every point in its arm moves in the arc of a circle of which C
is the centre. Let B be the point which divides the arm A C into equal
parts, A B and B C; and let D E be a straight rod equal in length to C
B, and playing on the fixed centre or pivot D. The end E of this rod
is connected by a straight bar, E B, with the point B, by pivots at B
and E on which the rod B E plays freely. If the beam be supposed to
move alternately on its axis C, the point B will move up and down in a
circular arc, of which C is the centre, and at the same time the point
E will move in an equal circular arc round the point D as a centre.
According to what we have just explained, the middle point F of the
rod B E will move up and down in a straight line.
Also, let a rod, A G, equal in length to B E, be attached to the end A
of the beam by a pivot on which it moves freely, and let its extremity
G be connected with E by a rod, G E, equal in length to A B, and
playing on pivots at G and E.
By this arrangement the joint A G being always parallel to B E, the
three points C, A, and G will be in circumstances precisely similar to
the points C, B, and F, except that the system C A G will be on a
scale of double the magnitude of C B F: C A being twice C B, and A G
twice B F, it is clear, then, that whatever course the point F may
follow, the point G must follow a similar line,[18] but will move
twice as fast. But, since the point F has been already shown to move
up and down in a straight line, the point G must also move up and down
in a straight line, but of double the length.[19]
[Footnote 18: It is, in fact, the principle of the pantograph. The
points C, F, and G evidently lie in the same straight line, since
C B : C A :: B F : A G, and the latter lines are parallel. Taking
C as the common _pole_ of the _loci_ of the points F G, the _radius
vector_ of the one will always be twice the corresponding _radius
vector_ of the other; and therefore these curves are similar,
similarly placed, and parallel. Hence, by the last note, the point G
must move in a line differing imperceptibly from a right line.]
[Footnote 19: It is not necessary that the rods, forming the parallel
motion, should have the proportions which we have assigned to them.
There are various proportions which answer the purpose, and which will
be seen by reference to practical works on the steam engine.]
By this arrangement the pistons of both the steam cylinder and
air-pump are worked; the rod of the latter being attached to the point
F, and that of the former to the point G.
This beautiful contrivance, which is incontestably one of the happiest
mechanical inventions of Watt, affords an example with what facility
the mind of a mere mechanician can perceive, as it were instinctively,
a result to obtain which by strict reasoning would require a very
complicated mathematical analysis. Watt, when asked, by persons whose
admiration was justly excited by this invention, to what process of
reasoning he could trace back his discovery, replied that he was aware
of none; that the conception flashed upon his mind without previous
investigation, and so as to excite in himself surprise at the
perfection of its action; and that on looking at it for the first
time, he experienced all that pleasurable sense of novelty which
arises from the first contemplation of the results of the invention
of others.
This and the other inventions of Watt seem to have been the pure
creations of his natural genius, very little assisted by the results
of practice, and not at all by the light of education. It does not
even appear that he was a dexterous mechanic; for he never assisted in
the construction of the first models of his own inventions. His
dwelling-house was two miles from the factory, to which he never went
more than once in a week, and then did not stay half an hour.
(_a_) However beautiful and ingenious in principle the parallel motion
may be, it has recently been shown in the United States that much
simpler means are sufficient to subserve the same purpose. In the
engines constructed recently, under the direction of Mr. R. L.
Stevens, a substitute for the parallel motion has been introduced that
performs the task equally well, and is much less complex. On the head
of the piston-rod a bar is fixed, at right angles to it, and to the
longitudinal section of the engine. The ends of this bar work in
guides formed of two parallel and vertical bars of iron, by which the
upper end of the piston-rod is constrained to move in a straight line.
The cross-bar that moves in the guides is connected with the end of
the working beam by an inflexible bar, having a motion on two circular
gudgeons, one of which is in the working beam, the other in the
cross-bar. This is therefore free to accommodate itself to the changes
in the respective position of the piston-rod and working beam, and yet
transmits the power exerted by the steam upon the former whether it be
ascending or descending, to the latter and through it to the other
parts of the machine.--A. E.
(_b_) The most improved form of Watt's engine was reached by
successive additions to the old atmospheric engine of Newcomen and
Cawley. Hence, the working-beam, derived from the pump brake of that
engine, always formed a part; and the parallel motion, or some
equivalent contrivance was absolutely necessary. In many American
engines, and particularly in those used in steam boats, the working
beam is no longer used for the purpose of transmitting motion to the
machinery. This is effected by applying a bar, called the cross-head,
at right angles to the upper ends of the piston-rod. The ends of the
cross-head work in iron guides, adapted to a gallows-frame of wood. On
each side of the cylinder, connecting rods are applied, which take
hold of the cranks of the shafts of the water-wheel. Two other
connecting rods give motion to a short beam, which works the air and
supply pumps.
The working beam is also suppressed in engines which work
horizontally. The connecting rod is in them merely a jointed
prolongation of the piston-rod, extending to the crank, whose axis
lies in the same horizontal plane with and at right angles to the axis
of the cylinder.--A. E.
(55.) A perfect motion being thus obtained of conveying the alternate
motion of the piston to the working beam, the use of a counterpoise to
lift the piston was discontinued, and the beam was made to balance
itself exactly on its centre. The next end to be obtained was to adapt
the reciprocating motion of the working end of the beam to machinery.
The motion most generally useful for this purpose is one of _continued
rotation_. The object, therefore, was by the _alternate_ motion of the
end of the beam to transmit to a shaft or axis a _continued circular_
motion. In the first instance, Watt proposed effecting this by a
_crank_, connected with the working end of the beam by a metal
connector or rod.
Let K be the centre or axis, or shaft by which motion is given to the
machinery, and to which rotation is to be imparted by the beam C H. On
the axle K, suppose a lever, K I, fixed, so that when K I is turned
round the centre K, the wheel must be turned with it. Let a connector
or rod, H I, be attached to the points H and I, playing freely on
pivots or joints. As the end H is moved upwards and downwards, the
lever K I is turned round the centre K, so as to give a continued
rotatory motion to the shaft which revolves on that centre. The
different positions which the connector and lever K I assume in the
different parts of a revolution are represented in fig. 16.
(56.) This was the first method which occurred to Watt for producing a
continued rotatory motion by means of the vibrating motion of the
beam, and is the method now universally used. A workman, however, from
Mr. Watt's factory, who was aware of the construction of a model of
this, communicated the method to Mr. Washborough of Bristol, who
anticipated Watt in taking out a patent; and although it was in his
power to have disputed the patent, yet rather than be involved in
litigation, he gave up the point, and contrived another way of
producing the same effect, which he called the _sun and planet_ wheel,
and which he used until the expiration of Washborough's patent, when
the crank was resumed.
The toothed wheel B (fig. 17.) is fixed on the end of the connector,
so that it does not turn on its axis. The teeth of this wheel work in
those of another wheel, A, which is the wheel to which rotation is to
be imparted, and which is turned by the wheel B revolving round it,
urged by the rod H I, which receives its motion from the working-beam.
The wheel A is called the sun-wheel, and B the planet-wheel, from the
obvious resemblance to the motion of these bodies.
This contrivance, although in the main inferior to the more simple one
of the crank, is not without some advantages; among others, it gives
to the sun-wheel double the velocity which would be communicated by
the simple crank, for in the simple crank one revolution only on the
axle is produced by one revolution of the crank, but in the sun and
planet-wheel two revolutions of the sun-wheel are produced by one of
the planet-wheel; thus a double velocity is obtained from the same
motion of the beam. This will be evident from considering that when
the planet-wheel is in its highest position, its lowest tooth is
engaged with the highest tooth of the sun-wheel; as the planet-wheel
passes from the highest position, its teeth drive those of the
sun-wheel before them, and when it comes into the lowest position, the
highest tooth of the planet-wheel is engaged with the lowest of the
sun-wheel: but then half of the sun-wheel has _rolled off_ the
planet-wheel, and, therefore, the tooth which was engaged with it in
its higher position, must now be distant from it by half the
circumference of the wheel, and must therefore be again in the highest
position, so that while the planet-wheel has been carried from the top
to the bottom, the sun-wheel has made a complete revolution. A little
reflection, however, on the nature of the motion, will render this
plainer than any description can. This advantage of giving an
increased velocity, may be obtained also by the simple crank, by
placing toothed wheels on its axle. Independently of the greater
expense attending the construction of the sun and planet-wheel, its
liability to go out of order, and the rapid wear of the teeth, and
other objections, rendered it decidedly inferior to the crank, which
has now entirely superseded it.
(58.) Whether the simple crank or the sun and planet wheel be used,
there still remains a difficulty of a peculiar nature attending the
continuance of the rotatory motion. There are two positions in which
the engine can give no motion whatever to the crank. These are when
the end of the beam, the axle of the crank, and the pivot which joins
the connector with the crank, are in the same straight line. This will
be easily understood. Suppose the beam, connector, and crank to assume
the position represented in fig. 15. If steam urge the piston
downwards, the point H and the connector H I will be drawn directly
upwards. But it must be very evident that in the present situation of
the connector H I, and the lever I K, the force which draws the point
I in the direction I K can have no effect whatever in turning I K
round the centre K, but will merely exert a pressure on the axle or
pivots of the wheel.
Again, suppose the crank and connector to be in the position H I K
(fig. 16.), the piston being consequently at the bottom of the
cylinder. If steam now press the piston _upwards_, the pivot H and the
connector H I will be pressed _downwards_, and this pressure will urge
the crank I K in the direction I K. It is evident that such a force
cannot turn the crank round the centre K, and can be attended with no
other effect than a pressure on the axle or pivots of the wheel.
Hence in these two positions, the engine can have no effect whatever
in turning the crank. What, then, it may be asked, extricates the
machine from this mechanical dilemma in which it is placed twice in
every revolution, on arriving at those positions in which the crank
escapes the influence of the power? There is a tendency in bodies,
when once put in motion, to continue that motion until stopped by some
opposing force, and this tendency carries the crank out of those two
critical situations. The velocity which is given to it, while it is
under the influence of the impelling force of the beam, is retained in
a sufficient degree to carry it through that situation in which it is
deserted by this impelling force. Although the rotatory motion
intended to be produced by the crank is, therefore, not absolutely
destroyed by this circumstance, yet it is rendered extremely
irregular, since, in passing through the two positions already
described, where the machine loses its power over the crank, the
motion will be very slow, and, in the positions of the crank most
remote from these, where the power of the beam upon it is greatest,
the motion will be very quick. As the crank revolves from each of
those positions where the power of the machine over it is greatest, to
where that power is altogether lost, it is continually diminished, so
that, in fact, the crank is driven by a varying power, and therefore
produces a varying motion. This will be easily understood by
considering the successive positions of the crank and connector
represented in fig. 16.
This variable motion becomes particularly objectionable when the
engine is employed to drive machinery. To remove this defect, we have
recourse to the property of bodies just mentioned, viz. their tendency
to retain a motion which is communicated to them. A large metal wheel
called a _fly-wheel_ is placed upon the axis of the crank (fig. 15.),
and is turned by it. The effect of this wheel is to equalize the
motion communicated by the action of the beam on the crank, that
action being just sufficient to sustain in the fly-wheel a uniform
velocity, and the tendency of this wheel to retain the velocity it
receives, renders its rotation sufficiently uniform for all practical
purposes.
This uniformity of motion, however, will only be preserved on two
conditions; _first_, that the supply of steam from the boiler shall be
uniform; and secondly, that the machine have always the same
resistance to overcome or be _loaded_ equally. If the supply of steam
from the boiler to the cylinder be increased, the motion of the piston
will be rendered more rapid, and, therefore, the revolution of the
fly-wheel will also be more rapid, and, on the other hand, a
diminished supply of steam will retard the fly-wheel. Again, if the
resistance or load upon the engine be diminished, the supply of steam
remaining the same, the velocity will be increased, since a less
resistance is opposed to the energy of the moving power; and, on the
other hand, if the resistance or load be increased, the speed will be
diminished, since a greater resistance will be opposed to the same
moving power. To insure a uniform velocity, in whatever manner the
load or resistance may be changed, it is necessary to proportion the
supply of steam to the resistance, so that, upon the least variation
in the velocity, the supply of steam will be increased or diminished,
so as to keep the engine going at the same rate.
(59.) One of the most striking and elegant appendages of the steam
engine is the apparatus contrived by Watt for effecting this purpose.
An apparatus, called a _regulator_ or _governor_, had been long known
to mill-wrights for rendering uniform the action of the stones in
corn-mills, and was used generally in machinery. Mr. Watt contrived a
beautiful application of this apparatus for the regulation of the
steam engine. In the pipe which conducts steam from the boiler to the
cylinder he placed a thin circular plate, so that when placed with its
face presented towards the length of the pipe, it nearly stopped it,
and allowed little or no steam to pass to the cylinder, but when its
edge was placed in the direction of the pipe, it offered no resistance
whatever to the passage of the steam. This circular plate, called the
throttle valve, was made to turn on a diameter as an axis, passing
consequently through the centre of the tube, and was worked by a lever
outside the tube. According to the position given to it, it would
permit more or less steam to pass. If the valve be placed with its
edge nearly in the direction of the tube, the supply of steam is
abundant; if it be placed with its face nearly in the direction of the
tube, the supply of steam is more limited, and it appears that, by the
position given to this valve, the steam may be measured in any
quantity to the cylinder.
At first it was proposed that the engine-man should adjust this valve
with his hand; when the engine was observed to increase its speed too
much, he would check the supply of steam by partially closing the
valve; but if, on the other hand, the motion was too slow, he would
open the valve and let in a more abundant supply of steam. Watt,
however, was not content with this, and desired to make the engine
itself discharge this task with more steadiness and regularity than
any attendant could, and for this purpose he applied the governor
already alluded to.
This apparatus is represented in fig. 15.; L is a perpendicular shaft
or axle to which a wheel, M, with a groove is attached. A strap or
rope, which is rolled upon the axle of the fly-wheel, is passed round
the groove in the wheel M, in the same manner as the strap acts in a
turning lathe. By means of this strap the rotation of the fly-wheel
will produce a rotation of the wheel M and the shaft L, and the speed
of the one will always increase or diminish in the same proportion as
the speed of the other. N, N are two heavy balls of metal placed at
the ends of rods, which play on an axis fixed on the revolving shaft
at O, and extend beyond the axis to Q Q. Connected with these by
joints at Q Q are two other rods, Q R, which are attached to a broad
ring of metal, moving freely up and down the revolving shaft. This
ring is attached to a lever whose centre is S, and is connected by a
series of levers with the throttle-valve T. When the speed of the
fly-wheel is much increased, the spindle L is whirled round with
considerable rapidity, and by their natural tendency[20] the balls N N
fly from the centre. The levers which play on the axis O, by this
motion, diverge from each other, and thereby depress the joints Q Q,
and draw down the joints R, and with them the ring of metal which
slides upon the spindle. By these means the end of the lever playing
on S is depressed, and the end V raised, and the motion is transmitted
to the throttle-valve, which is thereby partially closed, and the
supply of steam to the cylinder checked. If, on the contrary, the
velocity of the fly-wheel be diminished, the balls will fall towards
the axis, and the opposite effects ensuing, the supply of steam will
be increased, and the velocity restored.
[Footnote 20: The _centrifugal force_.]
[Illustration: Pl. IV. WATT'S SINGLE ACTING STEAM ENGINE.]
The peculiar beauty of this apparatus is, that in whatever position
the balls settle themselves, the velocity with which the governor
revolves must be the same,[21] and in this, in fact, consists its
whole efficacy as a regulator. Its regulating power is limited, and it
is only small changes of velocity that it will correct. It is evident
that such a velocity as, on the one hand, would cause the balls to fly
to the extremity of their play, or, on the other, would cause them to
fall down on their rests, would not be influenced by the governor.
[Footnote 21: Strictly speaking, this is only true when the divergence
of the rods from the spindle is not very great, and, in practice, this
divergence is never sufficient to render the above assertion untrue.
This property of the conical pendulum arises from the circumstance of
the centrifugal force, in this instance, varying as the radius of the
circle in which the balls are moved; and when this is the case, as is
well known, the _periodic time_ is constant. The time of one
revolution of the balls is equal to twice the time in which either
ball, as a common pendulum, would vibrate on the centre, and as all
its vibrations, though the arcs be unequal, are equal in time,
provided those arcs be small, so also is the periodic time of the
revolving ball invariable. These observations, however, only apply
when the balls settle themselves steadily into a circular motion; for
while they are ascending they describe a spiral curve with double
curvature, and the period will vary. This takes place during the
momentary changes in the velocity of the engine.]
We have thus described the principal parts of the double-acting steam
engine. The valves and the methods of working them have been reserved
for the next chapter, as they admit of considerable variety, and will
be better treated of separately. We have also reserved the
consideration of the boiler, which is far from being the least
interesting part of the modern steam engine, for a future chapter.
CHAPTER VII.
DOUBLE-ACTING STEAM ENGINE (_continued_.)
On the Valves of the Double-Acting Steam Engine. -- Original
Valves. -- Spindle Valves. -- Sliding Valve. -- D Valve. --
Four-Way Cock.
(60.) The various improvements described in the last chapter were
secured to Watt by patent in the year 1782. The engine now acquired an
enlarged sphere of action; for its dominion over manufactures was
decided by the _fly-wheel_, _crank_, and _governor_. By means of these
appendages, its motions were regulated with the most delicate
precision; so that while it retained a power whose magnitude was
almost unlimited, that power was under as exact regulation as the
motion of a time-piece. There is no species of manufacture, therefore,
to which this machine is not applicable, from the power which spins
the finest thread, or produces the most delicate web, to that which is
necessary to elevate the most enormous weights, or overcome the most
unlimited resistances. Although it be true, that in later times the
steam engine has received many improvements, some of which are very
creditable to the invention and talents of their projectors, yet it is
undeniable that all its great and leading perfections, all those
qualities by which it has produced such wonderful effects on the
resources of these countries, by the extension of manufactures and
commerce,--those qualities by which its influence is felt and
acknowledged in every part of the civilized globe, in increasing the
happiness, in multiplying the enjoyments, and cheapening the
pleasures of life,--that these qualities are due to the predominating
powers of one man, and that man one who possessed neither the
influence of wealth, rank, nor education, to give that first impetus
which is so often necessary to carry into circulation the earlier
productions of genius.
The method of working the valves of the double-acting steam engine, is
a subject which has much exercised the ingenuity of engineers, and
many elegant contrivances have been suggested, some of which we shall
now proceed to describe. But even in this the invention of Watt has
anticipated his successors; and the contrivances suggested by him are
those which are now almost universally used.
In order perfectly to comprehend the action of the several systems of
valves which we are about to describe, it will be necessary distinctly
to remember the manner in which the steam is to be communicated to the
cylinder, and withdrawn from it. When the piston is at the top of the
cylinder, the steam below it is to be drawn off to the condenser, and
the steam from the boiler is to be admitted above it. Again, when it
has arrived at the bottom of the cylinder, the steam above is to be
drawn off to the condenser, and the steam from the boiler is to be
admitted below it.
In the earlier engines constructed by Watt, this was accomplished by
four valves, which were opened and closed in pairs. Valve boxes were
placed at the top and bottom of the cylinder, each of which
communicated by tubes both with the steam-pipe from the boiler and the
condenser. Each valve-box accordingly contained two valves, one to
admit steam from the steam-pipe to the cylinder, and the other to
allow that steam to pass into the condenser. Thus each valve-box
contained a steam valve and an exhausting valve. The valves at the top
of the cylinder are called the _upper steam valve_ and the _upper
exhausting-valve_, and those at the bottom, the _lower steam valve_
and the _lower exhausting-valve_. In fig. 15. A´ is the upper steam
valve, which, when open, admits steam above the piston; B´ is the
upper exhausting-valve, which, when open, draws off the steam from the
piston to the condenser. C´ is the lower steam valve, which admits
steam below the piston; and D´, the lower exhausting-valve, which draws
off the steam from below the piston to the condenser.
Now, suppose the piston to be at the top of the cylinder, the cylinder
below it being filled with steam, which has just pressed it up. Let
the _upper steam valve_ A´, and the _lower exhausting-valve_ D´ be
opened, and the other two valves closed. The steam which fills the
cylinder below the piston will immediately pass through the valve D´
into the condenser, and a vacuum will be produced below the piston. At
the same time, steam is admitted from the steam-pipe through the valve
A´ above the piston, and its pressure will force the piston to the
bottom of the cylinder. On the arrival of the piston at the bottom of
the cylinder, the upper steam valve A´, and lower exhausting-valve D´,
are closed; and the lower steam valve C´, and upper exhausting-valve
B´ are opened. The steam which fills the cylinder above the piston now
passes off through B´ into the condenser, and leaves a vacuum above
the piston. At the same time, steam from the boiler is admitted
through the lower steam valve C´, below the piston, so that it will
press the piston to the top of the cylinder; and so the process is
continued.
It appears, therefore, that the upper steam valve, and the lower
exhausting-valve, must be opened together, on the arrival of the
piston at the top of the cylinder. To effect this, one lever, E´, is
made to communicate by jointed rods with both these valves, and this
lever is moved by a pin placed on the piston-rod of the air-pump; and
such a position may be given to this pin as to produce the desired
effect exactly at the proper moment of time. In like manner, another
lever, F´, communicates by jointed rods with the upper exhausting
valve and lower steam valve, so as to open them and close them
together; and this lever, in like manner, is worked by a pin on the
piston-rod of the air-pump.
(61.) This method of connecting the valves, and working them, has been
superseded by another, for which Mr. MURRAY of Leeds obtained a
patent, which was, however, set aside by Messrs. Bolton and Watt, who
showed that they had previously practised it. This method is
represented in figs. 18, 19. The stems of the valves are
perpendicular, and move in steam-tight sockets in the top of the
valve-boxes. The stem of the upper steam valve A is a tube through
which the stem of the upper exhausting-valve B passes, and in which it
moves steam-tight; both these stems moving steam-tight through the top
of the valve-box. The lower steam valve C, and exhausting-valve D, are
similarly circumstanced; the stem of the former being a tube through
which the stem of the latter passes. The stems of the upper steam
valve and lower exhausting-valve are then connected by a rod, E; and
those of the upper exhausting-valve and lower steam valve by another
rod, F. These rods, therefore, are capable of moving the valves in
pairs, when elevated and depressed. The motion which works the valves
is, however, not communicated by the rod of the air-pump, but is
received from the axis of the fly-wheel. This axis works an apparatus
called an _eccentric_; the principle which regulates the motion of
this may be thus explained:--
D E (figs. 20, 21.) is a circular metallic ring, the inner surface of
which is perfectly smooth. This ring is connected with a shaft, F B,
which communicates motion to the valves by levers which are attached
to it at B. A circular metallic plate is fitted in the ring so as to
be capable of turning within it, the surfaces of the ring and plate
which are in contact being smooth and lubricated with oil or grease.
This circular plate revolves, but not on its centre. It turns on an
axis C, at some distance from its centre A; the effect of which,
evidently, is that the ring within which it is turned is moved
alternately in opposite directions, and through a space equal to twice
the distance (C A) of the axis of the circular plate from the common
centre of it and the ring. The eccentric in its two extreme positions
is represented in figs. 20, 21. The plate and ring D E are placed on
the axis of the fly-wheel, or on the axis of some other wheel which is
worked by the fly-wheel. So that the motion of continued rotation in
the fly-wheel is thus made to produce an alternate motion in a
straight line in the shaft F B. This rod is made to communicate by
levers with the rods E and F (figs. 18, 19.), which work the valves in
such a manner, that, when the eccentric is in the position fig. 20.,
one pair of valves are opened, and the other pair closed; and when it
is brought to the position fig. 21., the other pair are opened and the
former closed and so on. It is by means of such an apparatus as this
that the valves are worked almost universally at present.
The piston being supposed to be at the top of the cylinder (fig. 18.),
and the rod E raised, the valves A and D are opened, and B and C
closed. The steam enters from the steam-pipe at an aperture
immediately above the valve A, and, passing through the open valve,
enters the cylinder above the piston. At the same time, the steam
which is below the piston, and which has just pressed it up, flows
through the open valve D, and through a tube immediately under it to
the condenser. A vacuum being thus produced below the piston, and
steam pressure acting above it, it descends; and when it arrives at
the bottom of the cylinder (fig. 19.) the rod F is drawn down, and the
valves A and D fall into their seats, and at the same time the rod F
is raised, and the valves B and C are opened. Steam is now admitted
through an aperture above the valve C, and passes below the piston,
while the steam above it passes through the open valve B into a tube
immediately under it, which leads to the condenser. A vacuum being
thus produced above the piston, and steam pressure acting below it,
the piston ascends, and thus the alternate ascent and descent is
continued by the motion communicated to the rods E F from the
fly-wheel.
[Illustration: Pl. V. WATT'S DOUBLE-ACTING STEAM ENGINE.]
[Illustration: Pl. VI.]
(_c_) An improvement has been made in the United States in the mode of
working the puppet valve. It consists in placing them by pairs in two
different vertical planes instead of one. The rods then work through
four separate stuffing boxes, and the necessity of making two of them
hollow cylinders is avoided.--A. E.
(62.) There are various other contrivances for regulating the
circulation of steam through the cylinder. In figs. 22, 23. is
represented a section of a slide valve suggested by Mr. Murray of
Leeds. The steam-pipe from the boiler enters the valve-box D E at S.
Curved passages, A A, B B, communicate between this valve-box and the
top and bottom of the cylinder; and a fourth passage leads to the tube
C, which passes to the condenser. A sliding piece within the valve-box
opens a communication alternately between each end of the cylinder and
the tube C, which leads to the condenser. In the position of the
apparatus in fig. 22. steam is passing from the steam-pipes, through
the curved passage A A above the piston, and at the same time the
steam below the piston is passing through the passage B B into the
tube C, and thence to the condenser. A vacuum is thus formed below the
piston, and steam is introduced above it. The piston, therefore,
descends; and when it arrives at the bottom of the cylinder, the slide
is moved into the position represented in fig. 23. Steam now passes
from S through B B below the piston, and the steam above it passes
through A A and C to the condenser. A vacuum is thus produced above
the piston, and steam pressure is introduced below it, and the piston
ascends; and in this way the motion is continued.
The slide is moved by a lever, which is worked by the eccentric from
the fly-wheel.
(63.) Watt suggested a method of regulating the circulation of steam,
which is called the D valve, from the resemblance which the horizontal
section of the valve has to the letter D. This method, which is very
generally used, is represented in section in figs. 24, 25. Steam from
the boiler enters through S. A rod of metal connects two solid plugs,
A B, which move steam-tight in the passage D. In the position of the
apparatus represented in fig. 24. the steam passes from S through the
passage D, and enters the cylinder above the piston; while the steam
below the piston passes through the open passage by the tube C to the
condenser. A vacuum is thus formed below the piston, while the
pressure of steam is introduced above it, and it accordingly descends.
When it has arrived at the bottom of the cylinder, the plugs A B are
moved into the position in fig. 25. Steam now passing from S through
D, enters the cylinder below the piston; while the steam which is
above the piston, and has just pressed it down, passes through the
open passage into the condenser. A vacuum is thus produced above the
piston, and the steam pressure below forces it up. When it has arrived
at the top of the cylinder, the position of the plugs A B is again
changed to that represented in fig. 24., and a similar effect to that
already described is produced, and the piston is pressed down; and so
the process is continued.
The plugs A B, and the rod which connects them, are moved up and down
by proper levers, which receive their motion from the eccentric.
This contrivance is frequently modified, by conducting the steam from
above the piston to the condenser, through a tube in the plugs A B,
and their connecting rod. In figs. 26, 27. a tube passes through the
plugs A B and the rod which joins them. In the position fig. 26. steam
entering at S passes through the tube to the cylinder above the
piston, while the steam below the piston passes through C into the
condenser. A vacuum being thus made below the piston, and steam
pressing above it, it descends; and when it has arrived at the bottom
of the cylinder, the position of the plugs A B and the tube is changed
to that represented at fig. 27. The steam now entering at S passes to
the cylinder below the piston, while the steam above the piston passes
through C into the condenser. A vacuum is thus produced above the
piston, and steam pressure introduced below it, so that it ascends.
When it has arrived at the top of the cylinder, the plugs are moved
into the position represented in fig. 26., and similar effects being
produced, the piston again descends; and so the motion is continued.
The motion of the sliding tube may be produced as in the former
contrivances, by the action of the eccentric. It is also sometimes
done by a bracket fastened on the piston-rod of the air-pump. This
bracket, in the descent of the piston, strikes a projection on the
valve-rod, and drives it down; and in the ascent meets a similar
projection, and raises it.
(64.) Another method, worthy of notice for its elegance and
simplicity, is the _four-way cock_. A section of this contrivance is
given in figs. 28, 29.: C T S B are four passages or tubes; S leads
from the boiler, and introduces steam; C, opposite to it, leads to the
condenser; T is a tube which communicates with the top of the
cylinder; and B one which communicates with the bottom of the
cylinder. These four tubes communicate with a cock, which is furnished
with two curved passages, as represented in the figures; and these
passages are so formed, that, according to the position given to the
cock, they may be made to open a communication between any two
adjacent tubes of the four just mentioned. When the cock is placed as
in fig. 28. communication is opened between the steam-pipe and the top
of the cylinder by one of the curved passages, and between the
condenser and the bottom of the cylinder by the other curved passage.
In this case the steam passes from below the piston to the condenser,
leaving a vacuum under it, and steam is introduced from the boiler
above the piston. The piston therefore descends; and when it has
arrived at the bottom of the cylinder, the position of the cock is
changed to that represented in fig. 29. This change is made by turning
the cock through one fourth of an entire revolution, which may be done
by a lever moved by the eccentric, or by various other means. One of
the curved passages in the cock now opens a communication between the
steam-pipe and the bottom of the cylinder; while the other opens a
communication between the condenser and the top of the cylinder. By
these means, the steam from the boiler is introduced below the piston,
while the steam above the piston is drawn off to the condenser. A
vacuum being thus made above the piston, and steam introduced below
it, it ascends; and when it has arrived at the top of the cylinder,
the cock being moved back, it resumes the position in fig. 28., and
the same consequences ensue, the piston descends; and so the process
is continued. In figs. 30, 31. the four-way cock with the passages to
the top and bottom of the cylinder is represented on a larger scale.
This beautiful contrivance is not of late invention. It was used by
Papin, and is also described by Leupold in his _Theatrum Machinarum_,
a work published about the year 1720, in which an engine is described
acting with steam of high pressure, on a principle which we shall
describe in a subsequent chapter.
The four-way cock is liable to some practical objections. The quantity
of steam which fills the tubes between the cock and the cylinder, is
wasted every stroke. This objection, however, also applies to the
sliding valve (figs. 22, 23.), and to the sliding tube or D valves
(figs. 24, 25, 26, 27.). In fact, it is applicable to every
contrivance in which means of shutting off the steam are not placed at
both top and bottom of the cylinder. Besides this, however, the
various passages and tubes cannot be conveniently made large enough to
supply steam in sufficient abundance; and consequently it becomes
necessary to produce steam in the boiler of a more than ordinary
strength to bear the attenuation which it suffers in its passage
through so many narrow tubes.
[Illustration: Pl. VII.]
One of the greatest objections, however, to the use of the four-way
cock, particularly in large engines, is its unequal wear. The parts of
it near the passages having smaller surfaces, become more affected
by the friction, and in a short time the steam leaks between the cock
and its case, and becomes wasted, and tends to vitiate the vacuum.
These cocks are seldom used in condensing engines, except they be
small engines, but are frequently adopted in high-pressure
steam-engines; for in these the leakage is not of so much consequence,
as will appear hereafter.
CHAPTER VIII.
BOILER AND ITS APPENDAGES.--FURNACE.
The Boiler and its Appendages. -- Level Gauges. -- Feeding
Apparatus. -- Steam Gauge. -- Barometer Gauge. -- Safety Valves.
-- Self-regulating Damper. -- Edelcrantz's Valve. -- Furnace. --
Self-consuming Furnace. -- Brunton's Self-regulating Furnace. --
Oldham's Modification.
(65.) The regular action of a steam engine, as well as the economy of
fuel, depends in a great degree on the construction of the boiler or
apparatus for generating the steam. The boiler may be conceived as a
great magazine of steam for the use of the engine; and care must be
taken not only that a sufficient quantity be always ready for the
supply of the machine, but also that it shall be of the proper
quality; that is, that its pressure shall not exceed that which is
required, nor fall short of it. Precautions should, therefore, be
taken that the production of steam should be exactly proportioned to
the work to be done, and that the steam so produced shall be admitted
to the cylinder in the same proportion.
To accomplish this, various contrivances, eminently remarkable for
their ingenuity, have been resorted to, and which we shall now proceed
to describe.
(_d_) It may be premised that boilers have been made of various
figures, each having its own peculiar advantages and defects. That
which possesses the greatest degree of strength, is one of the shape
of a cylinder. This form was originally introduced by Oliver Evans, in
the United States. It will have its entire superiority when the fire
is made beneath it in a furnace of masonry. But this method is not
applicable to steam boats or locomotive engines. In these instances
the weight of the separate furnace is an objection; when, therefore,
this form is applied in them, the fire is usually made in a chamber of
cylindric shape, within the boiler, and the smoke is conveyed to the
chimney by flues, which pass through the water. These flues have, in
some cases, been reduced to the size of small tubes, and of this
method an example will be found in a subsequent part of this work.--A.
E.
(66.) Different methods have been, from time to time, suggested for
indicating the level of the water in the boiler. We have already
mentioned the two gauge-pipes used in the earlier steam engines (31.);
and which are still generally continued. There are, however, some
other methods which merit our attention.
A weight, F (fig. 32.), half immersed in the water in the boiler, is
supported by a wire, which, passing steam-tight through a small hole
in the top, is connected by a flexible string or chain passing over a
wheel, W, with a counterpoise, A, which is just sufficient to balance
F, when half immersed. If F be raised above the water, A being
lighter, will no longer balance it, and F will descend, pulling up A,
and turning the wheel W. If, on the other hand, F be plunged deeper in
the water, A, will more than balance it, and will pull it up. So that
the only position in which F and A will balance each other is when F
is half immersed. The wheel W is so adjusted, that when two pins,
placed on its rim, are in the horizontal position, as in fig. 32., the
water is at its proper level. Consequently it follows, that if the
water rise above this level, the weight F is lifted, and A falls, so
that the pins P P´ come into the position in fig. 33. If, on the other
hand, the level of the water falls, F falls and A rises, so that the
pins P P´ assume the position in fig. 34. Thus, in general, the
position of the pins P P´ becomes an indication of the quantity of
water in the boiler.
Another method is to place a glass tube (fig. 35.) with one end, T,
entering the boiler above the proper level, and the other end, T´,
entering it below the proper level. It must be evident that the water
in the tube will always stand at the same level as the water in the
boiler; since the lower part has a free communication with that water,
while the surface is submitted to the pressure of the same steam as
the water in the boiler. This, and the last-mentioned gauge, have the
advantage of addressing the eye of the engineer at once, without any
adjustment; whereas the gauge cocks must be both opened, whenever the
depth is to be ascertained.
These gauges, however, require the frequent attention of the
engine-man; and it becomes desirable either to find some more
effectual means of awakening that attention, or to render the supply
of the boiler independent of any attention. In order to enforce the
attention of the engine-man to replenish the boiler when partially
exhausted by evaporation, a tube was sometimes inserted at the lowest
level to which it was intended that the water should be permitted to
fall. This tube was conducted from the boiler into the engine-house,
where it terminated in a mouth-piece or whistle, so that whenever the
water fell below the level at which this tube was inserted in the
boiler, the steam would rush through it, and, issuing with great
velocity at the mouth-piece, would summon the engineer to his duty
with a call that would rouse him even from sleep.
(67.) In the most effectual of these methods, the task of replenishing
the boiler should still be executed by the engineer; and the utmost
that the boiler itself was made to do was to give due notice of the
necessity for the supply of water. The consequence was, among other
inconveniences, that the level of the water was subject to constant
variation.
To remedy this, a method has been invented by which the engine is made
to feed its own boiler. The pipe G´ (fig. 15.), which leads from the
hot water pump H´, terminates in a small cistern, C (fig. 36.), in
which the water is received. In the bottom of this cistern a valve, V,
is placed, which opens upwards, and communicates with a feed-pipe,
which descends into the boiler below the level of the water in it. The
stem of the valve V is connected with a lever turning on the centre D,
and loaded with a weight, F, dipped in the water in the boiler in a
manner similar to that described in fig. 32., and balanced by a
counterpoise, A, in exactly the same way. When the level of the water
in the boiler falls, the float F falls with it, and, pulling down the
arm E of the lever, raises the valve V, and lets the water descend
into the boiler from the cistern C. When the boiler has thus been
replenished, and the level raised to its former place, F will again be
raised, and the valve V closed by the weight A. In practice, however,
the valve V adjusts itself by means of the effect of the water on the
weight F, so as to permit the water from the feeding cistern C to flow
in a continued stream, just sufficient in quantity to supply the
consumption from evaporation, and to maintain the level of the water
in the boiler constantly the same.
By this singularly felicitous arrangement, the boiler is made to
replenish itself, or, more properly speaking, it is made to receive
such a supply as that it never wants replenishing, an effect which no
effort of attention on the part of an engine-man could produce. But
this is not the only good effect produced by this contrivance. A part
of the steam which originally left the boiler, and having discharged
its duty in moving the piston, was condensed and reconverted into
water, and lodged by the air-pump in the hot-well (47.), is here
again restored to the source from which it came, bringing back all the
unconsumed portion of its heat preparatory to being once more put in
circulation through the machine.
The entire quantity of hot water pumped into the cistern C is not
always required for the boiler. A waste-pipe may be provided for
carrying off the surplus, which may be turned to any purpose for which
it may be required; or it may be discharged into a cistern to cool,
preparatory to being restored to the cold cistern (fig. 12.), in case
water for the supply of that cistern be not sufficiently abundant.
In cities and places in which it becomes an object to prevent the
waste of water, the waste-pipes proceeding from the feed-cistern C
(fig. 36.) and from the cold cistern containing the condenser and
air-pump, may be conducted to a cistern A B (fig. 37.). Let C be the
pipe from the feeding cistern, and D that from the cold cistern; by
these pipes the waste water, from both these cisterns is deposited in
A B. In the bottom of A B is a valve V, opening upwards, connected
with a float F. When the quantity of water collected in the cistern A
B is such that the level rises considerably, the float F is raised,
and lifts the valve V, and the water flows into the main pipe, which
supplies water for working the engine: G is the cold water-pump for
the supply of the cold cistern.
This arrangement for saving the water discharged from the feeding and
condensing cisterns has been adopted in the printing office of the
Bank of Ireland, and a very considerable waste of water is thereby
prevented.
(68.) It is necessary to have a ready method of ascertaining at all
times the strength of the steam which is used in working the engine.
For this purpose a bent tube containing mercury is inserted into some
part of the apparatus which has free communication with the steam. It
is usually inserted in the _jacket_ of the cylinder (44.). Let A B C
(fig. 38.) be such a tube. The pressure of the steam forces the
mercury down in the leg A B, and up in the leg B C. If the mercury in
both legs be at exactly the same level, the pressure of the steam must
be exactly equal to that of the atmosphere; because the steam pressure
on the mercury in A B balances the atmospheric pressure on the mercury
in B C. If, however, the level of the mercury in B C be above the
level of the mercury in B A, the pressure of the steam will exceed
that of the atmosphere. The excess of its pressure above that of the
atmosphere may be found by observing the difference of the levels of
the mercury in the tubes B C and B A; allowing a pressure of one pound
on each square inch for every two inches in the difference of the
levels.
If, on the contrary, the level of the mercury in B C should fall below
its level in A B, the atmospheric pressure will exceed that of the
steam, and the degree or quantity of the excess may be ascertained
exactly in the same way.
If the tube be glass, the difference of levels of the mercury would be
visible: but it is most commonly made of iron; and in order to
ascertain the level, a thin wooden rod with a float is inserted in the
open end of B C; so that the portion of the stick within the tube
indicates the distance of the level of the mercury from its mouth. A
bulb or cistern of mercury might be substituted for the leg A B, as in
the common barometer. This instrument is called the STEAM-GAUGE.
If the steam-gauge be used as a measure of the strength of the steam
which presses on the piston, it ought to be on the same side of the
throttle valve (which is regulated by the governor) as the cylinder;
for if it were on the same side of the throttle valve with the boiler,
it would not be affected by the changes which the steam may undergo in
passing through the throttle valve, when partially closed by the
agency of the governor.
(69.) The force with which the piston is pressed depends on two
things: 1º, the actual strength of the steam which presses on it; and
2º, on the actual strength of the vapour which resists it. For
although the vacuum produced by the method of separate condensation
be much more perfect than what had been produced in the atmospheric
engines, yet still some vapour of a small degree of elasticity is
found to be raised from the hot water in the bottom of the condenser
before it can be extracted by the air-pump. One of these pressures is
indicated by the steam-gauge already described; but still, before we
can estimate the force with which the piston descends, it is necessary
to ascertain the force of the vapour which remains uncondensed, and
resists the motion of the piston. Another gauge, called the
barometer-gauge, is provided for this purpose. A glass tube A B (fig.
39.), more than thirty inches long, and open at both ends, is placed
in an upright or vertical position, having the lower end B immersed in
a cistern of mercury C. To the upper end is attached a metal tube,
which communicates with the condenser, in which a constant vacuum, or
rather high degree of rarefaction, is sustained. The same vacuum must,
therefore, exist in the tube A B, above the level of the mercury; and
the atmospheric pressure on the surface of the mercury in the cistern
C will force the mercury up in the tube A B, until the column which is
suspended in it is equal to the difference between the atmospheric
pressure and the pressure of the uncondensed steam. The difference
between the column of mercury sustained in this instrument and in the
common barometer will determine the strength of the uncondensed steam,
allowing a force proportional to one pound per square inch for every
two inches of mercury in the difference of the two columns. In a
well-constructed engine, which is in good order, there is very little
difference between the altitude in the barometer-gauge and the common
barometer.
To compute the force with which the piston descends, thus becomes a
very simple arithmetical process. First ascertain the difference of
the levels of the mercury in the steam-gauge. This gives the excess of
the steam pressure above the atmospheric pressure. Then find the
height of the mercury in the barometer-gauge. This gives the excess
of the atmospheric pressure above the uncondensed steam. Hence, if
these two heights be added together, we shall obtain the excess of the
impelling force of the steam from the boiler on the one side of the
piston, above the resistance of the uncondensed steam on the other
side. This will give the effective impelling force. Now, if one pound
be allowed for every two inches of mercury in the two columns just
mentioned, we shall have the number of pounds of impelling pressure on
every square inch of the piston. Then if the number of square inches
in the section of the piston be found, and multiplied by the number of
pounds on each square inch, the whole effective force with which it
moves will be obtained.
In the computation of the power of the engine, however, all this
force, thus computed, is not to be allowed as the effective working
power. For it requires some force, and by no means an inconsiderable
portion, to move the engine itself, even when unloaded; all this,
therefore, which is spent in overcoming friction, &c. is to be left
out of account, and only the balance set down as the effective working
power.
From what we have stated, it appears that in order to estimate the
effective force with which the piston is urged, it is necessary to
refer to both the barometer and the steam-gauge. This double
computation may be obviated by making one gauge serve both purposes.
If the end C of the steam-gauge (fig. 38.) instead of communicating
with the atmosphere, were continued to the condenser, we should have
the pressure of the steam acting upon the mercury in the tube B A, and
the pressure of the uncondensed vapour which resists the piston acting
on the mercury in the tube B C. Hence the difference of the levels of
the mercury in the tubes will at once indicate the difference between
the force of the steam and that of the uncondensed vapour, which is
the effective force with which the piston is urged.
(70.) To secure the boiler from accidents arising from the steam
becoming too strong, a safety valve is used, similar to those
described in Papin's steam engine, loaded with a weight equal to the
strength which the steam is intended to have above the atmospheric
pressure; for it is found expedient, even in condensing engines, to
use the steam of a pressure somewhat above that of the atmosphere.
Besides this valve, another of the very opposite kind is sometimes
used. Upon stopping the engine, and extinguishing the fire, it is
found that the steam condensed within the boiler produces a vacuum; so
that the atmosphere, pressing on the external surface of the boiler,
has a tendency to crush it. To prevent this, a safety valve is
provided, which opens inwards, and which being forced open by the
atmospheric pressure when a vacuum is produced within, the air rushes
in, and a balance is obtained between the pressures within and
without.
(71.) We have already explained the manner in which the governor
regulates the supply of steam from the boiler to the cylinder,
proportioning the quantity to the work to be done, and thereby
sustaining a uniform motion. Since, then, the _consumption_ of steam
in the engine is subject to variation, owing to the various quantities
of work it may have to perform, it is evident that the PRODUCTION of
steam in the boiler should be subject to a proportional variation.
For, otherwise, one of two effects would ensue: the boiler would
either fail to supply the engine with steam, or steam would accumulate
in the boiler, from being produced in too great abundance, and would
escape at the safety valve, and thus be wasted. In order to vary the
production of steam in proportion to the demands of the engine, it is
necessary to increase or mitigate the furnace, as production is to be
augmented or diminished. To effect this by any attention on the part
of the engine-man would be impossible; but a most ingenious method has
been contrived, of making the boiler regulate itself in these
respects. Let T (fig. 40.) be a tube inserted in the top of the
boiler, and descending nearly to the bottom. The pressure of the
steam on the surface of the water in the boiler forces water up in the
tube T, until the difference of the levels is equal to the difference
between the pressure of the steam in the boiler and that of the
atmosphere. A weight F, half immersed in the water in the tube, is
suspended by a chain which passes over the wheels P P´, and is
balanced by a metal plate D, in the same manner as the float in fig.
32. is balanced by the weight A. The plate D passes through the mouth
of the flue E, as it issues finally from the boiler; so that when the
plate D falls, it stops the flue and thereby suspends the draught of
air through the furnace, mitigates the fire, and diminishes the
production of steam. If, on the contrary, the plate D be drawn up, the
draught is increased, the fire rendered more effective, and the
production of steam in the boiler stimulated. Now suppose that the
boiler is producing steam faster than the engine consumes it, either
because the load on the engine has been diminished, and therefore its
consumption of steam proportionally diminished, or because the fire
has become too intense. The consequence is, that the steam beginning
to accumulate, will press upon the surface of the water in the boiler
with increased force, and the water will rise in the tube T. The
weight F will therefore be lifted, and the plate D will descend, stop
the draught, mitigate the fire, and check the production of steam; and
it will continue so to do, until the production of steam becomes
exactly equal to the demands of the engine.
If, on the other hand, the production of steam be not equal to the
wants of the machine, either because of the increased load, or the
insufficiency of the fire, the steam in the boiler losing its
elasticity, the surface of the water rises, not sustaining a pressure
sufficient to keep it at its wonted level. Therefore, the surface in
the tube T falls, and the weight F falls, and the plate D rises. The
draught is thus increased, by opening the flue, and the fire rendered
more intense; and thus the production of steam is stimulated, until
it is sufficiently rapid for the purposes of the engine. This
apparatus is called the _self-acting damper_.
(72.) It has been proposed to connect this damper with the
safety-valve invented by the Chevalier Edelcrantz. A small brass
cylinder is fixed to the boiler, and is fitted with a piston which
moves in it, without much friction, and nearly steam-tight. The
cylinder is closed at top, having a hole through which the piston-rod
plays; so that the piston is thus prevented from being blown out of
the cylinder by the steam. The side of the cylinder is pierced with
small holes opening into the air, and placed at short distances above
each other. Let the piston be loaded with a weight proportional to the
pressure of the steam intended to be produced. When the steam has
acquired a sufficient elasticity, the piston will be lifted, and steam
will escape through the first hole. If the production of steam be not
too rapid, and that its pressure be not increasing, the piston will
remain suspended in this manner: but if it increase, the piston will
be raised above the second hole, and it will continue to rise until
the escape of the steam through the holes is sufficient to render the
weight of the piston a counterpoise for the steam. This safety valve
is particularly well adapted to cases where steam of an exactly
uniform pressure is required; for the pressure must necessarily be
always equal to the weight on the piston. Thus, suppose the section of
the piston be equal to a square inch; if it be loaded with 10 lbs.,
including its own weight, the steam which will sustain it in any
position in the cylinder, whether near the bottom or top, must always
be exactly equal in pressure to 10 lbs. per inch. In this respect it
resembles the quality already explained in the governor, and renders
the pressure of the steam uniform, exactly in the same manner as the
governor renders the velocity of the engine uniform.
(73.) The economy of fuel depends, in a great degree, on the
construction of the furnace, independently of the effects of the
arrangements we have already described.
The grate or fire-place of an ordinary furnace is placed under the
boiler; and the atmospheric air passing through the ignited fuel,
supplies sufficient oxygen to support a large volume of flame, which
is carried by the draught into a flue, which circulates twice or
oftener round the boiler, and in immediate contact with it, and
finally issues into the chimney. Through this flue the flame
circulates, so as to act on every part of the boiler near which the
flue passes; and it is frequently not until it passes into the
chimney, and sometimes not until it leaves the chimney, that it ceases
to exist in the state of flame.
The dense black smoke which is observed to issue from the chimneys of
furnaces is formed of a quantity of unconsumed fuel, and may be
therefore considered as so much fuel wasted. Besides this, in large
manufacturing towns, where a great number of furnaces are employed, it
is found that the quantity of smoke which thus becomes diffused
through the atmosphere, renders it pernicious to the health, and
destructive to the comforts of the inhabitants.
These circumstances have directed the attention of engineers to the
discovery of means whereby this smoke or wasted fuel may be consumed
for the use of the engine itself, or for whatever use the furnace may
be applied to. The most usual method of accomplishing this is by so
arranging matters that fuel in a state of high combustion, and,
therefore, producing no smoke, shall be always kept on that part of
the grate which is nearest to the mouth of the flue (and which we
shall call the back); by this means the smoke which arises from the
imperfectly ignited fuel which is nearer to the front of the grate
must pass over the surface of the red fuel, before it enters the flue,
and is thereby ignited, and passes in a state of flame into the flue.
A passage called the _feeding-mouth_ leads to the front of the grate,
and both this passage and the grate are generally inclined at a small
angle to the horizon, in order to facilitate the advance of the fuel
according as its combustion proceeds.
When fresh fuel for feeding the boiler is first introduced, it is
merely laid in the feeding-mouth. Here it is exposed to the action of
a part of the heat of the burning fuel on the grate, and undergoes, in
some degree, the process of coking. The door of the feeding-mouth is
furnished with small apertures for the admission of a stream of air,
which carries the smoke evolved by the coking of the fresh fuel over
the burning fuel on the grate, by which this smoke is ignited, and
becomes flame, and in this state enters the flue, and circulates round
the boiler. When the furnace is to be fed, the door of the
feeding-mouth is opened, and the fuel which had been laid in it, and
partially coked, is forced upon the front part of the grate. At first,
its combustion being imperfect, but proceeding rapidly, a dense black
smoke arises from it. The current of air from the open door through
the feeding-mouth carries this over the vividly burning fuel in the
back part of the grate, by which the smoke being ignited, passes in a
state of flame into the flue. When the furnace again requires feeding,
every part of this fuel will be in a state of active combustion, and
it is forced to the back part of the grate next the flue, preparatory
to the introduction of more fuel from the feeding-mouth.
The apertures in the door of the feeding-mouth are furnished with
covers, so that the quantity of air admitted through them can be
regulated by the workmen. The efficiency of these furnaces in a great
degree depends on the judicious admission of the air through the
feeding-mouth: for if less than the quantity necessary to support the
combustion of the fuel be admitted, a part of the smoke will remain
unconsumed; and if more than the proper quantity be admitted, it will
defeat the effects of the fuel by cooling the boiler. If the process
which we have just described be considered, it will not be difficult
to perceive the total impossibility in such a furnace of exactly
regulating the draught of air, so that too much shall not pass at one
time, and too little at another. When the door is open to introduce
fresh fuel into the feeding-mouth, and advance that which occupied it
upon the grate, the workman ceases to have any control whatever over
the draught of air; and even at other times when the door is closed,
his discretion and attention cannot be depended on. The consequence
is, that with these defects the proprietors of steam engines found,
that in the place of economising the fuel, the use of these furnaces
entailed on them such an increased expense that they were generally
obliged to lay them aside.
(74.) Mr. Brunton of Birmingham having turned his attention to the
subject, has produced a furnace which seems to be free from the
objections against those we have just mentioned. The advantages of his
contrivance, as stated by himself, are as follow:--
"First, I put the coal upon the grate by small quantities, and at very
short intervals, say every two or three seconds. 2dly, I so dispose of
the coals upon the grate, that the smoke evolved must pass over that
part of the grate upon which the coal is in full combustion, and is
thereby consumed. 3dly, As the introduction of coal is uniform in
short spaces of time, the introduction of air is also uniform, and
requires no attention from the fireman.
[Illustration: Pl. VIII.]
"As it respects economy: 1st, The coal is put upon the fire by an
apparatus driven by the engine, and so contrived that the quantity of
coal is proportioned to the quantity of work which the engine is
performing, and the quantity of air admitted to consume the smoke is
regulated in the same manner. 2dly, The fire-door is never opened,
excepting to clean the fire; the boiler of course is not exposed to
that continual irregularity of temperature which is unavoidable in the
common furnace, and which is found exceedingly injurious to boilers.
3dly, The only attention required is to fill the coal receiver, every
two or three hours, and clean the fire when necessary. 4thly, The coal
is more completely consumed than by the common furnace, as all the
effect of what is termed stirring up the fire (by which no
inconsiderable quantity of coal is passed into the ash-pit) is
attained without moving the coal upon the grate."
The fire-place is a circular grate placed on a vertical shaft in a
horizontal position. It is capable of revolving, and is made to do so
by the vertical shaft, which is turned by wheelwork, which is worked
by the engine itself; or this shaft or spindle may be turned by a
water-wheel, on which a stream of water is allowed to flow from a
reservoir into which it is pumped by the power of the engine; and by
regulating the quantity of water in the stream, the grate may be made
to revolve with a greater or less speed. In that part of the boiler
which is over the grate, there is an aperture, in which is placed a
hopper, through which fuel is let down upon the grate at the rate of
any quantity per minute that may be required. The apparatus which
admits the coals through this hopper is worked by the engine also, and
by the same means as the grate is turned so that the grate revolves
with a speed proportional to the rapidity with which the fuel is
admitted through the hopper, and by this ingenious arrangement the
fuel falls equally thick upon the grate.
The supply of water which turns the wheel which works the grate and
the machinery in the hopper, is regulated by a cock connected with the
self-regulating damper; so that when the steam is being produced too
fast, the supply of water will be diminished, and by that means the
supply of fuel to the grate will be diminished, and the grate will
revolve less rapidly: and when the steam is being produced too slowly
for the demands of the engine, the contrary effects take place. In
this way the fuel which is introduced into the furnace is exactly
proportioned to the work which the engine has to perform. The hopper
may be made large enough to hold coals for a day's work, so that the
furnace requires no other attendance than to deposite coals in the
hopper each morning.
The coals are let down from the hopper on the grate at that part
which is most remote from the flue; and as they descend in very small
quantities at a time, they are almost immediately ignited. But until
their ignition is complete, a smoke will arise, which, passing to the
flue over the vividly burning fuel, will be ignited. Air is admitted
through proper apertures, and its quantity regulated by the damper in
the same manner as the supply of fuel.
The superiority of this beautiful invention over the common
smoke-consuming furnaces is very striking. Its principle of
self-adjustment as to the supply of coals and atmospheric air, and the
proportioning of these to the quantity of work to be performed by the
engine, not only independent of human labour, but with a greater
degree of accuracy than any human skill or attention could possibly
effect, produces saving of expense, both in fuel and labour.
(75.) Mr. Oldham, engineer to the Bank of Ireland, has proposed
another modification of the self-regulating furnace, which seems to
possess several advantages, and evinces considerable ingenuity.
He uses a slightly inclined grate, at the back or lower end of which
is the flue, and at the front or higher end, the hopper for admitting
the coals. In the bottom or narrow end of the hopper is a moveable
shelf, worked by the engine. Upon drawing back this shelf, a small
quantity of fuel is allowed to descend upon a fixed shelf under it;
and upon the return of the moveable shelf, this fuel is protruded
forward upon the grate. Every alternate bar of the grate is fixed, but
the intermediate ones are connected with levers, by which they are
moved alternately up and down.[22] The effect is, that the coals upon
the bars are continually stirred, and gradually advanced by their own
weight from the front of the grate, where they fall from the hopper,
to the back, where they are deposited in the ash-pit. By the shape and
construction of the bars, the air is conducted upwards between them,
and rushes through the burning fuel, so as to act in the manner of a
blowpipe, and the entire surface of the fire presents a sheet of
flame.
[Footnote 22: Mr. Brunton used moveable bars in a furnace constructed
by him before he adopted the horizontal revolving grate. That plan,
however, does not appear to have been as successful as the latter, as
he has abandoned it. Mr. Oldham states that his furnace has been in
use for several years without any appearance of derangement in the
mechanism, and with a considerable saving of fuel.]
We cannot fail to be struck with the beauty of all these contrivances,
by which the engine is made to regulate itself, and supply its own
wants. It is in fact, all but alive. It was observed by Belidor, long
before the steam engine reached the perfection which it has now
acquired, that it resembled an animal, and that no mere work of man
ever approached so near to actual _life_. Heat is the principle of its
existence. The boiler acts the part of the heart, from which its
vivifying fluid rushes copiously through all the tubes, where having
discharged the various functions of life, and deposited its heat in
the proper places, it returns again to the source it sprung from, to
be duly prepared for another circulation. The healthfulness of its
action is indicated by the regularity of its pulsations; it procures
its own food by its own labour; it selects those parts which are fit
for its support, both as to quantity and quality; and has its natural
evacuations, by which all the useless and innutritious parts are
discharged. It frequently cures its own diseases, and corrects the
irregularity of its own actions, exerting something like moral
faculties. Without designing to carry on the analogy, Mr. Farey, in
speaking of the variations incident to the work performed by different
steam engines, states some further particulars in which it maybe
curiously extended. "We must observe," says he, "that the variation in
the performance of different steam engines, which are constructed on
the same principle, working under the same advantages, is the same as
would be found in the produce of the labour of so many different
horses or other animals when compared with their consumption of food;
for the effects of different steam engines will vary as much from
small differences in the proportion of their parts, as the strength
of animals from the vigour of their constitutions; and again, there
will be as great differences in the performance of the same engine
when in good and bad order, from all the parts being tight and well
oiled, so as to move with little friction, as there is in the labour
of an animal from his being in good or bad health, or excessively
fatigued: but in all these cases there will be a maximum which cannot
be exceeded, and an average which we ought to expect to obtain."
CHAPTER IX.
DOUBLE-CYLINDER ENGINES.
Hornblower's Engine. -- Woolf's Engine. -- Cartwright's Engine.
(76.) The expansive property of steam, of which Watt availed himself
in his single engine by cutting off the supply of steam before the
descent of the piston was completed, was applied in a peculiar manner
by an engineer named Hornblower, about the year 1781, and at a later
period by Woolf. Hornblower was the first who conceived the idea of
working an engine with two cylinders of different sizes, by allowing
the steam to flow freely from the boiler until it fills the smaller
cylinder, and then permitting it to expand into the greater one,
employing it thus to press down two pistons in the manner which we
shall presently describe. The condensing apparatus of Hornblower, as
well as the other appendages of the engine, do not differ materially
from those of Watt; so that it will be sufficient for our present
purpose to explain the manner in which the steam is made to act in
moving the piston.
Let C, fig. 41., be the centre of the great working beam, carrying
two arch heads, on which the chains of the piston rods play. The
distances of these arch heads from the centre C must be in the same
proportion as the length of the cylinders, in order that the same play
of the beam may correspond to the plays of both pistons. Let F be the
steam-pipe from the boiler, and G a valve to admit the steam above the
lesser piston, H is a tube by which a communication may be opened by
the valve I between the top and bottom of the lesser cylinder B. K is
a tube communicating, by the valve L, between the bottom of the lesser
cylinder B and the top of the greater cylinder A. M is a tube
communicating, by the valve N, between the top and bottom of the
greater cylinder A; and P a tube leading to the condenser by the
exhausting valve O.
At the commencement of the operation, suppose all the valves opened,
and steam allowed to flow through the entire engine until the air be
completely expelled, and then let all the valves be closed. To start
the engine, let the exhausting valve O and the steam valves G and L be
opened, as in fig. 41. The steam will flow freely from the boiler, and
press upon the lesser piston, and at the same time the steam below the
greater piston will flow into the condenser, leaving a vacuum in the
greater cylinder. The valve L being opened, the steam which is under
the piston in the lesser cylinder will flow through K, and press on
the greater piston, which, having a vacuum beneath it, will
consequently descend. At the commencement of the motion, the lesser
piston is as much resisted by the steam below it as it is urged by the
steam above it; but after a part of the descent has been effected, the
steam below the lesser piston passing into the greater, expands into
an increased space, and therefore loses its elastic force
proportionally. The steam above the lesser piston retaining its full
force by having a free communication with the boiler by the valve G,
the lesser piston will be urged by a force equal to the excess of the
pressure of this steam above the diminished pressure of the expanded
steam below it. As the pistons descend, the steam which is between it
continually increasing in its bulk, and therefore decreasing in its
pressure, from whence it follows, that the force which resists the
lesser piston is continually decreasing, while that which presses it
down remains the same, and therefore the effective force which impels
it must be continually increasing.
On the other hand, the force which urges the greater piston is
continually decreasing, since there is a vacuum below it, and the
steam which presses it is continually expanding into an increased
bulk.
Impelled in this way, let us suppose the pistons to have arrived at
the bottoms of the cylinders, as in fig. 42., and let the valves G, L,
and O be closed, and the valves I and N opened. No steam is allowed to
flow from the boiler, G being closed, nor any allowed to pass into the
condenser, since O is closed, and all communications between the
cylinders is stopped by closing L. By opening the valve I, a free
communication is made between the top and bottom of the lesser piston
through the tube H, so that the steam which presses above the lesser
piston will exert the same pressure below it, and the piston is in a
state of indifference. In the same manner the valve N being open, a
free communication is made between the top and bottom of the greater
piston, and the steam circulates above and below the piston, and
leaves it free to rise. A counterpoise attached to the pump-rods in
this case, draws up the piston, as in Watt's single engine; and when
they arrive at the top, the valves I and N are closed, and G, L, and O
opened, and the next descent of the pistons is produced in the manner
already described, and so the process is continued.
The valves are worked by the engine itself, by means similar to some
of those already described. By computation, we find the power of this
engine to be nearly the same as a similar engine on Watt's expansive
principle. It does not however appear, that any adequate advantage
was gained by this modification of the principle, since no engines of
this construction are now made.
(77.) The use of two cylinders was revived by _Arthur Woolf_, in 1804,
who, in this and the succeeding year, obtained patents for the
application of steam raised under a high pressure to double-cylinder
engines. The specification of his patent states, that he has proved by
experiment that steam raised under a safety-valve loaded with any
given number of pounds upon the square inch, will, if allowed to
expand into as many times its bulk as there are pounds of pressure on
the square inch, have a pressure equal to that of the atmosphere.
Thus, if the safety-valve be loaded with four pounds on the square
inch, the steam, after expanding into four times its bulk, will have
the atmospheric pressure. If it be loaded with 5, 6, or 10 lbs. on the
square inch, it will have the atmospheric pressure when it has
expanded into 5, 6, or 10 times its bulk, and so on. It was, however,
understood in this case, that the vessel into which it was allowed to
expand should have the same temperature as the steam before it
expands.
It is very unaccountable how a person of Mr. Woolf's experience in the
practical application of steam could be led into errors so gross as
those involved in the averments of this patent; and it is still more
unaccountable how the experiments could have been conducted which led
him to conclusions not only incompatible with all the established
properties of elastic fluids--properties which at that time were
perfectly understood--but even involving in themselves palpable
contradiction and absurdity. If it were admitted that every additional
pound avoirdupois which should be placed upon the safety-valve would
enable steam, by its expansion into a proportionally enlarged space,
to attain a pressure equal to the atmosphere, the obvious consequence
would be, that a physical relation would subsist between the
atmospheric pressure and the pound avoirdupois! It is wonderful that
it did not occur to Mr. Woolf, that, granting his principle to be true
at any given place, it would necessarily be false at another place
where the barometer would stand at a different height. Thus if the
principle were true at the foot of a mountain, it would be false at
the top of it; and if it were true in fair weather, it would be false
in foul weather, since these circumstances would be attended by a
change in the atmospheric pressure, without making any change in the
pound avoirdupois.[23]
[Footnote 23: It is strange that this absurdity has been repeatedly
given as unquestionable fact in various encyclopædias on the article
"Steam Engine," as well as in by far the greater number of treatises
expressly on the subject.]
The method by which Mr. Woolf proposed to apply the principle which he
imagined himself to have discovered was by an arrangement of cylinders
similar to those of Hornblower, but having their magnitudes
proportioned to the greater extent of expansion which he proposed to
use. Two cylinders, like those of Hornblower, were placed under the
working beam, having their piston-rods at distances from the axis
proportioned to the lengths of their respective strokes. The relative
magnitudes of the cylinders A and B must be adjusted according to the
extent to which the principle of expansion is intended to be used. The
valves C C´ were placed at each end of the lesser cylinder in tubes
communicating with the boiler, so as to admit steam on each side of
the lesser piston, and cut it off at pleasure. A tube, D´, formed a
communication between the upper end of the lesser and lower end of the
greater cylinder, which communication is opened and closed at pleasure
by the valve E´. In like manner, the tube D forms a communication
between the lower end of the lesser cylinder and the upper end of the
greater, which may be opened and closed by the valve E. The top and
bottom of the greater cylinder communicated with the condenser by
valves F´ F.
Let us suppose that the air is blown from the engine in the usual
way, all the valves closed, and the engine ready to start, the pistons
being at the top of the cylinders. Open the valves C, E, and F. The
steam which occupies the greater cylinder below the piston will now
pass into the condenser through F, leaving a vacuum below the piston.
The steam which is in the lesser cylinder below the piston will pass
through D and open the valve E, and will press down the greater
piston. The steam from the boiler will flow in at C, and press on the
lesser piston. At first the whole motion will proceed from the
pressure upon the greater piston, since the steam, both above and
below the lesser piston, has the same pressure. But, as the pistons
descend, the steam below the less passing into the greater cylinder,
expands into a greater space, and consequently exerts a diminished
pressure, and, therefore, the steam on the other side exerting an
undiminished pressure, acquires an impelling force exactly equal to
the pressure lost in the expansion of the steam between the two
pistons. Thus both pistons will be pressed to the bottoms of their
respective cylinders. It will be observed that in the descent the
greater piston is urged by a continually decreasing force, while the
lesser is urged by continually increasing force.
Upon the arrival of the pistons at the bottoms of the cylinders, let
the valves, C, E, F be closed, and C´, E´, F´ be opened, as in fig.
44. The steam which is above the greater piston now flows through F´
into the condenser, leaving the space above the piston a vacuum. The
steam which is above the lesser piston passes through E´ and D´ below
the greater, while the steam from the boiler is admitted through C´
below the lesser piston. The pressure of the steam entering through E´
below the greater piston, pressing on it against the vacuum above it,
commences the ascent. In the mean time the steam above the lesser
piston passing into the enlarged space of the greater cylinder, loses
gradually its elastic force, so that the steam entering from the
boiler at C´ becomes in part effective, and the ascent is completed
under exactly the same circumstances as the descent, and in this way
the process is continued.
It is evident that the valves may be easily worked by the mechanism of
the engine itself.
In this arrangement the pistons ascend and descend together, and their
rods must consequently be attached to the beam at the same side of the
centre. It is sometimes desirable that they should act on different
sides of the centre of the beam, and consequently that one should
ascend while the other descends. It is easy to arrange the valves so
as to effect this. In fig. 45., the lesser piston is at the bottom of
the cylinder, and the greater at the top. On opening the valves C´,
E´, F´, a vacuum is produced below the greater piston, and steam flows
from the lesser cylinder, through E´, _above_ the greater piston, and
presses it down. At the same time steam being admitted from the boiler
through C´ below the lesser piston, forces it up against the
diminishing force of the steam above it, which expands into the
greater cylinder. Thus as the greater piston descends the lesser
ascends. When each has traversed its cylinder, the valves C´, E´, F´
being closed, and C, E, F opened, the lesser piston will descend, and
the greater ascend, and so on.
(78.) The law according to which the elastic force of steam diminishes
as it expands, of which Mr. Woolf appears to have been entirely
ignorant, is precisely similar to the same property in air and other
elastic fluids. If steam expands into twice or thrice its volume, it
will lose its elastic force in precisely the same proportion as it
enlarges its bulk; and therefore will have only a half or a third of
its former pressure, supposing that as it expands its temperature is
kept up. Although Mr. Woolf's patent contained the erroneous principle
which we have noticed, yet, so far as his invention suggested the idea
of employing steam at a very high-pressure, and allowing it to expand
in a much greater degree than was contemplated either by Watt or
Hornblower, it became the means of effecting a considerable saving in
fuel; for engines used for pumping on a large scale, the steam being
produced under a pressure of forty or fifty pounds or more upon the
square inch, might be worked first through a small space with intense
force, and the communication with the boiler being then cut off, it
might be allowed, with great advantage, to expand through a very large
space. Some double-cylinder engines upon this principle have been
worked in Cornwall, with considerable economy. But the form in which
the expansive principle, combined with high pressure, is now applied
in the engines used for raising water from the mines, is that in which
it was originally proposed by Watt. A single cylinder of considerable
length is employed; the piston is driven through a small proportion of
this length by steam, admitted from the boiler at a very intense
pressure: the steam being then cut off, the piston is urged by the
expansive force of the steam which has been admitted, and is by that
means brought to the bottom of the cylinder.
It is evident, under such circumstances, that the pressure of the
steam admitted from the boiler must be much greater than the
resistance opposed to the piston, and that the motion of the piston
must, in the first instance, be accelerated and not uniform. If the
piston moved from the commencement with a uniform motion, the pressure
of the steam urging it must necessarily be exactly equal to the
resistance opposed to it, and then cutting off the supply of steam
from the boiler, the piston could only continue its motion by inertia,
the steam immediately becoming of less pressure than the resistance;
and after advancing through a very small space, the piston would
recoil upon the steam, and come to a state of rest. The steam,
however, at the moment it is cut off being of much greater pressure
than the amount of resistance upon the piston, will continue to drive
the piston forward, until by its expansion its force is so far
diminished as to become equal to the resistance of the piston. From
that point the impelling power of the steam will cease, and the
piston will move forward by its inertia only. The point at which the
steam is cut off should therefore be so regulated that it shall
acquire a pressure equal to the resistance on the piston by its
expansion, just at such a distance from the end of the stroke as the
piston may be able to move through by its inertia. It is evident the
adjustment of this will require great care and nicety of management.
(79.) In 1797 a patent was granted to the _Rev. Mr. Cartwright_, a
gentleman well known for other mechanical inventions, for some
improvements in the steam-engine. His contrivance is at once so
elegant and simple, that, although it has not been carried into
practice, we cannot here pass it over without notice.
The steam-pipe from the boiler is represented cut off at B (fig. 46.);
T is a spindle-valve for admitting steam above the piston, and R is a
spindle-valve in the piston; D is a curved pipe forming a
communication between the cylinder and the condenser, which is of very
peculiar construction. Cartwright proposed effecting a condensation
without a jet, by exposing the steam to contact with a very large
quantity of cold surface. For this purpose, he formed his condenser by
placing two cylinders nearly equal in size one within the other,
allowing the water of the cold cistern in which they were placed to
flow through the inner cylinder, and to surround the outer one. Thus
the thin space between the two cylinders formed the condenser.
[Illustration: Pl. IX.]
[Illustration: Pl. X.]
The air-pump is placed immediately under the cylinder, and the
continuation of the piston-rod works its piston, which is solid and
without a valve: F is the pipe from the condenser to the air-pump,
through which the condensed steam is drawn off through the valve G on
the ascent of the piston, and on the descent, this is forced through a
tube into a hot well, H, for the purpose of feeding the boiler through
the feed-pipe I. In the top of the hot well H is a valve which opens
inwards, and is kept closed by a ball floating on the surface of the
liquid. The pressure of the condensed air above the surface of the
liquid in H forces it through I into the boiler. When the air
accumulates in too great a degree in H, the surface of the liquid is
pressed so low that the ball falls and opens the valve, and allows it
to escape. The air in H is that which is pumped from the condenser
with the liquid, and which was disengaged from it.
Let us suppose the piston at the top of the cylinder; it strikes the
tail of the valve T, and raises it, while the stem of the piston-valve
R strikes the top of the cylinder, and is pressed into its seat. A
free communication is at the same time open between the cylinder,
below the piston and the condenser, through the tube D. The pressure
of the steam thus admitted above the piston, acting against the vacuum
below it, will cause its descent. On arriving at the bottom of the
cylinder, the tail of the piston-valve R will strike the bottom, and
it will be lifted from its seat, so that a communication will be
opened through it with the condenser. At the same moment a projecting
spring, K, attached to the piston-rod, strikes the stem of the
steam-valve T, and presses it into its seat. Thus, while the further
admission of steam is cut off, the steam above the piston flows into
the condenser, and the piston being relieved from all pressure, is
drawn up by the momentum of the fly-wheel, which continues the motion
it received from the descending force. On the arrival of the piston
again at the top of the cylinder, the valve T is opened and R closed,
and the piston descends as before, and so the process is continued.
The mechanism by which motion is communicated from the piston to the
fly-wheel is peculiarly elegant. On the axis of the fly-wheel is a
small wheel with teeth, which work in the teeth of another large wheel
L. This wheel is turned by a crank, which is worked by a cross-piece
attached to the end of the piston-rod. Another equal-toothed wheel, M,
is turned by a crank, which is worked by the other end of the
cross-arm attached to the piston-rod.
One of the peculiarities of this engine is, that the liquid which is
used for the production of steam in the boiler circulates through the
machine without either diminution or admixture with any other fluid,
so that the boiler never wants more feeding than what can be supplied
from the hot-well H. This circumstance forms a most important feature
in the machine, as it allows of ardent spirits being used in the
boiler instead of water, which, since they boil at low heats, promised
a saving of half the fuel. The inventor even proposed, that the engine
should be used as a still, as well as a mechanical power, in which
case the whole of the fuel would be saved.
In this engine, the ordinary method of rendering the piston
steam-tight, by oil or melted wax or tallow poured upon it, could not
be applied, since the steam above the piston must always have a free
passage through the piston-valve R. The ingenious inventor therefore
contrived a method of making the piston steam-tight in the cylinder,
without oil or stuffing, and his method has since been adopted with
success in other engines.
A ring of metal is ground into the cylinder, so as to fit it
perfectly, and is then cut into four equal segments. The inner surface
of this ring being slightly conical, another ring is ground into it,
so as to fit it perfectly, and this is also cut into four segments,
and one is placed within the other, but in such a manner that the
joints or divisions do not coincide. The arrangement of the two rings
is represented in fig. 47. Within the inner ring are placed four
springs, which press the pieces outward against the sides of the
cylinder, and are represented in the diagram. Four pairs of these
rings are placed one over another, so that their joints do not
coincide, and the whole is screwed together by plates placed at top
and bottom. A vertical section of the piston is given in fig. 48.
One of the advantages of this piston is, that the longer it is worked,
the more accurately it fits the cylinder, so that, as the machine
wears it improves.
Metallic pistons have lately come into very general use, and such
contrivances differ very little from the above.
CHAPTER X.
LOCOMOTIVE ENGINES ON RAILWAYS.
High-pressure Engines. -- Leupold's Engine. -- Trevithick and
Vivian. -- Effects of Improvement in Locomotion. -- Historical
Account of the Locomotive Engine. -- Blenkinsop's Patent. --
Chapman's Improvement. -- Walking Engine. -- Stephenson's First
Engines. -- His Improvements. -- Liverpool and Manchester Railway
Company. -- Their Preliminary Proceedings. -- The Great
Competition of 1829. -- The Rocket. -- The Sanspareil. -- The
Novelty. -- Qualities of the Rocket. -- Successive Improvements.
-- Experiments. -- Defects of the Present Engines. -- Inclined
Planes. -- Methods of surmounting them. -- Circumstances of the
Manchester Railway Company. -- Probable Improvements in
Locomotives. -- Their capabilities with respect to speed. --
Probable Effects of the Projected Railroads. -- Steam Power
compared with Horse Power. -- Railroads compared with Canals.
(80.) In the various modifications of the steam engine which we have
hitherto considered, the pressure introduced on one side of the piston
derives its efficacy either wholly or partially from the vacuum
produced by condensation on the other side. This always requires a
condensing apparatus, and a constant and abundant supply of cold
water. An engine of this kind must therefore necessarily have
considerable dimensions and weight, and is inapplicable to uses in
which a small and light machine only is admissible. If the condensing
apparatus be dispensed with, the piston will always be resisted by a
force equal to the atmospheric pressure, and the only part of the
steam pressure which will be available as a moving power, is that
part by which it exceeds the pressure of the atmosphere. Hence, in
engines which do not work by condensation, steam of a much higher
pressure than that of the atmosphere is indispensably necessary, and
such engines are therefore called _high-pressure engines_.
We are not, however, to understand that every engine, in which steam
is used of a pressure exceeding that of the atmosphere, is what is
meant by a _high-pressure engine_; for in the ordinary engines in
common use, constructed on Watt's principle, the safety-valve is
loaded with from 3 to 5 lbs. on the square inch; and in Woolf's
engines, the steam is produced under a pressure of 40 lbs. on the
square inch. These would therefore be more properly called _condensing
engines_ than _high-pressure engines_; a term quite inapplicable to
those of Woolf. In fact, by _high-pressure engines_ is meant engines
in which no vacuum is produced, and, therefore, in which the piston
works against a pressure equal to that of the atmosphere.
In these engines, the whole of the condensing apparatus, _viz._ the
cold-water cistern, condenser, air-pump, cold-water pump, &c. are
dispensed with, and nothing is retained except the boiler, cylinder,
piston, and valves. Consequently, such an engine is small, light, and
cheap. It is portable also, and may be moved, if necessary, along with
its load, and is therefore well adapted to locomotive purposes.
(81.) High-pressure engines were one of the earliest modifications of
the steam engine. The contrivance, which is obscurely described in the
article already quoted (27.), from the Century of Inventions, is a
high-pressure engine; for the power there alluded to is the elastic
force of steam working against the atmospheric pressure. _Newcomen_,
in 1705, applied the working-beam, cylinder, and piston to the
atmospheric engine; and _Leupold_, about 1720, combined the
working-beam and cylinder with the high-pressure principle, and
produced the earliest high-pressure engine worked by a cylinder and
piston. The following is a description of _Leupold's_ engine:--
A (fig. 49.) is the boiler, with the furnace beneath it; C C´ are two
cylinders with two solid pistons, P P´, connected with the
working-beams B B´, to which are attached the pump-rods, R R´, of two
forcing-pumps, F F´, which communicate with a great force-pipe S; G is
a _four-way cock_ (66.) already described. In the position in which it
stands in the figure, the steam is issuing from below the piston P
into the atmosphere, and the piston is descending by its own weight;
steam from the boiler is at the same time pressing up the piston P´,
with a force equal to the difference between the pressure of the steam
and that of the atmosphere. Thus the piston R of the forcing-pump is
being drawn up, and the piston P´ is forcing the piston R´ down, and
thereby driving water into the force-pipe S. On the arrival of the
piston P at the bottom of the cylinder C, and P´ at the top of the
cylinder C´, the position of the cock is changed as represented in
fig. 50. The steam, which has just pressed up the piston P´, is
allowed to escape into the atmosphere, while the steam, passing from
the boiler below the piston P, presses it up, and thus P ascends by
the steam pressure, and P´ descends by its own weight. By these means
the piston R is forced down, driving before it the water in the pump
cylinder into the force-pipe S, and the piston R´ is drawn up to allow
the other pump-cylinder to be re-filled; and so the process is
continued.
A valve is placed in the bottom of the force-pipes, to prevent the
water which has been driven into it from returning. This valve opens
upwards; and, consequently, the weight of the water pressing upon it
only keeps it more effectually closed. On each descent of the piston,
the pressure transmitted to the valve acting upwards being greater
than the weight of the water resting upon it, forces it open, and an
increased quantity of water is introduced.
(82.) From the date of the improvement of Watt until the commencement
of the present century, high-pressure engines were altogether
neglected in these countries. In the year 1802, Messrs. _Trevithick_
and _Vivian_ constructed the first high-pressure engine which was ever
brought into extensive practical use in this kingdom. A section of
this machine, made by a vertical plane, is represented in fig. 51.
The boiler A B is a cylinder with flat circular ends. The fire-place
is constructed in the following manner:--A tube enters the cylindrical
boiler at one end; and, proceeding onwards, near the other extremity,
is turned and recurved, so as to be carried back parallel to the
direction in which it entered. It is thus conducted out of the boiler,
at another part of the same end at which it entered. One of the ends
of this tube communicates with the chimney E, which is carried
upwards, as represented in the figure. The other mouth is furnished
with a door; and in it is placed the grate, which is formed of
horizontal bars, dividing the tube into two parts; the upper part
forming the fire-place, and the lower the ash-pit. The fuel is
maintained in a state of combustion, on the bars, in that part of the
tube represented at C D; and the flame is carried by the draft of the
chimney round the curved flue, and issues at E into the chimney. The
flame is thus conducted through the water, so as to expose the latter
to as much heat as possible.
A section of the cylinder is represented at F, immersed in the boiler,
except a few inches of the upper end, where the four-way cock G is
placed for regulating the admission of the steam. A tube is
represented at H, which leads from this four-way cock into the
chimney; so that the waste steam, after working the piston, is carried
off through this tube, and passes into the chimney. The upper end of
the piston-rod is furnished with a cross-bar, which is placed in a
direction at right angles to the length of the boiler, and also to the
piston-rod. This bar is guided in its motion by sliding on two iron
perpendicular rods fixed to the sides of the boiler, and parallel to
each other. To the ends of this cross-bar are joined two connecting
rods, the lower ends of which work two cranks fixed on an axis
extending across and beneath the boiler, and immediately under the
centre of the cylinder. This axis is sustained in bearings formed in
the legs which support the boiler, and upon its extremity is fixed the
fly-wheel as represented at B. A large-toothed wheel is placed on this
axis; which, being turned with the cranked axle, communicates motion
to other wheels; and, through them, to any machinery which the engine
may be applied to move.
As the four-way cock is represented in the figure, the steam passes
from the boiler through the curved passage G above the piston, while
the steam below the piston is carried off through a tube which does
not appear in the figure, by which it is conducted to the tube H, and
thence to the chimney. The steam, therefore, which passes above the
piston presses it downwards; while the pressure upwards does not
exceed that of the atmosphere. The piston will therefore descend with
a force depending on the excess of the pressure of the steam produced
in the boiler above the atmospheric pressure. When the piston has
arrived at the bottom of the cylinder, the cock is made to assume the
position represented in the figure 52. This effect is produced by the
motion of the piston-rod. The steam now passes from above the piston,
through the tube H, into the chimney, while the steam from the boiler
is conducted through another tube below the piston. The pressure above
the piston, in this case, does not exceed that of the atmosphere;
while the pressure below it will be that of the steam in the boiler.
The piston will therefore ascend with the difference of these
pressures. On the arrival of the piston at the top of the cylinder,
the four-way cock is again turned to the position represented in fig.
51., and the piston again descends; and in the same manner the process
is continued. A safety-valve is placed on the boiler at V, loaded
with a weight W, proportionate to the strength of the steam with which
it is proposed to work.
[Illustration: Pl. XI.]
In the engines now described, this valve was frequently loaded at the
rate of from 60 to 80 lbs. on the square inch. As the boilers of
high-pressure engines were considered more liable to accidents from
bursting than those in which steam of a lower pressure was used,
greater precautions were taken against such effects. A second
safety-valve was provided, which was not left in the power of the
engine-man. By this means he had a power to diminish the pressure of
the steam, but could not increase it beyond the limit determined by
the valve which was removed from his interference. The greatest cause
of danger, however, arose from the water in the boiler being consumed
by evaporation faster than it was supplied; and therefore falling
below the level of the tube containing the furnace. To guard against
accidents arising from this circumstance, a hole was bored in the
boiler, at a certain depth, below which the water should not be
allowed to fall; and in this hole a plug of metal was soldered with
lead, or with some other metal, which would fuse at that temperature
which would expose the boiler to danger. Thus, in the event of the
water being exhausted, so that its level would fall below the plug,
the heat of the furnace would immediately melt the solder, and the
plug would fall out, affording a vent for the steam, without allowing
the boiler to burst. The mercurial steam-gauge, already described, was
also used as an additional security. When the force of the steam
exceeded the length of the column of mercury which the tube would
contain, the mercury would be blown out, and the tube would give vent
to the steam. The water by which the boiler was replenished was forced
into it by a pump worked by the engine. In order to economise the
heat, this water was contained in a tube T, which surrounded the pipe
H. As the waste steam, after working the piston, passed off through H,
it imparted a portion of its heat to the water contained in the tube
T, which was thus warmed to a certain temperature before it was
forced into the boiler by the pump. Thus a part of the heat, which was
originally carried from the boiler in the form of steam, was returned
again to the boiler with the water with which it was fed.
It is evident that engines constructed in this manner may be applied
to all the purposes to which the condensing engines are applicable.
(_e_) To the plates of the English edition has been added one, (plate
A) representing a high-pressure engine as constructed by the West
Point Foundry in the state of New York. The principal parts will be
readily distinguished from their resemblance to the analogous parts of
a condensing engine. The condenser and air-pump of that engine, it
will be observed, are suppressed. At _v x_ and _y z_ are forcing-pumps
by which a supply of water is injected into the boiler at each motion
of the engine. For the four-way cock, used in the English
high-pressure engines, a slide valve at _r s_, is substituted, and is
found to work to much greater advantage. It is set in motion by an
eccentric, in a manner that will be more obvious from an inspection of
the plate than from any description.--A. E.
(_f_) A very safe and convenient boiler for a high-pressure engine has
been invented in the United States by Mr. Babcock. The boiler consists
of small tubes, into which water is flashed by a small forcing-pump at
every stroke of the engine. The tubes are kept so hot in a furnace, as
to generate steam of the required temperature, but not hot enough to
cause any risk of the decomposition of the water. The strength of the
apparatus is such, and the quantity of water exposed to heat at one
time so small, as to leave hardly any risk of danger.--A. E.
(83.) Two years after the date of the patent of this engine, its
inventor constructed a machine of the same kind for the purpose of
moving carriages on railroads; and applied it successfully, in the
year 1804, on the railroad at Merthyr Tydvil, in South Wales. It was
in principle the same as that already described. The cylinder however
was in a horizontal position, the piston-rod working in the direction
of the line of road: the extremity of the piston-rod, by means of a
connecting rod, worked cranks placed on the axletree, on which were
fixed two cogged wheels: these worked in others, by which their motion
was communicated finally to cogged wheels fixed on the axle of the
hind wheels of the carriage, by which this axle was kept in a state of
revolution. The hind wheels being fixed on the axletree, and turning
with it, were caused likewise to revolve; and so long as the weight of
the carriage did not exceed that which the friction of the road was
capable of propelling, the carriage would thus be moved forwards. On
this axle was placed a fly-wheel to continue the rotatory motion at
the termination of each stroke. The fore wheels are described as being
capable of turning like the fore wheels of a carriage, so as to guide
the vehicle. The projectors appear to have contemplated, in the first
instance, the use of this carriage on turnpike roads; but that notion
seems to have been abandoned, and its use was only adopted on the
railroad before mentioned. On the occasion of its first trial, it drew
after it as many carriages as contained 10 tons of iron a distance of
nine miles; which stage it performed without any fresh supply of
water, and travelled at the rate of five miles an hour.
(84.) Capital and skill have of late years been directed with
extraordinary energy to the improvement of inland transport; and this
important instrument of national wealth and civilisation has received
a proportionate impulse. Effects are now witnessed, which, had they
been narrated a few years since, could only have been admitted into
the pages of fiction or volumes of romance. Who could have credited
the possibility of a ponderous engine of iron, loaded with several
hundred passengers, in a train of carriages of corresponding
magnitude, and a large quantity of water and coal, taking flight from
Manchester and arriving at Liverpool, a distance of about thirty
miles, in little more than an hour? And yet this is a matter of daily
and almost hourly occurrence. Neither is the road, on which this
wondrous performance is effected, the most favourable which could be
constructed for such machines. It is subject to undulations and
acclivities, which reduce the rate of speed much more than similar
inequalities affect the velocity on common roads. The rapidity of
transport thus attained is not less wonderful than the weights
transported. Its capabilities in this respect far transcend the
exigencies even of the two greatest commercial marts in Great Britain.
Loads, varying from 50 to 150 tons are transported at the average rate
of 15 miles an hour; but the engines in this case are loaded below
their power; and in one instance we have seen a load--we should rather
say a _cargo_--of wagons, conveying merchandise to the amount of 230
tons gross, transported from Liverpool to Manchester at the average
rate of 12 miles an hour.
The astonishment with which such performances must be viewed, might be
qualified, if the art of transport by steam on railways had been
matured, and had attained that full state of perfection which such an
art is always capable of receiving from long experience, aided by
great scientific knowledge, and the unbounded application of capital.
But such is not the present case. The art of constructing locomotive
engines, so far from having attained a state of maturity, has not even
emerged from its infancy. So complete was the ignorance of its powers
which prevailed, even among engineers, previous to the opening of the
Liverpool railway, that the transport of heavy goods was regarded as
the chief object of the undertaking, and its principal source of
revenue. The incredible speed of transport, effected even in the very
first experiments in 1830, burst upon the public, and on the
scientific world, with all the effect of a new and unlooked-for
phenomenon. On the unfortunate occasion which deprived this country
of Mr. Huskisson, the wounded body of that statesman was transported a
distance of about fifteen miles in twenty-five minutes, being at the
rate of thirty-six miles an hour. The revenue of the road arising from
passengers since its opening, has, contrary to all that was foreseen,
been nearly double that which has been derived from merchandise. So
great was the want of experience in the construction of engines, that
the company was at first ignorant whether they should adopt large
steam-engines fixed at different stations on the line, to pull the
carriages from station to station, or travelling engines to drag the
loads the entire distance. Having decided on the latter, they have,
even to the present moment, laboured under the disadvantage of the
want of that knowledge which experience alone can give. The engines
have been constantly varied in their weight and proportions, in their
magnitude and form, as the experience of each successive month has
indicated. As defects became manifest they were remedied; improvements
suggested were adopted; and each quarter produced engines of such
increased power and efficiency, that their predecessors were
abandoned, not because they were worn out, but because they had been
outstripped in the rapid march of improvement. Add to this, that only
one species of travelling engine has been effectively tried; the
capabilities of others remain still to be developed; and even that
form of engine which has received the advantage of a course of
experiments on so grand a scale to carry it towards perfection, is
far short of this point, and still has defects, many of which,
it is obvious, time and experience will remove. If then travelling
steam engines, with all the imperfections of an incipient
invention--with the want of experience, the great parent of
practical improvements--with the want of the common advantage of the
full application of the skill and capital of the country--subjected to
but one great experiment, and that experiment limited to one form of
engine; if, under such disadvantages, the effects to which we have
referred have been produced, what may we not expect from this
extraordinary power, when the enterprise of the country shall be
unfettered, when greater fields of experience are opened, when time,
ingenuity, and capital have removed the existing imperfections, and
have brought to light new and more powerful principles? This is not
mere speculation on possibilities, but refers to what is in a state of
actual progression. Railways are in progress between the points of
greatest intercourse in the United Kingdom, and travelling steam
engines are in preparation for the common turnpike roads; the
practicability and utility of that application of the steam engine
having not only been established by experiment to the satisfaction of
their projectors, but proved before the legislature in a committee of
inquiry on the subject.
The important commercial and political effects attending such
increased facility and speed in the transport of persons and goods,
are too obvious to require any very extended notice here. A part of
the price (and in many cases a considerable part) of every article of
necessity or luxury, consists of the cost of transporting it from the
producer to the consumer; and consequently every abatement or saving
in this cost must produce a corresponding reduction in the price of
every article transported; that is to say, of everything which is
necessary for the subsistence of the poor, or for the enjoyment of the
rich, of every comfort, and of every luxury of life. The benefit of
this will extend, not to the consumer only, but to the producer: by
lowering the expense of transport of the produce, whether of the soil
or of the loom, a less quantity of that produce will be spent in
bringing the remainder to market, and consequently a greater surplus
will reward the labour of the producer. The benefit of this will be
felt even more by the agriculturist than by the manufacturer; because
the proportional cost of transport of the produce of the soil is
greater than that of manufactures. If 200 quarters of corn be
necessary to raise 400, and 100 more be required to bring the 400 to
market, then the net surplus will be 100. But if by the use of steam
carriages the same quantity can be brought to market with an
expenditure of 50 quarters, then the net surplus will be increased
from 100 to 150 quarters; and either the profit of the farmer, or the
rent of the landlord, must be increased by the same amount.
But the agriculturist would not merely be benefited by an increased
return from the soil already under cultivation. Any reduction in the
cost of transporting the produce to market would call into cultivation
tracts of inferior fertility, the returns from which would not at
present repay the cost of cultivation and transport. Thus land would
become productive which is now waste, and an effect would be produced
equivalent to adding so much fertile soil to the present extent of the
country. It is well known, that land of a given degree of fertility
will yield increased produce by the increased application of capital
and labour. By a reduction in the cost of transport, a saving will be
made which may enable the agriculturist to apply to tracts already
under cultivation the capital thus saved, and thereby increase their
actual production. Not only, therefore, would such an effect be
attended with an increased extent of cultivated land, but also with an
increased degree of cultivation in that which is already productive.
It has been said, that in Great Britain there are above a million of
horses engaged in various ways in the transport of passengers and
goods, and that to support each horse requires as much land as would,
upon an average, support eight men. If this quantity of animal power
were displaced by steam engines, and the means of transport drawn from
the bowels of the earth, instead of being raised upon its surface,
then, supposing the above calculation correct, as much land would
become available for the support of human beings as would suffice for
an additional population of eight millions; or, what amounts to the
same, would increase the means of support of the present population by
about one-third of the present available means. The land which now
supports horses for transport would then support men, or produce corn
for food.
The objection that a quantity of land exists in the country capable of
supporting horses alone, and that such land would be thrown out of
cultivation, scarcely deserves notice here. The existence of any
considerable quantity of such land is extremely doubtful. What is the
soil which will feed a horse and not feed oxen or sheep, or produce
food for man? But even if it be admitted that there exists in the
country a small portion of such land, that portion cannot exceed, nor
indeed equal, what would be sufficient for the number of horses which
must after all continue to be employed for the purposes of pleasure,
and in a variety of cases where steam must necessarily be
inapplicable. It is to be remembered, also, that the displacing of
horses in one extensive occupation, by diminishing their price must
necessarily increase the demand for them in others.
The reduction in the cost of transport of manufactured articles, by
lowering their price in the market, will stimulate their consumption.
This observation applies of course not only to home but to foreign
markets. In the latter we already in many branches of manufacture
command a monopoly. The reduced price which we shall attain by
cheapness and facility of transport will still further extend and
increase our advantages. The necessary consequence will be, an
increased demand for manufacturing population; and this increased
population again reacting on the agricultural interests, will form an
increased market for that species of produce. So interwoven and
complicated are the fibres which form the texture of the highly
civilized and artificial community in which we live, that an effect
produced on any one point is instantly transmitted to the most remote
and apparently unconnected parts of the system.
The two advantages of increased cheapness and speed, besides extending
the amount of existing traffic, call into existence new objects of
commercial intercourse. For the same reason that the reduced cost of
transport, as we have shown, calls new soils into cultivation, it also
calls into existence new markets for manufactured and agricultural
produce. The great speed of transit which has been proved to be
practicable, must open a commerce between distant points in various
articles, the nature of which does not permit them to be preserved so
as to be fit for use beyond a certain time. Such are, for example,
many species of vegetable and animal food, which at present are
confined to markets at a very limited distance from the grower or
feeder. The truth of this observation is manifested by the effects
which have followed the intercourse by steam on the Irish Channel. The
western towns of England have become markets for a prodigious quantity
of Irish produce, which it had been previously impossible to export.
If animal food be transported alive from the grower to the consumer,
the distance of the market is limited by the power of the animal to
travel, and the cost of its support on the road. It is only particular
species of cattle which bear to be carried to market on common roads
and by horse carriages. But the peculiar nature of a railway, the
magnitude and weight of the loads which may be transported on it, and
the prodigious speed which may be attained, render the transport of
cattle, of every species, to almost any distance, both easy and cheap.
In process of time, when the railway system becomes extended, the
metropolis and populous towns will therefore become markets, not as at
present to districts within limited distances of them, but to the
whole country.
The moral and political consequences of so great a change in the
powers of transition of persons and intelligence from place to place
are not easily calculated. The concentration of mind and exertion
which a great metropolis always exhibits, will be extended in a
considerable degree to the whole realm. The same effect will be
produced as if all distances were lessened in the proportion in which
the speed and cheapness of transit are increased. Towns, at present
removed some stages from the metropolis, will become its suburbs;
others, now a day's journey, will be removed to its immediate
vicinity; business will be carried on with as much ease between them
and the metropolis, as it is now between distant points of the
metropolis itself. Let those who discard speculations like these as
wild and improbable, recur to the state of public opinion, at no very
remote period, on the subject of steam navigation. Within the memory
of persons who have not yet passed the meridian of life, the
possibility of traversing by the steam engine the channels and seas
that surround and intersect these islands, was regarded as the dream
of enthusiasts. Nautical men and men of science rejected such
speculations with equal incredulity, and with little less than scorn
for the understanding of those who could for a moment entertain them.
Yet we have witnessed steam engines traversing not these channels and
seas alone, but sweeping the face of the waters round every coast in
Europe. The seas which interpose between our Asiatic dominions and
Egypt, and those which separate our own shores from our West Indian
possessions, have offered an equally ineffectual barrier to its
powers. Nor have the terrors of the Pacific prevented the "Enterprise"
from doubling the Cape, and reaching the shores of India. If steam be
not used as the only means of connecting the most distant points of
our planet, it is not because it is inadequate to the accomplishment
of that end, but because the supply of the material from which at the
present moment it derives its powers, is restricted by local and
accidental circumstances.[24]
[Footnote 24: Some of the preceding observations on inland transport,
as well as other parts of the present chapter, appeared in articles
written by me in the Edinburgh Review for October, 1832, and October,
1834.]
We propose in the present chapter to lay before our readers some
account of the means whereby the effects above referred to have been
produced; of the manner and degree in which the public have availed
themselves of these means; and of the improvements of which they seem
to us to be susceptible.
(85.) It is a singular fact, that in the history of this invention
considerable time and great ingenuity were vainly expended in
attempting to overcome a difficulty, which in the end turned out to be
purely imaginary. To comprehend distinctly the manner in which a wheel
carriage is propelled by steam, suppose that a pin or handle is
attached to the spoke of the wheel at some distance from its centre,
and that a force is applied to this pin in such a manner as to make
the wheel revolve. If the face of the wheel and the surface of the
road were absolutely smooth and free from friction, so that the face
of the wheel would slide without resistance upon the road, then the
effect of the force thus applied would be merely to cause the wheel to
turn round, the carriage being stationary, the surface of the wheel
would slip or slide upon the road as the wheel is made to revolve. But
if, on the other hand, the pressure of the face of the wheel upon the
road is such as to produce between them such a degree of adhesion as
will render it impossible for the wheel to slide or slip upon the road
by the force which is applied to it, the consequence will be, that the
wheel can only turn round in obedience to the force which moves it by
causing the carriage to advance, so that the wheel will roll upon the
road, and the carriage will be moved forward, through a distance equal
to the circumference of the wheel, each time it performs a complete
revolution.
It is obvious that both of these effects may be partially produced;
the adhesion of the wheel to the road may be insufficient to prevent
slipping altogether, and yet it may be sufficient to prevent the wheel
from slipping as fast as it revolves. Under such circumstances the
carriage would advance and the wheel would slip. The progressive
motion of the carriage during one complete revolution of the wheel
would be equal to the difference between the complete circumference
of the wheel and the portion through which in one revolution it has
slipped.
When the construction of travelling steam engines first engaged the
attention of engineers, and for a considerable period afterwards, a
notion was impressed upon their minds that the adhesion between the
face of the wheel and the surface of the road must necessarily be of
very small amount, and that in every practical case the wheels thus
driven would either slip altogether, and produce no advance of the
carriage, or that a considerable portion of the impelling power would
be lost by the partial slipping or sliding of the wheels. It is
singular that it should never have occurred to the many ingenious
persons who for several years were engaged in such experiments and
speculations, to ascertain by experiment the actual amount of adhesion
in any particular case between the wheels and the road. Had they done
so, we should probably now have found locomotive engines in a more
advanced state than that to which they have attained.
To remedy this imaginary difficulty, Messrs. _Trevithick_ and _Vivian_
proposed to make the external rims of the wheels rough and uneven, by
surrounding them with projecting heads of nails or bolts, or by
cutting transverse grooves on them. They proposed, in cases where
considerable elevations were to be ascended, to cause claws or nails
to project from the surface during the ascent, so as to take hold of
the road.
In seven years after the construction of the first locomotive engine
by these engineers, another locomotive engine was constructed by Mr.
_Blinkensop_, of Middleton Colliery, near Leeds. He obtained a patent,
in 1811, for the application of a rack-rail. The railroad thus,
instead of being composed of smooth bars of iron, presented a line of
projecting teeth, like those of a cog-wheel, which stretched along the
entire distance to be travelled. The wheels on which the engine
rolled were furnished with corresponding teeth, which worked in the
teeth of the railroad; and, in this way, produced a progressive motion
in the carriage.
The next contrivance for overcoming this fictitious difficulty, was
that of Messrs. _Chapman_, who, in the year 1812, obtained a patent
for working a locomotive engine by a chain extending along the middle
of the line of railroad, from the one end to the other. This chain was
passed once round a grooved wheel under the centre of the carriage; so
that, when this grooved wheel was turned by the engine, the chain
being incapable of slipping upon it, the carriage was consequently
advanced on the road. In order to prevent the strain from acting on
the whole length of the chain, its links were made to fall upon
upright forks placed at certain intervals, which between those
intervals sustained the tension of the chain produced by the engine.
Friction-rollers were used to press the chain into the groove of the
wheel, so as to prevent it from slipping. This contrivance was soon
abandoned, for the very obvious reason that a prodigious loss of force
was incurred by the friction of the chain.
The following year, 1813, produced a contrivance, of singular
ingenuity, for overcoming the supposed difficulty arising from the
want of adhesion between the wheels and the road. This was no other
than a pair of mechanical legs and feet, which were made to walk and
propel in a manner somewhat resembling the feet of an animal.
[Illustration: _Fig. 53._]
A sketch of these propellers is given in fig. 53. A is the carriage
moving on the railroad, L and L´ are the legs, F and F´ the feet. The
foot F has a joint at O, which corresponds to the ankle; another joint
is placed at K, which corresponds to the knee; and a third is placed
at L, which corresponds to the hip. Similar joints are placed at the
corresponding letters in the other leg. The knee-joint K is attached
to the end of the piston of the cylinder. When the piston, which is
horizontal, is pressed outwards, the leg L presses the foot F
against the ground, and the resistance forces the carriage A onwards.
As the carriage proceeds, the angle K at the knee becomes larger, so
that the leg and thigh take a straighter position; and this continues
until the piston has reached the end of its stroke. At the hip L there
is a short lever, L M, the extremity of which is connected by a cord
or chain with a point, S, placed near the shin of the leg. When the
piston is pressed into the cylinder, the knee, K, is drawn towards the
engine, and the cord, M S, is made to lift the foot, F, from the
ground; to which it does not return until the piston has arrived at
the extremity of the cylinder. On the piston being again driven out of
the cylinder, the foot, F, being placed on the road, is pressed
backwards by the force of the piston-rod at K; but the friction of the
ground preventing its backward motion, the re-action causes the engine
to advance: and in the same manner this process is continued.
Attached to the thigh, at N, above the knee, by a joint, is a
horizontal rod, N R, which works a rack, R. This rack has beneath it a
cog-wheel. This cog-wheel acts in another rack below it. By these
means, when the knee K is driven _from_ the engine, the rack R is
moved _backwards_; but the cog-wheel, acting on the other rack beneath
it, will move the latter _in the contrary direction_. The rack R being
then moved _in the same direction with the knee_, K, it follows that
the other rack will always be moved _in a contrary direction_. The
lower rack is connected by another horizontal rod with the thigh of
the leg, L´ F´, immediately above the knee, at N´. When the piston is
forced _inwards_, the knee, K´, will thus be forced _backwards_; and
when the piston is forced _outwards_, the knee, K´, will be drawn
_forwards_. It therefore follows that the two knees, K and K´, are
pressed alternately backwards and forwards. The foot, F´, when the
knee, K´, is drawn forward, is lifted by the means already described
for the foot, F.
It will be apparent, from this description, that the piece of
mechanism here exhibited is a contrivance derived from the motion of
the legs of an animal, and resembling in all respects the fore legs of
a horse. It is however to be regarded rather as a specimen of great
ingenuity than as a contrivance of practical utility.
(86.) It was about this period that the important fact was first
ascertained that the adhesion or friction of the wheels with the rails
on which they moved was amply sufficient to propel the engine, even
when dragging after it a load of great weight; and that in such case,
the progressive motion would be effected without any slipping of the
wheels. The consequence of this fact rendered totally useless all the
contrivances for giving wheels a purchase on the road, such as racks,
chains, feet, &c. The experiment by which this was determined appears
to have been first tried on the Wylam railroad; where it was proved,
that, when the road was level, and the rails clean, the adhesion of
the wheels was sufficient, in all kind of weather, to propel
considerable loads. By manual labour it was first ascertained how much
weight the wheels of a common carriage would overcome without slipping
round on the rail; and having found the proportion which that bore to
the weight, they then ascertained that the weight of the engine would
produce sufficient adhesion to drag after it, on the railroad, the
requisite number of wagons.[25]
[Footnote 25: Wood on Railroads, 2d edit.]
In 1814, an engine was constructed at Killingworth, by Mr.
_Stephenson_, having two cylinders with a cylindrical boiler, and
working two pair of wheels, by cranks placed at right angles; so that
when the one was in full operation, the other was at its dead points.
By these means the propelling power was always in action. The cranks
were maintained in this position by an endless chain, which passed
round two cogged wheels placed under the engine, and which were fixed
on the same axles on which the wheels were placed. The wheels in this
case were fixed on the axles, and turned with them.
[Illustration: _Fig. 54._]
This engine is represented in fig. 54., the sides being open, to
render the interior mechanism visible. A B is the cylindrical boiler;
C C are the working cylinders; D E are the cogged wheels fixed on the
axle of the wheels of the engine, and surrounded by the endless chain.
These wheels being equal in magnitude, perform their revolutions in
the same time; so that, when the crank, F, descends to the lowest
point, the crank, G, rises from the lowest point to the horizontal
position, D; and, again, when the crank, F, rises from the lowest
point to the horizontal position, E, the other crank rises to the
highest point; and so on. A very beautiful contrivance was adopted in
this engine, by which it was suspended on springs of steam. Small
cylinders, represented at H, are screwed by flanges to one side of the
boiler, and project within it a few inches; they have free
communication at the top with the water or steam of the boiler. Solid
pistons are represented at I, which move steam-tight in these
cylinders; the cylinders are open at the bottom, and the piston-rods
are screwed on the carriage of the engine, over the axle of each pair
of wheels, the pistons being presented upwards. As the engine is
represented in the figure, it is supported on four pistons, two at
each side. The pistons are pressed upon by the water or steam which
occupies the upper chamber of the cylinder; and the latter being
elastic in a high degree, the engine has all the advantage of spring
suspension. The defect of this method of supporting the engine is,
that when the steam loses that amount of elasticity necessary for the
support of the machine, the pistons are forced into the cylinders, and
the bottoms of the cylinders bear upon them. All spring suspension is
then lost. This mode of suspension has consequently since been laid
aside.
In an engine subsequently constructed by Mr. _Stephenson_, for the
Killingworth railroad, the mode adopted of connecting the wheels by an
endless chain and cog-wheels was abandoned; and the same effect was
produced by connecting the two cranks by a straight rod. All such
contrivances, however, have this great defect, that, if the fore and
hind wheels be not constructed with dimensions accurately equal, there
must necessarily be a slipping or dragging on the road. The nature of
the machinery requires that each wheel should perform its revolution
exactly in the same time; and consequently, in doing so, must pass
over exactly equal lengths of the road. If, therefore, the
circumference of the wheels be not accurately equal, that wheel which
has the lesser circumference must be dragged along so much of the road
as that by which it falls short of the circumference of the greater
wheel; or, on the other hand, the greater must be dragged in the
opposite direction, to compensate for the same difference. As no
mechanism can accomplish a perfect equality in four, much less in six,
wheels, it may be assumed that a great portion of that dragging
effect is a necessary consequence of the principle of this machine;
and even were the wheels, in the first instance, accurately
constructed, it is not possible that their wear could be so exactly
uniform as to continue equal.
(87.) The next stimulus which the progress of this invention received,
proceeded from the great national work undertaken at Liverpool, by
which that town and the extensive commercial mart of Manchester were
connected by a double line of railway. When this project was
undertaken, it was not decided what moving power it might be most
expedient to adopt as a means of transport on the proposed road: the
choice lay between horse-power, fixed steam engines, and locomotive
engines; but the first, for many obvious reasons, was at once rejected
in favour of one or other of the last two.
The steam engine may be applied, by two distinct methods, to move
wagons either on a turnpike road or on a railway. By the one method
the steam engine is fixed, and draws the carriage or train of
carriages towards it by a chain extending the whole length of road on
which the engine works. By this method the line of road over which the
transport is conducted is divided into a number of short intervals, at
the extremity of each of which an engine is placed. The wagons or
carriages, when drawn by any engine to its own station, or detached,
and connected with the extremity of the chain worked by the next
stationary engine; and thus the journey is performed, from station to
station, by separate engines. By the other method the same engine
draws the load the whole journey, travelling with it.
The Directors of the Liverpool and Manchester railroad, when that work
was advanced towards its completion, employed, in the spring of the
year 1829, Messrs. _Stephenson_ and _Lock_ and Messrs. _Walker_ and
_Rastrick_, experienced engineers, to visit the different railways
where practical information respecting the comparative effects of
stationary and locomotive engines was likely to be obtained; and from
these gentlemen they received reports on the relative merits,
according to their judgment, of the two methods. The particulars of
their calculations are given at large in the valuable work of Mr.
_Nicholas Wood_ on railways; to which we refer the reader, not only on
this, but on many other subjects connected with the locomotive steam
engine into which it would be foreign to our subject to enter. The
result of the comparison of the two systems was, that the capital
necessary to be advanced to establish a line of stationary engines was
considerably greater than that which was necessary to establish an
equivalent power in locomotive engines; that the annual expense by the
stationary engines was likewise greater; and that, consequently, the
expense of transport by the latter was greater, in a like proportion.
The subjoined table exhibits the results numerically:--
+------------------------+------------+-------------+------------------+
| | | Annual | Expense of |
| | Capital. | Expense. | taking a Ton of |
| | | | Goods One Mile. |
+------------------------+------------+-------------+------------------+
| | £ s. d.| £ s. d.| |
| Locomotive Engines | 58,000 0 0 | 25,517 8 2 | 0·164 of a penny.|
| Stationary Engines |121,496 7 0 | 42,031 16 5 | 0·269 |
| +------------+-------------+------------------+
| Locomotive System--less| 63,496 7 0 | 16,514 8 3 | 0·105 |
+------------------------+------------+-------------+------------------+
On the score of economy, therefore, the system of locomotive engines
was entitled to a preference; but there were other considerations
which conspired with this to decide the choice of the Directors in its
favour. An accident occurring in any part of a road worked by
stationary engines must necessarily produce a total suspension of work
along the entire line. The most vigilant and active attention on the
part of every workman, however employed, in every part of the line,
would therefore be necessary; but, independently of this, accidents
arising from the fracture or derangement of any of the chains, or from
the suspension of the working of any of the fixed engines, would be
equally injurious, and would effectually stop the intercourse along
the line. On the other hand, in locomotive engines an accident could
only affect the particular train of carriages drawn by the engine to
which the accident might occur; and even then the difficulty could be
remedied by having a supply of spare engines at convenient stations
along the line. It is true that the _probability_ of accident is,
perhaps, less in the stationary than in the locomotive system; but the
_injurious consequences_, when accident _does_ happen, are
prodigiously greater in the former. "The one system," says Mr.
_Walker_, "is like a chain extending from Liverpool to Manchester, the
failure of a single link of which would destroy the whole; while the
other is like a number of short and unconnected chains," the
destruction of any one of which does not interfere with the effect of
the others, and the loss of which may be supplied with facility.
The decision of the Directors was, therefore, in favour of locomotive
engines; and their next measure was to devise some means by which the
inventive genius of the country might be stimulated to supply them
with the best possible form of engines for this purpose. With this
view, it was proposed and carried into effect to offer a prize for the
best locomotive engine which might be produced under certain proposed
conditions, and to appoint a time for a public trial of the claims of
the candidates. A premium of 500_l._ was accordingly offered for the
best locomotive engine to run on the Liverpool and Manchester Railway;
under the condition that it should produce no smoke; that the pressure
of the steam should be limited to 50 lbs. on the inch; and that it
should draw at least three times its own weight, at the rate of not
less than ten miles an hour; that the engine should be supported on
springs, and should not exceed fifteen feet in height. Precautions
were also proposed against the consequences of the boiler bursting;
and other matters, not necessary to mention more particularly here.
This proposal was announced in the spring of 1829, and the time of
trial was appointed in the following October. The engines which
underwent the trial were, the Rocket, constructed by Mr. _Stephenson_;
the Sanspariel, by _Hackworth_; and the Novelty, by Messrs.
_Braithwait_ and _Ericson_. Of these, the Rocket obtained the premium.
A line of railway was selected for the trial, on a level piece of road
about two miles in length, near a place called Rainhill, between
Liverpool and Manchester; the distance between the two stations was a
mile and a half, and the engine had to travel this distance backwards
and forwards ten times, which made altogether a journey of 30 miles.
The Rocket performed this journey twice: the first time in 2 hours 14
minutes and 8 seconds; and the second time, in 2 hours 6 minutes and
49 seconds. Its speed at different parts of the journey varied: its
greatest rate of motion was rather above 29 miles an hour; and its
least, about 11-1/2 miles an hour. The average rate of the one journey
was 13-4/10 miles an hour; and of the other, 14-2/20 miles. This was
the only engine which performed the complete journey proposed, the
others having been stopped from accidents which occurred to them in
the experiment. The Sanspariel performed the distance between the
stations eight times, travelling 22-1/2 miles in 1 hour 37 minutes and
16 seconds. The greatest velocity to which this engine attained was
something less than 23 miles per hour. The Novelty had only passed
twice between the stations when the joints of the boiler gave way, and
put an end to the experiment.
(88.) The great object to be attained in the construction of these
engines was, to combine with sufficient lightness the greatest
possible heating power. The fire necessarily acts on the water in two
ways: first, by its radiant heat; and, second, by the current of
heated air which is carried by the draft through the fire, and finally
passes into the chimney. To accomplish this object, therefore, it is
necessary to expose to both these sources of heat the greatest
possible quantity of surface in contact with the water. These ends
were attained by the following admirable arrangement in the Rocket:--
[Illustration: _Fig. 55._]
[Illustration: _Fig. 56._]
This engine is represented in fig. 55. It is supported on four wheels;
the principal part of the weight being thrown on one pair, which are
worked by the engine. The boiler consists of a cylinder 6 feet in
length, with flat ends; the chimney issues from one end, and to the
other end is attached a square box, B, the bottom of which is
furnished with the grate on which the fuel is placed. This box is
composed of two casings of iron, one contained within the other,
having between them a space about 3 inches in breadth; the magnitude
of the box being 3 feet in length, 2 feet in width, and 3 feet in
depth. The casing which surrounds the box communicates with the lower
part of the boiler by a pipe marked C; and the same casing at the top
of the box communicates with the upper part of the boiler by another
pipe marked D. When water is admitted into the boiler, therefore, it
flows freely through the pipe C, into the casing which surrounds the
furnace or fire-box, and fills this casing to the same level as that
which it has in the boiler. When the engine is at work, the boiler is
kept about half filled with water; and, consequently, the casing
surrounding the furnace is completely filled. The steam which is
generated in the water contained in the casing finds its exit through
the pipe D, and escapes into the upper part of the boiler. A section
of the engine, taken at right angles to its length is represented at
fig. 56. Through the lower part of the boiler pass a number of copper
tubes of small size, which communicate at one end with the fire-box,
and at the other with the chimney, and form a passage for the heated
air from the furnace to the chimney. The ignited fuel spread on the
grate at the bottom of the fire-box disperses its heat by radiation,
and acts in this manner on the whole surface of the casing surrounding
the fire-box; and thus raises the temperature of the thin shell of
water contained in that casing. The chief part of the water in the
casing, being lower in its position than the water in the boiler,
acquires a tendency to ascend when heated, and passes into the boiler;
so that a constant circulation of the heated water is maintained, and
the water in the boiler must necessarily be kept at nearly the same
temperature as the water in the casing. The air which passes through
the burning fuel, and which fills the fire-box, is carried by the
draft through the tubes which extend through the lower part of the
boiler; and as these tubes are surrounded on every side with the water
contained in the boiler, this air transmits its heat through these
tubes to the water. It finally issues into the chimney, and rises by
the draft. The power of this furnace must necessarily depend on the
power of draft in the chimney; and to increase this, and at the same
time to dispose of the waste steam after it has worked the piston,
this steam is carried off by a pipe L, which passes from the cylinder
to the chimney, and escapes there in a jet which is turned upwards. By
the velocity with which it issues from this jet, and by its great
comparative levity, it produces a strong current upwards in the
chimney, and thus gives force to the draft of the furnace. In fig. 56.
the grate-bars are represented at the bottom of the fire-box at F.
There are two cylinders, one of which works each wheel; one only
appearing in the drawing, fig. 55., the other being concealed by the
engine. The spokes which these cylinders work are placed at right
angles on the wheels; the wheels being fixed on a common axle, with
which they turn.
In this engine, the surface of water surrounding the fire-box, exposed
to the action of radiant heat, amounted to 20 square feet, which
received heat from the surface of 6 square feet of burning fuel on the
bars. The surface exposed to the action of the heated air amounted to
118 square feet. The engine drew after it another carriage, containing
fuel and water; the fuel used was coke, for the purpose of avoiding
the production of smoke.
(89.) The Sanspareil of Mr. _Hackworth_ is represented in fig. 57.;
the horizontal section being exhibited in fig. 58.
[Illustration: _Fig. 57._]
[Illustration: _Fig. 58._]
The draft of the furnace is produced in the same manner as in the
Rocket, by ejecting the waste steam coming from the cylinder into the
chimney; the boiler, however, differs considerably from that of the
Rocket. A recurved tube passes through the boiler, somewhat similar to
that already described in the early engine of Messrs. _Trevithick_ and
_Vivian_. In the horizontal section (fig. 58.), D expresses the
opening of the furnace at the end of the boiler, beside the chimney.
The grate-bars appear at A, supporting the burning fuel; and a curved
tube passing through the boiler, and terminating in the chimney, is
expressed at B, the direction of the draft being indicated by the
arrow; C is a section of the chimney. The cylinders are placed, as in
the Rocket, on each side of the boiler; each working a separate
wheel, but acting on spokes placed at right angles to each other. The
tube in which the grate and flue are placed diminishes in diameter as
it approaches the chimney. At the mouth where the grate was placed,
its diameter was 2 feet; and it was gradually reduced, so that, at the
chimney, its diameter was only 15 inches. The grate-bars extended 5
feet into the tube. The surface of water exposed to the radiant heat
of the fire was 16 square feet; and that exposed to the action of the
heated air and flame was about 75 square feet. The magnitude of the
grate or sheet of burning fuel which radiated heat, was 10 square
feet.
(90.) The Novelty, of Messrs. _Braithwait_ and _Ericson_, is
represented in fig. 59.; and a section of the generator and boiler is
exhibited in fig. 60.: the corresponding parts in the two figures are
marked by the same letters.
[Illustration: _Fig. 59._]
A is the generator or receiver, containing the steam which works the
engine; this communicates with a lower generator, B, which extends in
a horizontal direction the entire length of the carriage. Within the
generator, A, is contained the furnace, F, which communicates in a
tube, C; carried up through the generator, and terminated at the top
by sliding shutters, which exclude the air, and which are only opened
to supply fuel to the grate, F. Below the grate the furnace is not
open, as usual, to the atmosphere, but communicates by a tube, E, with
a bellows, D; which is worked by the engine, and which forces a
constant stream of air, by the tube E, through the fuel on F, so as to
keep that fuel in vivid combustion. The heated air, contained in the
furnace, F, is driven on, by the same force, through a small curved
tube marked _e_, which circulates like a worm (as represented in fig.
60.) through the horizontal generator or receiver; and, tapering
gradually, until reduced to very small dimensions, it finally issues
into the chimney, G. The air, in passing along this tube, imparts its
heat to the water by which the tube is surrounded, and is brought to a
considerably reduced temperature when discharged into the chimney. The
cylinder, which is represented at K, works one pair of wheels, by
means of a bell-crank; the other pair, when necessary, being connected
with them.
[Illustration: _Fig. 60._]
In this engine, the magnitude of the surface of burning fuel on the
grate-bars is less than 2 square feet; the surface exposed to radiant
heat is 9-1/2 square feet; and the surface of water exposed to heated
air is about 33 square feet.
The superiority of the Rocket may be attributed chiefly to the greater
quantity of surface of the water which is exposed to the action of the
fire. With a less extent of grate-bars than the Sanspareil, in the
proportion of 3 to 5, it exposes a greater surface of water to radiant
heat, in the proportion of 4 to 3; and a greater surface of water to
heated air, in the proportion of more than 3 to 2. It was found that
the Rocket, compared with the Sanspareil, consumed fuel, in the
evaporation of a given quantity of water, in the proportion of 11 to
28. The suggestion of using the tubes to conduct through the water the
heated air to the chimney is due to Mr. _Booth_, treasurer of the
Liverpool and Manchester Railway Company; and, certainly, nothing has
been more conducive to the efficiency of the engines since used than
this improvement. It is much to be regretted that the ingenious
gentleman who suggested this has reaped none of the advantages to
which a patentee would be legally entitled.[26]
[Footnote 26: Mr. _Booth_ received a part of the premium of 500_l._,
but has not participated in any degree in the profits of the
manufacture of the engines.]
(91.) The great object to be effected in the boilers of these engines
is, to keep a small quantity of water at an excessive temperature, by
means of a small quantity of fuel kept in the most active state of
combustion. To accomplish this, it is necessary, first, so to shape
the boiler, furnace, and flues, that the water shall be in contact
with as extensive a surface as possible, every part of which is acted
on either immediately, by the heat radiating from the fire, or
mediately, by the air which has passed through the fire, and which
finally rushes into the chimney: and, secondly, that such a forcible
draught should be maintained in the furnace, that a quantity of heat
shall be extricated from the fuel, by combustion, sufficient to
maintain the water at the necessary temperature, and to produce the
steam with sufficient rapidity. To accomplish these objects,
therefore, the chamber containing the grate should be completely
surrounded by water, and should be below the level of the water in the
boiler. The magnitude of the surface exposed to radiation should be as
great as is consistent with the whole magnitude of the machine. The
comparative advantage which the Rocket possessed in these respects
over the other engines will be evident on inspection. In the next
place, it is necessary that the heat, which is absorbed by the air
passing through the fuel, and keeping it in a state of combustion,
should be transferred to the water before the air escapes into the
chimney. Air being a bad conductor of heat, to accomplish this it is
necessary that the air in the flues should be exposed to as great an
extent of surface in contact with the water as possible. No
contrivance can be less adapted for the attainment of this end than
one or two large tubes traversing the boiler, as in the earliest
locomotive engines: the body of air which passes through the centre of
these tubes had no contact with their surface, and, consequently,
passed into the chimney at nearly the same temperature as that which
it had when it quitted the fire. The only portion of air which
imparted its heat to the water was that portion which passed next to
the surface of the tube.
Several methods suggest themselves to increase the surface of water in
contact with a given quantity of air passing through it. This would be
accomplished by causing the air to pass between plates placed near
each other, so as to divide the current into thin strata, having
between them strata of water, or it might be made to pass between
tubes differing slightly in diameter, the water passing through an
inner tube, and being also in contact with the external surface of the
outer tube. Such a method would be similar in principle to the
steam-jacket used in _Watt's_ steam engines, or to the condenser of
_Cartwright's_ engine already described. But, considering the facility
of constructing small tubes, and of placing them in the boiler, that
method, perhaps, is, on the whole, the best in practice; although the
shape of a tube, geometrically considered, is most unfavourable for
the exposure of a fluid contained in it to its surface. The air which
passes from the fire-chamber, being subdivided as it passes through
the boiler by a great number of very small tubes, may be made to
impart all its excess of heat to the water before it issues into the
chimney. This is all which the most refined contrivance can effect.
The Rocket engine was traversed by 25 tubes, each 3 inches in
diameter; and the principle has since been carried to a much greater
extent.
The abstraction of a great quantity of heat from the air before it
reaches the chimney is attended with one consequence, which, at first
view, would present a difficulty apparently insurmountable; the
chimney would, in fact, lose its power of draught. This difficulty,
however, was removed by using the waste steam, which had passed from
the cylinder after working the engine, for the purpose of producing a
draught. This steam was urged through a jet presented upwards in the
chimney, and driven out with such force in that direction as to create
a sufficient draught to work the furnace.
It will be observed that the principle of draught in the Novelty is
totally distinct from this: in that engine the draught is produced by
a bellows worked by the engine. The question, as far as relates to
these two methods, is, whether more power is lost in supplying the
steam through the jet, as in the Rocket, or in working the bellows, as
in the Novelty. The force requisite to impel the steam through the jet
must be exerted by the returning stroke of the piston, and,
consequently, must rob the working effect to an equivalent amount. On
the other hand, the power requisite to work the bellows in the Novelty
must be subducted from the available power of the engine. The former
method is found to be the more effectual and economical.
The importance of these details will be understood, when it is
considered that the only limit to the attainment of speed by
locomotive engines is the power to produce in a given time a certain
quantity of steam. Each stroke of the piston causes one revolution of
the wheels, and consumes two cylinders full of steam: consequently, a
cylinder of steam corresponds to a certain number of feet of road
travelled over: hence it is that the production of a rapid and
abundant supply of heat, and the imparting of that heat quickly and
effectually to the water, is the key to the solution of the problem to
construct an engine capable of rapid motion.
The method of subdividing the flue into tubes was carried much further
by Mr. _Stephenson_ after the construction of the Rocket; and, indeed,
the principle was so very obvious, that it is only surprising that, in
the first instance, tubes of smaller diameter than 3 inches were not
used. In engines since constructed, the number of tubes vary from 90
to 120, the diameter being reduced to 2 inches or less, and in some
instances tubes have been introduced, even to the number of 150, of
1-1/2 inch diameter. In the Meteor, 20 square feet are exposed to
radiation, and 139 to the contact of heated air; in the Arrow, 20
square feet to radiation, and 145 to the contact of heated air. The
superior economy of fuel gained by this means will be apparent by
inspecting the following table, which exhibits the consumption of fuel
which was requisite to convey a ton weight a mile in each of four
engines, expressing also the rate of the motion:--
+------------------+---------------+-----------------+
| |Average rate of|Consumption of |
| Engines. | speed in miles|Coke in pounds |
| | per hour. |per ton per mile.|
+------------------+---------------+-----------------+
|No. 1. Rocket | 14 | 2·41 |
| 2. Sanspareil | 15 | 2·47 |
| 3. Phoenix | 12 | 1·42 |
| 4. Arrow | 12 | 1·25 |
+------------------+---------------+-----------------+
(92.) Since the period at which the railway was opened for the actual
purposes of transport, the locomotive engines have been in a state of
progressive improvement. Scarcely a month has passed without
suggesting some change in the details, by which fuel might be
economised, the production of steam rendered more rapid, the wear of
the engine rendered slower, the proportionate strength of the
different parts improved, or some other desirable end obtained. The
consequence of this has been, that the particular engines to which we
have alluded, and others of the same class, without having, as it
were, lived their natural life, or without having been worn out by
work, have been laid aside to give place to others of improved powers.
By the exposure of the cylinders to the atmosphere in the Rocket, and
engines of a similar form, a great waste of heat was incurred, and it
was accordingly determined to remove them from the exterior of the
boiler, and to place them within a casing immediately under the
chimney: this chamber was necessarily kept warm by its proximity to
the end of the boiler, but more by the current of heated air which
constantly rushed into it from the tubes. This change, also, rendered
necessary another, which improved the working of the engine. In the
earlier engines the motion of the piston was communicated to the wheel
by a connecting rod attached to one of the spokes on the exterior of
the wheel, as represented in fig. 55. By the change to which we have
just alluded, the cylinders being placed between the wheels under the
chimney, this mode of working became inapplicable, and it was
considered better to connect the piston-rods with two cranks placed at
right angles on the axles of the great wheels. By this means, it was
found that the working of the machine was more even, and productive of
less strain than in the former arrangement. On the other hand, a
serious disadvantage was incurred by the adoption of a cranked axle.
The weakness necessarily arising from such a form of axle could only
be counterbalanced by great thickness and weight of metal; and even
this precaution does not prevent the occasional fracture of such axles
at the angles of the cranks. The advantages, however, of this plan, on
the whole, are considered to predominate.
In the most improved engines in present use two safety-valves are
provided, of which only one is in the power of the engine-man. The
tubes being smaller and more numerous than in the earlier engines, the
heat is more completely extracted from the air before it enters the
chimney. A powerful draft is rendered still more necessary by the
smallness of the tubes: this is effected by forcing the steam which
has worked the pistons through a contracted orifice, presented upwards
in the chimney, by the regulation of which any degree of draft may be
obtained.
One of the most improved engines at present in use is represented in
fig. 61.
A represents the cylindrical boiler, the lower half of which is
traversed by tubes, as described in the Rocket. They are usually from
80 to 100 in number, and about 1-1/2 inch in diameter; the boiler is
about 7 feet in length; the fire-chamber is attached to one end of it,
at F, as in the Rocket, and similar in construction; the cylinders are
inserted in a chamber at the other end, immediately under the chimney.
The piston-rods are supported in the horizontal position by guides;
and connecting rods extend from them, under the engine, to the two
cranks placed on the axle of the large wheels. The effects of any
inequality in the road are counteracted by springs, on which the
engine rests; the springs being below the axle of the great wheels,
and above that of the less. The steam is supplied to the cylinders,
and withdrawn, by means of the common sliding valves, which are worked
by an eccentric wheel placed on the axle of the large wheels of the
carriage. The motion is communicated from this eccentric wheel to the
valve by sliding rods. The stand is placed for the attendant at the
end of the engine, next the fire-place, F; and two levers, L, project
from the end, which communicate with the valves by means of rods, by
which the engine is governed, so as to stop or reverse the motion.
[Illustration: _Fig. 61._]
The wheels of these engines have been commonly constructed of wood,
with strong iron tires, furnished with flanges adapted to the rails.
But Mr. _Stephenson_ has recently substituted, in some instances,
wheels of iron with hollow spokes. The engine draws after it a tender
carriage containing the fuel and water; and, when carrying a light
load, is capable of performing the whole journey from Liverpool to
Manchester without a fresh supply of water. When a heavy load of
merchandise is drawn, it is usual to take in water at the middle of
the trip.
(93.) In reviewing all that has been stated, it will be perceived that
the efficiency of the locomotive engines used on this railway is
mainly owing to three circumstances: 1st, The unlimited power of draft
in the furnace, by projecting the waste steam into the chimney; 2d,
The unlimited abstraction of heat from the air passing from the
furnace, by Mr. _Booth's_ ingenious arrangement of tubes traversing
the boiler; and, 3d, Keeping the cylinders warm, by immersing them in
the chamber under the chimney.[27] There are many minor details which
might be noticed with approbation, but these constitute the main
features of the improvements, and should never, for a moment, be lost
sight of by projectors of locomotive engines.
[Footnote 27: Mr. Robert Stephenson, whose experience and skill in the
construction of locomotives attaches great importance to this
condition. It has lately, however, been abandoned by some other
engine-makers, for the purpose of getting rid of the cranked axle
which must accompany it.]
The successive introduction of improvements in the engines, some of
which we have mentioned, has been accompanied by corresponding
accessions to their practical power, and to the economy of fuel; and
they have now arrived at a point which is as far beyond the former
expectations of the most sanguine locomotive projectors, as it
assuredly is short of the perfection of which these wonderful machines
are still susceptible.
In the spring of the year 1832, I made several experiments on the
Manchester railway, with a view to determine, in the actual state of
the locomotive engines at that time, their powers with respect to the
amount of load and the economy of fuel. Since that time I am not aware
that, in these respects, the engine has received any material
improvement. The following are the particulars of three experiments
thus made:--
I.
On Saturday, the 5th of May, the engine called the "Victory" took 20
wagons of merchandise, weighing gross 92 tons 19 cwt. 1 qr., together
with the tender containing fuel and water, of the weight of which I
have no account, from Liverpool to Manchester (30 miles,) in 1 h. 34
min. 45 sec. The train stopped to take in water half-way, for 10
minutes, not included in the above mentioned time. On the inclined
plane rising 1 in 96, and extending 1-1/2 mile, the engine was
assisted by another engine called the "Samson," and the ascent was
performed in 9 minutes. At starting, the fire-place was well filled
with coke, and the coke supplied to the tender accurately weighed. On
arriving at Manchester the fire-place was again filled, and the coke
remaining in the tender weighed. The consumption was found to amount
to 929 pounds net weight, being at the rate of one third of a pound
per ton per mile.
Speed on the level was 18 miles an hour; on a fall of 4 feet in a
mile, 21-1/2 miles an hour; fall of 6 feet in a mile, 25-1/2 miles an
hour; on the rise over Chatmoss, 8 feet in a mile 17-5/8 miles an
hour; on level ground sheltered from the wind, 20 miles an hour. The
wind was moderate, but direct ahead. The working wheels slipped three
times on Chatmoss, and the train was retarded from 2 to 3 minutes.
The engine, on this occasion, was not examined before or after the
journey, but was presumed to be in good working order.
II.
On Tuesday, the 8th of May, the same engine performed the same
journey, with 20 wagons, weighing gross 90 tons 7 cwt. 2 qrs.,
exclusive of the unascertained weight of the tender. The time of the
journey was 1 h. 41 min. The consumption of coke 1040 lbs. net weight,
estimated as before. Rate of speed:--
Level 17-5/8 miles per hour.
Fall of 4 feet in a mile 22
---- 6 22-1/2
Rise of 8 15
On this occasion there was a high wind ahead on the quarter, and the
connecting rod worked hot, owing to having been keyed too tight. On
arriving at Manchester, I caused the cylinders to be opened, and found
that the pistons were so loose, that the steam blew through the
cylinders with great violence. By this cause, therefore, the machine
was robbed of a part of its power during the journey; and this
circumstance may explain the slight decrease in speed, and increase in
the consumption of fuel, with a lighter load in this journey compared
with that performed on the 5th of May.
The Victory weighs 8 tons 2 cwt., of which 5 tons 4 cwt. rest on the
drawing wheels. The cylinders are 11 inches diameter, and 16 inches
stroke; and the diameter of the drawing wheels is 5 feet.
III.
On the 29th of May, the engine called the "Samson," (weighing 10 tons
2 cwt., with 14-inch cylinders, and 16-inch stroke; wheels 4 feet 6
inches diameter, both pairs being worked by the engine; steam 50 lbs.
pressure, 130 tubes) was attached to 50 wagons, laden with
merchandise; net weight about 150 tons; gross weight, including
wagons, tender, &c., 223 tons 6 cwt. The engine with this load
travelled from Liverpool to Manchester (30 miles) in 2 h. and 40 min.,
exclusive of delays upon the road for watering, &c., being at the
rate of nearly 12 miles an hour. The speed varied according to the
inclinations of the road. Upon a level, it was 12 miles an hour; upon
a descent of 6 feet in a mile, it was 16 miles an hour: upon a rise of
8 feet in a mile, it was about 9 miles an hour. The weather was calm,
the rails very wet; but the wheels did not slip, even in the slowest
speed, except at starting, the rails being at that place soiled and
greasy with the slime and dirt to which they are always exposed at the
stations. The coke consumed in this journey, exclusive of what was
raised in getting up the steam, was 1762 lbs., being at the rate of a
quarter of a pound per ton per mile.
(94.) From the above experiments it appears that a locomotive engine,
in good working order, with its full complement of load, is capable of
transporting weights at an expense in fuel amounting to about 4 ounces
of coke per ton per mile. The attendance required on the journey is
that of an engine-man and a fire-boy; the former being paid 1_s._
6_d._ for each trip of 30 miles, and the latter 1_s._ In practice,
however, we are to consider, that it rarely happens that the full
complement of load can be sent with the engines; and when lesser loads
are transported, the _proportionate expense_ must for obvious reasons,
be greater.
The practical expenditure of fuel on the Liverpool and Manchester line
may, perhaps, be fairly estimated at half a pound of coke per ton per
mile.
(95.) Having explained the power and efficiency of these locomotive
engines, it is now right to notice some of the defects under which
they labour.
The great original cost, and the heavy expense of keeping the engines
used on the railway in repair, have pressed severely on the resources
of the undertaking. One of the best constructed of the later engines
costs originally about 800_l._ It may be hoped that, by the excitement
of competition, the facilities derived from practice, and from the
manufacture of a greater number of engines of the same kind, some
reduction of this cost may be effected. The original cost, however, is
far from being the principal source of expense: the wear and tear of
these machines, and the occasional fracture of those parts on which
the greatest strain has been laid, have greatly exceeded what the
directors had anticipated. Although this source of expense must be in
part attributed to the engines not having yet attained that state of
perfection, in the proportion and adjustment of their parts, of which
they are susceptible, and to which experience alone can lead, yet
there are some obvious defects which demand attention.
The heads of the boilers are flat, and formed of iron, similar to the
material of the boilers themselves. The tubes which traverse the
boiler were, until recently, copper, and so inserted into the flat
head or ends as to be water-tight. When the boiler is heated, the
tubes are found to expand in a greater degree than the other parts of
the boiler; which frequently causes them either to be loosened at the
extremities, so as to cause leakage, or to bend from want of room for
expansion. The necessity of removing and refastening the tubes causes,
therefore, a constant expense.
It will be recollected that the fire-place is situated at one end of
the boiler, immediately below the mouths of the tubes: a powerful
draught of air, passing through the fire, carries with it ashes and
cinders, which are driven violently through the tubes, and especially
the lower ones, situated near the fuel. These tubes are, by this
means, subject to rapid wear, the cinders continually acting upon
their interior surface. After a short time it becomes necessary to
replace single tubes, according as they are found to be worn, by new
ones; and it not unfrequently happens, when this is neglected, that
tubes burst. After a certain length of time the engines require new
tubing, which is done at the expense of about 70_l._, allowing for the
value of the old tubes. This wear of the tubes might possibly be
avoided by constructing the fire-place in a lower position, so as to
be more removed from their mouths; or, still more effectually, by
interposing a casing of metal, which might be filled with water,
between the fire-place and those tubes which are the most exposed to
the cinders and ashes. The unequal expansion of the tubes and boilers
appears to be an incurable defect, if the present form of the engine
be retained. If the fire-place and chimney could be placed at the same
end of the boiler, so that the tubes might be recurved, the unequal
expansion would then produce no injurious effect; but it would be
difficult to clean the tubes if they were exposed, as they are at
present, to the cinders. The next source of expense arises from the
wear of the boiler-head, which is exposed to the action of the fire.
These require constant patching and frequent renewal.
A considerable improvement has lately been introduced into the method
of tubing, by substituting brass for copper tubes. We are not aware
that the cause of this improvement has been discovered; but it is
certain, whatever be the cause, that brass tubes are subject to
considerably slower wear than copper.
It has been said by some whose opinions are adverse to the advantage
of railways, but more especially to the particular species of
locomotive engines now under consideration, that the repairs of one of
these engines cost so great a sum as 1500_l._ per annum, and that the
directors now think of abandoning them, or adopting either stationary
engines or horse-power. As to the first of these statements I must
observe, that the expense of repairs of such machines should never be
computed in reference to _time_, but rather to the work done, or the
distance travelled over. I have ascertained that engines frequently
travel a distance of from 25,000 to 30,000 miles before they require
new tubing. During that work, however, single tubes are, of course,
occasionally renewed, and other repairs are made, the expense of which
may safely be stated as under the original cost of the engine. The
second statement, that the company contemplate substituting stationary
engines, or horses, for locomotives, is altogether at variance with
the truth. Whatever improvements may be contemplated in locomotives,
the directors assuredly have not the slightest intention of going back
in the progress of improvement, in the manner just mentioned.
The expense of locomotive power having so far exceeded what was
anticipated at the commencement of the undertaking, it was thought
advisable, about the beginning of the year 1834, to institute an
inquiry into the causes which produced the discrepancy between the
estimated and actual expenses, with a view to the discovery of some
practical means by which they could be reduced. The directors of the
company, for this purpose, appointed a sub-committee of their own
body, assisted by Mr. _Booth_, their treasurer, to inquire and report
respecting the causes of the amount of this item of their expenditure,
and to ascertain whether any and what measures could be devised for
the attainment of greater economy. A very able and satisfactory report
was made by this committee, or, to speak more correctly by Mr.
_Booth_.
It appears that, previous to the establishment of the railway, Messrs.
_Walker_ and _Rastrick_, engineers, were employed by the company to
visit various places where steam power was applied on railways, for
the purpose of forming an estimate of the probable expense of working
the railway by locomotive and by fixed power. These engineers
recommended the adoption of locomotive power, and their estimate was,
that the transport might be effected at the rate of .278 of a penny,
or very little more than a farthing per ton per mile. In the year
1833, five years after this investigation took place, it was found
that the actual cost was .625 of a penny, or something more than a
halfpenny per ton per mile, being considerably above double the
estimated rate. Mr. _Booth_ very properly directed his inquiries to
ascertain the cause of this discrepancy, by comparing the various
circumstances assumed by Messrs. _Walker_ and _Rastrick_, in making
their estimate, with those under which the transport was actually
effected. The first point of difference which he observed was the
speed of transport: the estimate was founded on an assumed _speed_ of
ten miles an hour, and it was stated that a fourfold speed would
require an addition of 50 per cent. to the power, without taking into
account wear and tear. Now the actual speed of transport being double
the speed assumed in the statement, Mr. _Booth_ holds it to be
necessary to add 25 per cent. on that score.
The next point of difference is in the amount of the loads: the
estimate is founded upon the assumption, that every engine shall start
with its full complement of load, and that with this it shall go the
whole distance. "The facts, however, are," says Mr. _Booth_, "that,
instead of a _full load_ of profitable carriage _from_ Manchester,
about half the wagons _come back empty_, and, instead of the tonnage
being conveyed the whole way, many thousand tons are conveyed only
half the way; also, instead of the daily work being uniform, it is
extremely fluctuating." It is further remarked, that in order to
accomplish the transport of goods from the branches and from
intermediate places, engines are despatched several times a day, from
both ends of the line, _to clear the road_; the object of this
arrangement being rather to lay the foundation of a beneficial
intercourse in future, than with a view to any immediate profit. Mr.
_Booth_ makes a rough estimate of the disadvantages arising from these
circumstances by stating them at 33 per cent. in addition to the
original estimate.
The next point of difference is the fuel. In the original estimate
_coal_ is assumed as the fuel, and it is taken at the price of five
shillings and tenpence per ton: now the act of parliament forbids the
use of coal which would produce smoke; the company have, therefore,
been obliged to use _coke_, at seventeen shillings and sixpence a
ton. Taking coke, then, to be equivalent to coal, ton for ton, this
would add .162 to the original estimate.
These several discrepancies being allowed for, and a proportional
amount being added to the original estimate, the amount would be
raised to .601 of a penny per ton per mile, which is within one
fortieth of a penny of the actual cost. This difference is considered
to be sufficiently accounted for by the wear and tear produced by the
very rapid motion, more especially when it is considered that many of
the engines were constructed before the engineer was aware of the
great speed that would be required.
"What then," says Mr. _Booth_, in the Report already alluded to, "is
the result of these opposite and mutually counteracting circumstances?
and what is the present position of the company in respect of their
moving power? Simply, that they are still in a course of experiment,
to ascertain practically the best construction, and the most durable
materials, for engines required to transport greater weights, and at
greater velocities, than had, till very recently, been considered
possible; and which, a few years ago, it had not entered into the
imagination of the most daring and sanguine inventor to conceive: and,
farther, that these experiments have necessarily been made, not with
the calm deliberation and quiet pace which a salutary caution
recommends,--making good each step in the progress of discovery before
advancing another stage,--but amidst the bustle and responsibilities
of a large and increasing traffic; the directors being altogether
ignorant of the time each engine would last before it would be laid up
as inefficient, but compelled to have engines, whether good or bad;
being aware of various defects and imperfections, which it was
impossible at the time to remedy, yet obliged to keep the machines in
motion, under all the disadvantages of heavy repairs, constantly going
on during the night, in order that the requisite number of engines
might be ready for the morning's work. Neither is this great
experiment yet complete; it is still going forward. But the most
prominent difficulties have been in a great measure surmounted, and
your committee conceive, that they are warranted in expecting, that
the expenditure in this department will, ere long, be materially
reduced, more especially when they consider the relative performances
of the engines at the _present time_ compared with what it was two
years ago."
In the half year ending 31st December, 1831, the six best engines
performed as follows:--
Miles.
Planet 9,986
Mercury 11,040
Jupiter 11,618
Saturn 11,786
Venus 12,850
Etna 8,764
------
Making in all 66,044
------
In the half year ending 31st December, 1833, the six best engines
performed as follows:--
Miles.
Jupiter 16,572
Saturn 18,678
Sun 15,552
Etna 17,763
Ajax 11,678
Firefly 15,608
------
Making in all 95,851
------
(96.) The advantages derivable from railroads are greatly abridged by
the difficulty arising from those changes of level to which all roads
are necessarily liable; but in the case of railroads, from causes
peculiar to themselves, these changes of level occasion great
inconvenience. To explain the nature of these difficulties, it will be
necessary to consider the relative proportion which must subsist
between the power of traction on a level and on an inclined plane. On
a level railroad the force of traction necessary to propel any load,
placed on wheel carriages of the construction now commonly used, may
perhaps be estimated at 7-1/2 pounds,[28] for every ton gross in the
load; that is to say, if a load of one ton gross were placed upon
wheel carriages upon a level railroad, the traces of horses drawing it
would be stretched with a force equivalent to 7-1/2 pounds. If the
load amounted to two or three tons, the tension of the traces would be
increased to 15 or 22-1/2 pounds, and so on. The necessity of this
force of traction, arising from the want of perfect smoothness in the
road, and from the friction of the wheels and axles of the carriages,
must be the same whether the road be level or inclined; and
consequently, in ascending an inclined plane, the same force of
traction will be necessary in addition to that which arises from the
tendency of the load to fall down the plane. This latter tendency is
always in the proportion of the elevation of the plane to its length;
that is to say, a plane which rises 1 foot in 100 will give a weight
of 100 tons a tendency to fall down the plane amounting to 1 ton, and
would therefore add 1 ton to the force of traction necessary for such
a load on a level.
[Footnote 28: The estimate commonly adopted by engineers at present is
9 pounds per ton. I have no doubt, however, that this is too high. I
am now (November, 1835,) engaged in an extensive course of experiments
on different railways, with a view to determine with precision this
and other points connected with the full developement of their theory;
and I have reason to believe, from the observations I have already
made, that even 7-1/2 pounds per ton is above the average force of
traction upon the level.]
Now since 7-1/2 pounds is very nearly the 300th part of a ton, it
follows that if an inclination upon a railroad rises at the rate of 1
foot in 300, or, what is the same, 17-1/2 feet in a mile, such an
acclivity will add 7-1/2 pounds per ton to the force of traction. This
acclivity therefore would require a force of traction twice as great
as a level. In like manner a rise of 35 feet in a mile would require
three times the force of traction of a level, 52-1/2 feet in a mile
four times that force, and so on. In fact, for every 7 feet in a mile
which an acclivity rises, 3 pounds per ton will be added to the force
of traction. If we would then ascertain the power necessary to pull a
load up any given acclivity upon a railroad, we must first take 7-1/2
pounds as the force necessary to overcome the common resistance of the
road, and then add 3 pounds for every 7 feet which the acclivity rises
per mile. For example, suppose an acclivity to rise at the rate of 70
feet in a mile, the force of traction necessary to draw a ton up it
would be thus calculated:--
Friction 7-1/2 lbs.
70 feet = 10 times 3 lbs. 30
------
Total force 37-1/2
It will be apparent, therefore, that if a railroad undulates by
inclined planes, even of the most moderate inclinations, the
propelling power to be used upon it must be of such a nature as to be
capable of increasing its intensity in a great degree, according to
the elevation of the planes which it has to encounter. A plane which
rises 52-1/2 feet per mile presents to the eye scarcely the appearance
of an ascent, and yet requires the power of traction to be increased
in a fourfold proportion.
It is the property of animal power, that within certain limits its
energy can be put forth at will, according to the exigency of the
occasion; but the intensity of mechanical power, in the instance now
considered, cannot so conveniently be varied, except indeed within
narrow limits.
In the application of locomotive engines upon railways the difficulty
arising from inclined planes has been attempted to be surmounted by
several methods, which we shall now explain.
1. Upon arriving at the foot of the plane the load is divided, and the
engine carries it up in several successive trips, descending the
plane unloaded after each trip. The objection to this method is the
delay which it occasions,--a circumstance which is incompatible with a
large transport of passengers. From what has been stated, it would be
necessary, when the engine is fully loaded on a level, to divide its
load into four parts, to be successively carried up when the incline
rises 52 feet per mile. This method has been practised in the
transport of merchandise occasionally, when heavy loads were carried
on the Liverpool and Manchester line, upon the Rainhill incline.
2. A subsidiary or assistant locomotive engine may be kept in constant
readiness at the foot of each incline, for the purpose of aiding the
different trains, as they arrive, in ascending. The objection to this
mode is the cost of keeping such an engine with its boiler continually
prepared, and its steam up. It would be necessary to keep its fire
continually lighted, whether employed or not; otherwise, when the
train would arrive at the foot of the incline, it should wait until
the subsidiary engine was prepared for work. In cases where trains
would start and arrive at stated times, this objection, however, would
have less force. This method is at present generally adopted on the
Liverpool and Manchester line. This method, however, cannot be
profitably applied to planes of any considerable length.
3. A fixed steam engine may be erected on the crest of the incline, so
as to communicate by ropes with the train at the foot. Such an engine
would be capable of drawing up one or two trains together, with their
locomotives, according as they would arrive, and no delay need be
occasioned. This method requires that the fixed engine should be kept
constantly prepared for work, and the steam continually up in the
boiler. This expedient is scarcely compatible with a large transit of
passengers, except at the terminus of a line.
4. In working on the level, the communication between the boiler and
the cylinder in the locomotives may be so restrained by partially
closing the throttle valve, as to cause the pressure upon the piston
to be less in a considerable degree than the pressure of steam in the
boiler. If under such circumstances a sufficient pressure upon the
piston can be obtained to draw the load on the level, the throttle
valve may be opened on approaching the inclined plane, so as to throw
on the piston a pressure increased in the same proportion as the
previous pressure in the boiler was greater than that upon the piston.
If the fire be sufficiently active to keep up the supply of steam in
this manner during the ascent, and if the rise be not greater in
proportion than the power thus obtained, the locomotive will draw the
load up the incline without further assistance. It is, however, to be
observed, that in this case the load upon the engine must be less than
the amount which the adhesion of its working wheels with the railroad
is capable of drawing; for this adhesion must be adequate to the
traction of the same load up the incline, otherwise whatever increase
of power might be obtained by opening the throttle valve, the drawing
wheels would revolve without causing the load to advance. This method
has been generally practised upon the Liverpool and Manchester line in
the transport of passengers; and, indeed, it is the only method yet
discovered, which is consistent with the expedition necessary for that
species of traffic. The objections to this method are, the necessity
of maintaining a much higher pressure in the boiler than is sufficient
for the purposes of the load upon more level parts of the line.
In the practice of this method considerable aid may be derived also by
suspending the supply of feeding water during the ascent. It will be
recollected that a reservoir of cold water is placed in the tender
which follows the engine, and that the water is driven from this
reservoir into the boiler by a forcing-pump, which is worked by the
engine itself. This pump is so constructed that it will supply as much
cold water as is equal to the evaporation, so as to maintain
constantly the same quantity of water in the boiler. But it is
evident, on the other hand, that the supply of this water has a
tendency to check the rate of evaporation, since in being raised to
the temperature of the water with which it mixes, it must absorb a
considerable portion of the heat supplied by the fire. With a view to
accelerate the production of steam, therefore, in ascending the
inclines, the engine-man may suspend the action of the forcing-pump,
and thereby stop the supply of cold water to the boiler; the
evaporation will go on with increased rapidity, and the exhaustion of
water produced by it will be repaid by the forcing-pump on the next
level, or still more effectually on the next descending incline.
Indeed the feeding pump may be made to act in descending an incline if
necessary, when the action of the engine itself is suspended, and when
the train descends by its own gravity, in which case it will perform
the part of a brake upon the descending train.
This method, on railroads intended for passengers, may be successfully
applied on inclines which do not exceed 18 feet in a mile; and, with a
sacrifice of the expense of locomotive power, inclines so steep as 36
feet in a mile may be worked in this manner. As, however, the
sacrifice is considerable, it will, perhaps, be always better to work
the more steep inclines by assistant engines.
5. The mechanical connexion between the piston of the cylinder and the
points of contact of the working wheels with the road may be so
altered, upon arriving at the incline, as to give the piston a greater
power over the working wheels. This may be done in an infinite variety
of ways, but hitherto no method has been suggested sufficiently simple
to be applicable in practice; and even were any means suggested which
would accomplish this, unless the intensity of the impelling power
were at the same time increased, it would necessarily follow that the
speed of the motion would be diminished in exactly the same proportion
as the power of the piston over the working wheels would be increased.
Thus, on the inclined plane, which rises 55 feet per mile, upon the
Liverpool line, the speed would be diminished to nearly one fourth of
its amount upon the level.
Whatever be the method adopted to surmount inclined planes upon a
railway, inconvenience attends the descent upon them. The motion down
the incline by the force of gravity is accelerated; and if the train
be not retarded, a descent of any considerable length, even at a small
elevation, would produce a velocity which would be attended with great
danger. The shoe used to retard the descent down hills on turnpike
roads cannot be used upon railroads, and the application of brakes to
the faces of the wheels is likewise attended with some uncertainty.
The friction produced by the rapid motion of the wheel sometimes sets
fire to wood, and iron would be inadmissible. The action of the steam
on the piston may be reversed, so as to oppose the motion of the
wheels; but even this is attended with peculiar difficulty.
From all that has been stated, it will be apparent that, with our
present knowledge, considerable inclines are fatal to the profitable
performance on a railway, and even small inclinations are attended
with great inconvenience.[29]
[Footnote 29: A contrivance might be applied in changes of level in
railroads somewhat similar to locks in a canal. The train might be
rolled upon a platform which might be raised by machinery; and thus at
the change of level there would be as it were _steps_ from one level
to another, up which the loads would be lifted by any power applied to
work the machinery. The advantage in this case would be, that the
trains might be adapted to work always upon a level.]
(97.) To obtain from the locomotive steam engines now used on the
railway the most powerful effects, it is necessary that the load
placed on each engine should be very considerable. It is not possible,
with our present knowledge, to construct and work three locomotive
engines of this kind, each drawing a load of 30 tons, at the same
expense and with the same effect as one locomotive engine drawing 90
tons. Hence arises what must appear an inconvenience and difficulty in
applying these engines to one of the most profitable species of
transport--the transport of passengers. It is impracticable, even
between places of the most considerable intercourse to obtain loads of
passengers sufficiently great at each trip to maintain such an engine
working on a railway.[30] The difficulty of collecting so considerable
a number of persons, at any stated hour, to perform the journey, is
obvious; and therefore, the only method of removing the inconvenience
is _to cause the same engine which transports passengers also to
transport goods_, so that the goods may make up the requisite
supplement to the load of passengers. In this way, provided the
traffic in goods be sufficient, such engines may start with their full
complement of load, whatever be the number of passengers.
[Footnote 30: On the occasion of races held at Newton, a place about
15 miles from Liverpool, two engines were sent, with trains of
carriages, to take back to Liverpool the visitors to the races. Some
accident prevented one of the engines from working on the occasion,
and both trains were attached to the same engine: 800 persons were on
this occasion drawn by the single engine to Liverpool in the space of
about an hour.]
(98.) In comparing the extent of capital, and the annual expenditure
of the Liverpool and Manchester line, and adopting it as a modulus in
estimating the expenses of similar undertakings projected elsewhere,
there are several circumstances to which it is important to attend. I
have already observed on the large waste of capital in the item of
locomotive engines which ought to be regarded as little more than
experimental machines, leading to a rapid succession of improvements.
Most of these engines are still in good working order, but have been
abandoned for the reasons already assigned. Other companies will, of
course, profit by the experience which has been thus purchased at a
high price by the Liverpool Company. This advantage in favour of
future companies will go on increasing until such companies have their
works completed.
A large portion of the current expense of a line of railway is
independent of its length; and is little less for the line connecting
Liverpool and Manchester, than it would be for a line connecting
Birmingham with Liverpool or London.
The establishments of resident engineers, coach and wagon yards, &c.
at the extremities of the line, would be little increased by a very
great increase in the length of the railway; and the same observation
will apply to other heads of expenditure.
It has been the practice of the canal companies between Liverpool and
Manchester to warehouse the goods transported between these towns,
without any additional charge beyond the price of transport. The
Railway Company, in competing with the canals, were, of course,
obliged to offer like advantages: this compelled them to invest a
considerable amount of capital in the building of extensive
warehouses, and to incur the annual expense of porterage, salaries,
&c. connected with the maintenance of such storage. In a longer line
of railway such expenses (if necessary at all) would not be
proportionally increased.
(99.) The comparison of steam-transport with the transport by horses,
even when working on a railway, exhibits the advantage of this new
power in a most striking point of view. To comprehend these advantages
fully, it will be necessary to consider the manner in which animal
power is expended as a means of transport. The portion of the strength
of a horse available for the purpose of a load depends on the speed of
a horse's motion. To this speed there is a certain limit, at which the
whole power of the horse will be necessary to move his own body, and
at which, therefore, he is incapable of carrying any load; and, on the
other hand, there is a certain load which the horse is barely able to
support, but incapable of moving with any useful speed. Between these
two limits there is a certain rate of motion at which the useful
effect of the animal is greatest. In horses of the heavier class, this
rate of motion may be taken on the average as that of 2 miles an hour;
and in the lighter description of horses, 2-1/2 miles an hour. Beyond
this speed, the load which they are capable of transporting diminishes
in a very rapid ratio as the speed increases: thus, if 121 express the
load which a horse is able to transport a given distance in a day,
working at the rate of four miles an hour, the same horse will not be
able to transport more than the load expressed by 64, _the same
distance_, at 7 miles an hour; and, at 10 miles an hour, the load
which he can transport will be reduced to 25. The most advantageous
speed at which a horse can work being 2 miles an hour, it is found
that, at this rate, working for 10 hours daily, he can transport 12
tons, on a level railway, a distance of 20 miles; so that the whole
effect of a day's work may be expressed by 240 tons carried 1 mile.
But this rate of transport is inapplicable to the purposes of
travelling; and therefore it becomes necessary, when horses are the
moving power, to have carriages for passengers distinct from those
intended for the conveyance of goods; so that the goods may be
conveyed at that rate of speed at which the whole effect of the horse
will be the greatest possible; while the passengers are conveyed at
that speed which, whatever the cost, is indispensably necessary. The
weight of an ordinary mail-coach is about two tons; and, on a
tolerably level turnpike road, it travels at the rate of 10 miles an
hour. At this rate, the number of horses necessary to keep it
constantly at work, including the spare horses indispensably necessary
to be kept at the several stages, is computed at the rate of a horse
per mile. Assuming the distance between London and Birmingham at 100
miles, a mail-coach running between these two places would require 100
horses; making the journey to and from Birmingham daily. The
performance, therefore, of a horse working at this rate may be
estimated at 2 tons carried 2 miles per day, or 4 tons carried 1 mile
in a day. The force of traction on a good turnpike road is at least 20
times its amount on a level railroad. It therefore follows, that the
performance of a horse on a railroad will be 20 times the amount of
its performance on a common road under similar circumstances. We may,
therefore, take the performance of a horse working at 10 miles an
hour, on a level railroad, at 80 tons conveyed 1 mile daily.
The best locomotive engines used on the Liverpool railway are capable
of transporting 150 tons on a level railroad at the same rate; and,
allowing the same time for stoppage, its work per day would be 150
tons conveyed 200 miles, or 30,000 tons conveyed 1 mile; from which it
follows, that the performance of one locomotive engine of this kind is
equivalent to that of 7500 horses working on a good turnpike road, or
to 375 horses working on a railway. The consumption of fuel requisite
for this performance, with the most improved engines used at present
on the Manchester and Liverpool line, would be at the rate of
eight[31] ounces of coke per ton per mile, including the waste of fuel
incurred by the stoppages. Thus the daily consumption of fuel, under
such circumstances, would amount to 15000 lbs. of coke; and 2 lbs. of
coke daily would perform the work of one horse on a good turnpike
road; and 40 lbs. of coke daily would perform the work of one horse on
a railway.
[Footnote 31: In an experimental trip with a heavy train at 12 miles
an hour, I found the consumption of coke to be only _four_ ounces per
ton per hour. I believe, however, the practical consumption in
ordinary work to be very nearly eight ounces.]
In this comparison, the engine is taken at its most advantageous
speed, while horse-power is taken at its least advantageous speed, if
regard be only had to the total quantity of weight transported to a
given distance. But, in the case above alluded to, speed is an
indispensable element; and steam, therefore, possesses this great
advantage over horse power, that _its most advantageous speed is that
which is at once adapted to all the purposes of transport, whether of
passengers or of goods_.
(100.) The effects of steam compared with horse-power, at lower rates
of motion, will exhibit the advantages of the former, though in a
less striking degree. An eight-horse wagon commonly weighs 8 tons, and
travels at the rate of 2-1/2 miles an hour. Strong horses working in
this way can travel 8 hours daily; thus each horse performs 20 miles a
day. The performance, therefore, of each horse may be taken as
equivalent to 20 tons transported 1 mile; and his performance on a
railway being 20 times this amount, may be taken as equivalent to 400
tons transported 1 mile a day. The performance of a horse working in
this manner is, therefore, 5 times the performance of a horse working
at 10 miles an hour; the latter effecting only the performance of 4
tons transported 1 mile per day on a good turnpike road, or 80 tons on
a railway. We shall hence obtain the proportion of the performance of
horses working in wagons to that of a locomotive steam engine. Since 2
lbs. of coke are equivalent to the daily performance of a horse in a
mail-coach, and 40 lbs. on a railway, at 10 miles an hour, it follows
that 10 lbs. will be equivalent to the performance of a horse on a
turnpike road, and 200 lbs. on a railway, at 2-1/2 miles an hour.
Since a locomotive engine can perform the daily work of 7500
mail-coach horses, it follows that it performs the work of 1500 wagon
horses.
These results must be understood to be subject to modifications in
particular cases, and to be only average calculations. Different
steam-engines, as well as different horses, varying in their
performance to a considerable extent; and the roads on which horses
work being in different states of perfection, and subject to different
declivities, the performance must vary accordingly.
In the practical comparison, also, of the results of so powerful an
agent as steam applied on railways, with so slight a power as that of
horses on common roads, it must be considered that the great
subdivision of load, and frequent times of starting, operate in favour
of the performance of horses; inasmuch as it would oftener occur that
engines capable of transporting enormous weights would start with
loads inferior to their power, than would happen in the application of
horse-power, where small loads may start at short intervals. This, in
fact, constitutes a practical difficulty in the application of steam
engines on railroads; and will, perhaps, for the present, limit their
application to lines connecting places of great intercourse.
The most striking effect of steam power, applied on a railroad, is the
extreme speed of transport which is attained by it; and it is the more
remarkable, as this advantage never was foreseen before experience
proved it. When the Liverpool and Manchester line was projected, the
transport of heavy goods was the object chiefly contemplated; and
although an intercourse in passengers was expected, it was not
foreseen that this would be the greatest source of revenue to the
proprietors. The calculations of future projectors will, therefore, be
materially altered, and a great intercourse in passengers will be
regarded as a necessary condition for the prosperity of such an
undertaking.
If this advantage of speed be taken into account, horse-power can
scarcely admit of any comparison whatever with steam-power on a
railway. In the experiments which I have already detailed, it appears
that a steam engine is capable of drawing 90 tons at the rate of about
20 miles an hour, and that it could transport that weight twice
between Liverpool and Manchester in about 3 hours. Two hundred and
seventy horses working in wagons would be necessary to transport the
same load the same distance in a day. It may be objected, that this
was an experiment performed under favourable circumstances, and that
assistance was obtained at the difficult point of the inclined plane.
In the ordinary performance, however, of the engines drawing
merchandise, where great speed is not attempted, the rate of motion is
not less than 15 miles an hour. In the trains which draw passengers,
the chief difficulty of maintaining a great speed arises from the
stoppages on the road to take up and let down passengers. There are
two classes of carriages at present used: the first class stops but
once, at a point half-way between Liverpool and Manchester, for the
space of a few minutes. This class performs the thirty miles in an
hour and a half, and sometimes in 1 hour and 10 minutes. On the level
part of the road its common rate of motion is 27 miles an hour; and I
have occasionally marked its rate, and found it above 30 miles an
hour.
But these, which are velocities obtained in the regular working of the
engines for the transport of passengers and goods, are considerably
inferior to the power of the present locomotives with respect to
speed. I have made some experimental trips, in which more limited
loads were placed upon the engines, by which I have ascertained that
very considerably increased rates of motion are quite practicable. In
one experiment I placed a carriage containing 36 persons upon an
engine, with which I succeeded in obtaining the velocity of about 48
miles an hour, and I believe that an engine loaded only with its own
tender has moved over 15 miles in 15 minutes.
It will then perhaps be asked, if the engines possess these great
capabilities of speed, why they have not been brought into practical
operation on the railroad, where, on the other hand, the average speed
when actually in motion, does not exceed 25 miles an hour? In answer
to this it may be stated, that the distance of 30 miles between
Liverpool and Manchester is performed in an hour and a half, and that
10 trains of passengers pass daily between these places: the mail,
also, is transmitted three times a day between them. It is obvious
that any greater speed than this, in so short a distance, would be
quite needless. When, however, more extended lines of road shall be
completed, the circumstances will be otherwise, and the despatch of
mails especially will demand attention. Full trains of passengers,
commonly transported upon the Manchester railroad, weigh about 50 tons
gross: with a lighter load, a lighter and more expeditious engine
might be used. The expense of transport with such an engine would of
course be increased; but for this the increased expedition there
would be ample compensation. When, therefore, London shall have been
connected with Liverpool, by a line of railroad through Birmingham,
the commercial interest of these places will naturally direct
attention to the greatest possible expedition of intercommunication.
For the transmission of mails, doubtless, peculiar engines will be
built, adapted to lighter loads and greater speed. With such engines,
the mails, with a limited number of passengers, will be despatched;
and, apart from any possible improvement which the engines may
hereafter receive, and looking only at their present capabilities, I
cannot hesitate to express my conviction that such a load may be
transported at the rate of above 60 miles an hour. If we may indulge
in expectations of what the probable improvements of locomotive steam
engines may effect, I do not think that even double that speed is
beyond the limits of mechanical probability. On the completion of the
line of road from the metropolis to Liverpool we may, therefore,
expect to witness the transport of mails and passengers in the short
space of three hours. There will probably be about three posts a day
between these and intermediate places.
The great extension which the application of steam to the purpose of
inland transport is about to receive from the numerous railroads which
are already in progress, and from a still greater number of others
which are hourly projected, impart to these subjects of inquiry
considerable interest. Neither the wisdom of the philosopher, nor the
skill of the statistician, nor the foresight of the statesman is
sufficient to determine the important consequences by which the
realization of these schemes must affect the progress of the human
race. How much the spread of civilization, the diffusion of knowledge,
the cultivation of taste, and the refinement of habits and manners
depend upon the easy and rapid inter-mixture of the constituent
elements of society, it is needless to point out. Whilst population
exists in detached and independent masses, incapable of transfusion
amongst each other, their dormant affinities are never called into
action, and the most precious qualities of each are never imparted to
the other. Like solids in physics, they are slow to form combinations;
but when the quality of fluidity has been imparted to them, when their
constituent atoms are loosened by fusion, and the particles of each
flow freely through and among those of the other, then the affinities
are awakened, new combinations are formed, a mutual interchange of
qualities takes place, and compounds of value far exceeding those of
the original elements are produced. Extreme facility of intercourse is
the fluidity and fusion of the social masses, from whence such an
activity of the affinities results, and from whence such an
inestimable interchange of precious qualities must follow. We have,
accordingly, observed, that the advancement in civilization and the
promotion of intercourse between distant masses of people have ever
gone on with contemporaneous progress, each appearing occasionally to
be the cause or the consequence of the other. Hence it is that the
urban population is ever in advance of the rural in its intellectual
character. But, without sacrificing the peculiar advantages of either,
the benefits of intercourse may be extended to both, by the
extraordinary facilities which must be the consequence of the
locomotive projects now in progress. By the great line of railroad
which is in progress from London to Birmingham, the time and expense
of passing between these places will probably be halved, and the
quantity of intercourse at least quadrupled, if we consider only the
direct transit between the terminal points of the line; but if the
innumerable tributary streams which will flow from every adjacent
point be considered, we have no analogies on which to build a
calculation of the enormous increase of intercommunication which must
ensue.
Perishable vegetable productions necessary for the wants of towns must
at present be raised in their immediate suburbs; these, however, where
they can be transported with a perfectly smooth motion at the rate of
twenty miles an hour, will be supplied by the agricultural labourer
of more distant points. The population engaged in towns, no longer
limited to their narrow streets, and piled story over story in
confined habitations, will be free to reside at distances which would
now place them far beyond reach of their daily occupations. The
salubrity of cities and towns will thus be increased by spreading the
population over a larger extent of surface, without incurring the
inconvenience of distance. Thus the advantages of the country will be
conferred upon the town, and the refinement and civilization of the
town will spread their benefits among the rural population.[32]
[Footnote 32: Some of the preceding observations appeared in an
article contributed by me to the British and Foreign Review.]
(101.) The quantity of canal property in these countries gives
considerable interest to every inquiry which has for its object the
relative advantage of this mode of transport, compared with that of
railways, whether worked by horses or by steam-power; and this
interest has been greatly increased by the recent extension of railway
projects. This is a subject which I shall have occasion, in another
work, to examine in all its details; and, therefore, in this place I
shall advert to it but very briefly.
When a floating body is moved on a liquid, it will suffer a
resistance, which will depend partly upon the transverse section of
the part immersed, and partly on the speed with which it is moved. It
is evident that the quantity of the liquid which it must drive before
it will depend upon that transverse section, and the velocity with
which it will impel the liquid will depend upon its own speed. Now, so
long as the depth of its immersion remains the same, it is
demonstrable that the resistance will increase in proportion to the
square of the speed; that is, with a double velocity there will be a
fourfold resistance, with a triple velocity a ninefold resistance, and
so on. Again, if the part immersed should be increased or diminished
by any cause, the resistance, on that account alone, will be increased
or diminished in the same proportion.
From these circumstances it will be apparent that a vessel floating on
water, if moved with a certain speed, will require four times the
impelling force to carry it forward with double the speed, unless the
depth of its immersion be diminished as its speed is increased.
Some experiments which have been made upon canals with boats of a
peculiar construction, drawn by horses, have led to the unexpected
conclusion, that, after a certain speed has been attained, the
resistance, instead of being increased, has been diminished. This fact
is not at variance with the law of resistance already explained. The
cause of the phenomenon is found in the fact, that when the velocity
has attained a certain point, the boat gradually rises out of the
water; so that, in fact, the immersed part is diminished. The two
conditions, therefore, which determine the resistance, thus modify
each other: while the resistance is, on the one hand, increased in
proportion to the square of the speed, it is, on the other hand,
diminished in proportion to the diminution of the transverse section
of the immersed part of the vessel. It would appear that, at a certain
velocity, these two effects neutralise each other; and, probably, at
higher velocities the immersed part may be so much diminished as to
diminish the resistance in a greater degree than it is increased by
the speed, and thus actually to diminish the power of traction.
It is known that boats are worked on some of the Scottish canals, and
also on the canal which connects Kendal with Preston, by which
passengers are transported, at the rate of about ten miles an hour,
exclusive of the stoppages at the locks, &c. The power of horses,
exerted in this way, is, of course, exerted more economically than
they could be worked at the same speed on common roads; and, probably,
it is as economical as they would be worked by railroad. It is,
probably, more economical than the transport of passengers by steam
upon railroads; but the speed is considerably less, nor, from the
nature of the impelling power, is it possible that it can be
increased.
There is reason to suppose that a like effect takes place with steam
vessels. Upon increasing the power of the engines in some of the Post
Office steam packets, it has been found, that, while the time of
performing the same voyage is diminished, the consumption of fuel is
also diminished. Now, since the consumption of fuel is in the direct
ratio of the moving power, and the latter in the direct ratio of the
resistance, it follows that the resistance must in this case be
likewise diminished.
(102.) When a very slow rate of travelling is considered, the useful
effects of horse-power applied on canals is somewhat greater than the
effect of the same power applied on railways; but at all speeds above
three miles an hour, the effect on railways is greater; and when the
speed is considerable, the canal becomes wholly inapplicable, while
the railway loses none of its advantages. At three miles an hour, the
performance of a horse on a canal and a railway is in the proportion
of four to three to the advantage of the canal; but at four miles an
hour his performance on a railway has the advantage in very nearly the
same proportion. At six miles an hour, a horse will perform three
times more work on a railway than on a canal. At eight miles an hour,
he will perform nearly five times more work.
But the circumstance which, so far as respects passengers, must give
railways, as compared with canals, an advantage which cannot be
considered as less than fatal to the latter, is the fact, that the
great speed and cheapness of transit attainable upon a railway by the
aid of steam-power will always secure to such lines not only a
monopoly of the travelling, but will increase the actual amount of
that source of profit in an enormous proportion, as has been already
made manifest between Liverpool and Manchester. Before the opening of
the railway there were about twenty-five coaches daily running between
Liverpool and Manchester. If we assume these coaches on the average to
take ten persons at each trip, it will follow that the number of
persons passing daily between these towns was about 500. Let us, then,
assume that 3000 persons passed weekly. This gives in six months
78,000. In the six months which terminated on the 31st of December
1831, the number of passengers between the same towns, exclusive of
any taken up on the road, was 256,321; and if some allowance be made
for those taken up on the road, the number may be fairly stated at
300,000. At present there is but one coach on the road between
Liverpool and Manchester; and it follows, therefore, that, besides
taking the monopoly of the transit in travellers, the actual number
has been already increased in a fourfold proportion.
The monopoly of the transit of passengers thus secured to the line of
communication by railroad will always yield so large a profit as to
enable merchandise to be carried at a comparatively low rate.
In light goods, which require despatch, it is obvious that the
railroad will always command the preference; and the question between
that mode of communication and canals is circumscribed to the transit
of those classes of heavy goods in which even a small saving in the
cost of transport is a greater object than despatch.
(103.) The first effect which the Liverpool railroad produced on the
Liverpool and Manchester canals was a fall in the price of transport;
and at this time, I believe, the cost of transport per ton on the
railroads and on the canals is the same. It will, therefore, be
naturally asked, this being the case, why the greater speed and
certainty of the railroad does not in every instance give it the
preference, and altogether deprive the canals of transport? This
effect, however, is prevented by several local and accidental causes,
as well as by direct influence and individual interest. A large
portion of the commercial and manufacturing population of Liverpool
and Manchester have property invested in the canals, and are deeply
interested to sustain them in opposition to the railway. Such persons
will give the preference to the canals in their own business, and will
induce those over whom they have influence to do so in every case
where speed of transport is not absolutely indispensable.
Besides these circumstances, the canal communicates immediately with
the shipping at Liverpool, and it ramifies in various directions
through Manchester, washing the walls of many of the warehouses and
factories for which the goods transported are destined. The
merchandise is thus transferred from the shipping to the boat, and
brought directly to the door of its owner, or _vice versâ_. If
transported by the railway, on the other hand, it must be carried to
the station at one extremity; and, when transported to the station at
the other, it has still to be carried to its destination in different
parts of the town.
These circumstances will sufficiently explain why the canals still
retain, and may probably continue to retain, a share of the traffic
between these great marts.
CHAPTER XI.
LOCOMOTIVE ENGINES ON TURNPIKE ROADS.
Railways and Turnpike Roads compared. -- Mr. Gurney's inventions.
-- His Locomotive Steam Engine. -- Its performances. --
Prejudices and errors. -- Committee of the House of Commons. --
Convenience and safety of Steam Carriages. -- Hancock's Steam
Carriage. -- Mr. N. Ogle. -- Trevithick's invention. --
Proceedings against Steam Carriages. -- Turnpike Bills. -- Steam
Carriage between Gloucester and Cheltenham. -- Its
discontinuance. -- Report of the Committee of the Commons. --
Present State and Prospects of Steam Carriages.
(104.) We have hitherto confined our observations to steam-power as a
means of transport applied on railways, but modern speculation has not
stopped here. Several attempts have been made, and some of them
attended with considerable success, to work steam-carriages on
turnpike roads. The practicability of this project has been hitherto
generally considered to be very questionable; but if we carry back our
view to the various epochs in the history of the invention of the
steam engine, we shall find the same doubt, and the same difficulty,
started at almost every important step in its progress. In comparing
the effect of a turnpike road with that of a railway, there are two
circumstances which obviously give facility and advantage to the
railway. One is, that the obstructions to the rolling motion of the
wheels, produced by the inequalities of the surface, are very
considerably less on a railway than on a road; less in the proportion
of at least 1 to 20. This proportion, however, must depend much on the
nature of the road with which the railway is compared. It is obvious
that a well-constructed road will offer less resistance than one ill
constructed; and it is ascertained that the resistance of a
Macadamised road is considerably more than that of a road well paved
with stones: the decision of this question, therefore, must involve
the consideration of another, viz. whether roads may not be
constructed by pavement or otherwise, smoother and better adapted to
carriages moved in the manner of steam-carriages than the roads now
used for horse-power?
But besides the greater smoothness of railroads compared with turnpike
roads, they have another advantage, which we suspect to have been
considerably exaggerated by those who have opposed the project for
steam-carriages on turnpike roads. One of the laws of adhesion, long
since developed by experiment, and known to scientific men, is, that
it is greater between the surfaces of bodies of the same nature than
between those of a different nature. Thus between two metals of the
same kind it is greater than between two metals of different kinds.
Between two metals of any kind it is greater than between metal and
stone, or between metal and wood. Hence, the wheels of steam-carriages
running on a railroad have a greater adhesion with the road, and
therefore offer a greater resistance to slip round without the advance
of the carriage, than wheels would offer on a turnpike road; for on a
railroad the iron tire of the wheel rests in contact with the iron
rail, while on a common road the iron tire rests in contact with the
surface of stone, or whatever material the road may be composed of.
Besides this, the dust and loose matter which necessarily collect on a
common road, when pressed between the wheels and the solid base of the
road act somewhat in the manner of rollers, and give the wheels a
greater facility to slip than if the road were swept clean, and the
wheels rested in immediate contact with its hard surface. The truth of
this observation is illustrated on the railroads themselves, where the
adhesion is found to be diminished whenever the rails are covered with
any extraneous matter, such as dust or moist clay. Although the
adhesion of the wheels of a carriage with a common road, however, be
less than those of the wheels of a steam-carriage with a railroad, yet
still the actual adhesion on turnpike roads is greater in amount than
has been generally supposed, and is quite sufficient to propel
carriages dragging after them loads of large amount.
The relative facility with which carriages are propelled on railroads
and turnpike roads equally affects any moving power, whether that of
horses or steam engines; and whether loads be propelled by the one
power or the other, the railroad, as compared with the turnpike road,
will always possess the same proportionate advantage; and a given
amount of power, whether of the one kind or the other, will always
perform a quantity of work less in the same proportion on a turnpike
road than on a railroad. But, on the other hand, the expense of
original construction, and of maintaining the repairs of a railroad,
is to be placed against the certain facility which it offers to
draught.
In the attempts which have been made to adapt locomotive engines to
turnpike roads, the projectors have aimed at the accomplishment of two
objects: first, the construction of lighter and smaller engines; and,
secondly, increased power. These ends, it is plain, can only be
attained, with our present knowledge, by the production of steam of
very high temperature and pressure, so that the smallest volume of
steam shall produce the greatest possible mechanical effect. The
methods of propelling the carriage have been in general similar to
that used in the railroad engines, viz. either by cranks placed on the
axles, the wheels being fixed upon the same axles, or by connecting
the piston-rods with the spokes of the wheels, as in the engine
represented in fig. 55. In some carriages, the boiler and moving
power, and the body of the carriage which bears the passengers, are
placed on the same wheels. In others, the engine is placed on a
separate carriage, and draws after it the carriage which transports
the passengers, as is always the case on railways.
The chief difference between the steam engines used on railways, and
those adapted to propel carriages on turnpike roads, is in the
structure of the boiler. In the latter it is essential that, while
the power remains undiminished, the boiler should be lighter and
smaller. The accomplishment of this has been attempted by various
contrivances for so distributing the water, as to expose a
considerable quantity of surface in contact with it to the action of
the fire; spreading it in thin layers on flat plates; inserting it
between plates of iron placed at a small distance asunder, the fire
being admitted between the intermediate plates; dividing it into small
tubes, round which the fire has play; introducing it between the
surfaces of cylinders placed one within another, the fire being
admitted between the alternate cylinders,--have all been resorted to
by different projectors.
(105.) First and most prominent in the history of the application of
steam to the propelling of carriages on turnpike roads, stands the
name of Mr. GOLDSWORTHY GURNEY, a medical gentleman and scientific
chemist, of Cornwall. In 1822, Mr. _Gurney_ succeeded Dr. Thompson as
lecturer on chemistry at the Surrey Institution; and, in consequence
of the results of some experiments on heat, his attention was directed
to the project of working steam carriages on common roads; and since
1825 he has devoted his exertions in perfecting a steam engine capable
of attaining the end he had in view. Numerous other projectors, as
might have been expected, have followed in his wake. Whether they, or
any of them, by better fortune, greater public support, or more
powerful genius, may outstrip him in the career on which he has
ventured, it would not, perhaps, at present, be easy to predict. But
whatever be the event, to Mr. _Gurney_ is due, and will be paid, the
honour of first proving the practicability of the project; and in the
history of the adaptation of the locomotive engine to common roads,
his name will stand before all others in point of time, and the
success of his attempts will be recorded as the origin and cause of
the success of others in the same race.
The incredulity, opposition, and even ridicule, with which the project
of Mr. _Gurney_ was met, are very remarkable. His views were from the
first opposed by engineers, without one exception. The contracted
habit of mind, sometimes produced by an education chiefly, if not
exclusively, directed to a merely practical object, subsequently
confirmed by exclusively practical pursuits, may, perhaps, in some
degree, account for this. But, I confess, it has not been without
surprise that I have observed, during the last ten years, the utter
incredulity which has prevailed among men of general science on this
subject,--an incredulity which the most unequivocal practical proof
has scarcely yet dispelled. "Among scientific men," says Mr. _Gurney_,
"my opinion had not a single supporter, with the exception of the late
Dr. _Wollaston_."
The mistake which so long prevailed in the application of locomotives
on railroads, and which, as we have shown, materially retarded the
progress of that invention, was shared by Mr. _Gurney_. Without
reducing the question to the test of experiment, he took for granted,
in his first attempts, that the adhesion of the wheels with the road
was too slight to propel the carriage. He was assured, he says, by
eminent engineers, that this was a point settled by actual experiment.
It is strange, however, that a person of his quickness and sagacity
did not inquire after the particulars of these "actual experiments."
So, however, it was; and, taking for granted the inability of the
wheels to propel, he wasted much labour and skill in the contrivance
of levers and propellers, which acted on the ground in a manner
somewhat resembling the feet of horses, to drive the carriage forward.
After various fruitless attempts of this kind, the experience acquired
in the trials to which they gave rise at last forced the truth upon
his notice, and he found that the adhesion of the wheels was not only
sufficient to propel the carriage heavily laden on level roads, but
was capable of causing it to ascend all the hills which occur on
ordinary turnpike roads. In this manner it ascended all the hills
between London and Barnet, London and Stanmore, Stanmore Hill,
Brockley Hill, and mounted Old Highgate Hill, the last at one point
rising one foot in nine.
It would be foreign to my present object to detail minutely all the
steps by which Mr. _Gurney_ gradually improved his contrivance. This,
like other inventions, has advanced by a series of partial failures;
but it has at length attained that state, in which, by practice alone,
on a more extensive scale, a further degree of perfection can be
obtained.
(106.) The boiler of this engine is so constructed that there is no
part of it, not even excepting the grate-bars, in which metal exposed
to the action of the fire is out of contact with water. If it be
considered how rapidly the action of an intense furnace destroys metal
when water is not present to prevent the heat from accumulating, the
advantage of this circumstance will be appreciated. I have seen the
bars of a new grate, never before used, melted in a single trip
between Liverpool and Manchester; and the inventor of another form of
locomotive engine has admitted to me that his grate-bars, though of a
considerable thickness, would not last more than a week. In the boiler
of Mr. _Gurney_, the grate-bars themselves are tubes filled with
water, and form, in fact, a part of the boiler itself. This boiler
consists of three strong metal cylinders placed in a horizontal
position one above the other. A section, made by a perpendicular or
vertical plane, is represented in fig. 62. The ends of the three
cylinders, just mentioned, are represented at D, H, and I. In the side
of the lowest cylinder D are inserted a row of tubes, a ground plan of
which is represented in fig. 63. These tubes, proceeding from the side
of the lowest cylinder D, are inclined slightly upwards, for a reason
which I shall presently explain. From the nature of the section, only
one of these tubes is visible in fig. 62. at C. The other extremities
of these tubes at A are connected with the same number of upright
tubes, one of which is expressed at E. The upper extremities G of
these upright tubes are connected with another set of tubes K, equal
in number, proceeding from G, inclining slightly upwards, and
terminating in the second cylinder H.
[Illustration: _Fig. 62._]
[Illustration: _Fig. 63._]
An end view of the boiler is exhibited in fig. 64., where the three
cylinders are expressed by the same letters. Between the cylinders D
and H there are two tubes of communication B, and two similar tubes
between the cylinders H and I. From the nature of the section these
appear only as a single tube in fig. 62. From the top of the cylinder
I proceeds a tube N, by which steam is conducted to the engine.
[Illustration: _Fig. 64._]
It will be perceived that the space F is enclosed on every side by a
grating of tubes, which have free communication with the cylinders D
and H, which cylinders have also a free communication with each other
by the tubes B. It follows, therefore, that if water be supplied to
the cylinder I, it will descend through the tubes, and first filling
the cylinder D and the tubes C, will gradually rise in the tubes B and
E, will next fill the tubes K and the cylinder H. The grating of water
pipes C E K forms the furnace, the pipes C being the fire-bars, and
the pipes E and K being the back and roof of the stove. The fire-door,
for the supply of fuel, appears at M, fig. 64. The flue issuing
between the tubes F is conducted over the tubes K, and the flame and
hot air are carried off through a chimney. That portion of the heat of
the burning fuel, which in other furnaces destroys the bars of the
grate, is here expended in heating the water contained in the tubes C.
The radiant heat of the fire acts upon the tubes K, forming the roof
of the furnace, on the tube E at the back of it, and partially on the
cylinders D and H, and the tubes B. The draft of hot air and flame
passing into the flue at A, acts upon the posterior surfaces of the
tubes E, and the upper sides of the tubes K, and finally passes into
the chimney.
As the water in the tubes C E K is heated, it becomes specifically
lighter than water of a less temperature, and consequently acquires a
tendency to ascend. It passes, therefore, rapidly into H. Meanwhile
the colder portions descend, and the inclined positions of the tubes C
and K give play to this tendency of the heated water, so that a
prodigiously rapid circulation is produced, when the fire begins to
act upon the tubes. When the water acquires such a temperature that
steam is rapidly produced, steam bubbles are constantly formed in the
tubes surrounding the fire; and if these remained stationary in the
tubes, the action of the fire would not only decompose the steam, but
render the tubes red hot, the water not passing through them to carry
off the heat. But the inclined position of the tubes, already noticed,
effectually prevents this injurious consequence. A steam bubble which
is formed either in the tubes C or K, having a tendency to ascend
proportional to its lightness as compared with water, necessarily
rushes upwards; if in C towards A, and if in K towards H. But this
motion of the steam is also aided by the rapid circulation of the
water which is continually maintained in the tubes, as already
explained, otherwise it might be possible, notwithstanding the levity
of steam compared with water, that a bubble might remain in a narrow
tube without rising. I notice this more particularly, because the
burning of the tubes is a defect which has been erroneously, in my
opinion, attributed to this boiler. To bring the matter to the test of
experiment, I have connected two cylinders, such as D and H, by a
system of glass tubes, such as represented at C E K. The rapid and
constant circulation of the water was then made evident: bubbles of
steam were formed in the tubes, it is true; but they passed with great
rapidity into the upper cylinder, and rose to the surface, so that the
glass tubes never acquired a higher temperature than that of the water
which passed through them.
This I conceive to be the chief excellence of Mr. _Gurney's_ boiler.
It is impossible that any part of the metal of which it is formed can
receive a greater temperature than that of the water which it
contains; and that temperature, as is obvious, can be regulated with
the most perfect certainty and precision. I have seen the tubes of
this boiler, while exposed to the action of the furnace, after that
action has continued for a long period of time, and I have never
observed the soot which covers them to redden, as it would do if the
tube attained a certain temperature.
Every part of the boiler being cylindrical, it has the form which,
mechanically considered, is most favourable to strength, and which,
within given dimensions, contains the greatest quantity of water. It
is also free from the defects arising from unequal expansion, which
are found to be most injurious in tubular boilers. The tubes C and K
can freely expand in the direction of their length, without being
loosened at their joints, and without straining any part of the
apparatus; the tubes E, being short, are subject to a very slight
degree of expansion; and it is obvious that the long tubes, with which
they are connected, will yield to this without suffering a strain, and
without causing any part of the apparatus to be loosened.
When water is converted into steam, any foreign matter which may be
combined with it is disengaged, and is deposited on the bottom of the
vessel in which the water is evaporated. All boilers, therefore,
require occasional cleansing, to prevent the crust thus formed from
accumulating; and this operation, for obvious reasons, is attended
with peculiar difficulty in tubular boilers. In the case before us,
the crust of deposited matter would gather and thicken in the tubes C
and K, and if not removed, would at length choke them. But besides
this, it would be attended with a still worse effect; for, being a bad
conductor, it would intercept the heat in its transit from the fire to
the water and would cause the metal of the tube to become unduly
heated. Mr. _Gurney_ of course foresaw this inconvenience, and
contrived an ingenious chemical method of removing it by occasionally
injecting through the tubes such an acid as would combine with the
deposite, and carry it away. This method was perfectly effectual; and
although its practical application was found to be attended with
difficulty in the hands of common workmen, Mr. _Gurney_ was persuaded
to adhere to it by the late Dr. _Wollaston_, until experience proved
the impossibility of getting it effectually performed, under the
circumstances in which boilers are commonly used. Mr. _Gurney_ then
adopted the more simple, but not less effectual, method of removing
the deposite by mechanical means. Opposite the mouths of the tubes,
and on the other side of the cylinders D and H, are placed a number of
holes, which, when the boiler is in use, are stopped by pieces of
metal screwed into them. When the tubes require to be cleaned these
stoppers are removed, and an iron scraper is introduced through the
holes into the tubes, which being passed backwards and forwards,
removes the deposite. The boiler may be thus cleaned by a common
labourer in half a day, at an expense of about 1_s._ 6_d._
The frequency of the periods at which a boiler of this kind requires
cleaning must depend, in a great degree, on the nature of the water
which is used; one in daily use with the water of the river Thames
would not require cleaning more than once in a month. Mr. _Gurney_
states that with water of the most unfavourable description, once a
fortnight would be sufficient.
(107.) In the more recent boilers constructed by Mr. _Gurney_, he has
maintained the draught through the furnace, by the method of
projecting the waste steam into the chimneys; a method so perfectly
effectual, that it is unlikely to be superseded by any other. The
objection which has been urged against it in locomotive engines,
working on turnpike roads, is, that the noise which it produces has a
tendency to frighten horses.
In the engines on the Liverpool road, the steam is allowed to pass
directly from the eduction pipe of the cylinder to the chimney, and it
there escapes in puffs corresponding with the alternate motion of the
pistons, and produces a noise, which, although attended with no
inconvenience on the railroad, would certainly be objectionable on
turnpike roads. In the engine used in Mr. _Gurney's_ steam-carriage,
the steam which passes from the cylinders is conducted to a
receptacle, which he calls a blowing box. This box serves the same
purpose as the upper chamber of a smith's bellows. It receives the
steam from the cylinders in alternate puffs, but lets it escape into
the chimney in a continued stream by a number of small jets. Regular
draught is by this means produced, and no noise is perceived. Another
exit for the steam is also provided, by which the conductor is enabled
to increase or diminish, or to suspend altogether, the draught in the
chimney, so as to adapt the intensity of the fire to the exigencies of
the road. This is a great convenience in practice; because, on some
roads, a draught is scarcely required, while on others a powerful
blast is indispensable.
Connected with this blowing box, is another ingenious apparatus of
considerable practical importance. The pipe through which the water
which feeds the boiler is conducted to it from the tank is carried
through this blowing box, within which it is coiled in a spiral form,
so that an extensive thread of the feeding water is exposed to the
heat of the waste steam which has escaped from the cylinders, and
which is enclosed in this blowing box. In passing through this pipe
the feeding water is raised from the ordinary temperature of about 60°
to the temperature of 212°. The fuel necessary to accomplish this is,
therefore, saved, and the amount of this is calculated at 1/6th of all
that is necessary to evaporate the water. Thus, 1/6th of the expense
of fuel is saved. But, what is much more important in a locomotive
engine, a portion of the weight of the engine is saved without any
sacrifice of its power. There is still another great advantage
attending this process. The feeding water in the worm just mentioned,
while it takes up the heat from the surrounding steam in the blowing
box, condenses 1/6th of the waste steam, which is thence conducted to
the tank, from which the feeding water is pumped, saving in this
manner 1/6th in weight and room of the water necessary to be carried
in the carriage for feeding the boiler.[33]
[Footnote 33: In boilers constructed for stationary purposes, or for
steam navigation, the steam-pipe, after it has passed through the
blowing box, is continued and made to form a series of returned flues
over the boiler, so as to take up the waste heat after it has passed
the boiler, and before it reaches the chimney. But in locomotive
engines for common roads, it has been found by experience, that the
power gained by the waste heat is not sufficient to propel the weight
of the material necessary for taking it up.]
So far as the removal of all inconvenience arising from noise, this
contrivance has been proved by experience to be perfectly
effectual.[34]
[Footnote 34: See Report of the Commons.]
In all boilers, the process of violent ebullition causes a state of
agitation in the water, and a number of counter currents, by which, as
the steam is disengaged from the surface of the water, it takes with
it a considerable quantity of water in mechanical mixture. If this be
carried through the cylinders, since it possesses none of the
qualities of steam, and adds nothing to the power of the vapour with
which it is combined, it causes an extensive waste of heat and water,
and produces other injurious effects. In every boiler therefore, some
means should be provided for the separation of the water thus
suspended in the steam, before the steam is conducted to the cylinder.
In ordinary plate boilers, the large space which remains above the
surface of the water serves this purpose. The steam being there
subject to no agitation or disturbance, the water mechanically
suspended in it descends by its own gravity, and leaves pure steam in
the upper part. In the small tubular boilers, this has been a matter,
however, of greater difficulty. The contracted spaces in which the
ebullition takes place, causes the water to be mixed with the steam in
a greater quantity than could happen in common plate boilers: and the
want of the same steam-room renders the separation of the water from
the steam a matter of some difficulty. These inconveniences have been
overcome by a succession of contrivances of great ingenuity. I have
already described the rapid and regular circulation effected by the
arrangement of the tubes. By this a regularity in the currents is
established, which alone has a tendency to diminish the mixture of
water with the steam. But in addition to this, a most effectual method
of separation is provided in the vessel I, which is a strong iron
cylinder of some magnitude, placed out of the immediate influence of
the fire. A partial separation of the steam from the water takes place
in the cylinder H; and the steam with the water mechanically suspended
in it, technically called moist steam, rises into the _separator_ I.
Here, being free from all agitation and currents, and being, in fact,
quiescent, the particles of water fall to the bottom, while the pure
steam remains at the top. This separator, therefore, serves all the
purposes of the steam-room above the surface of the water in the large
plate boilers. The dry steam is thus collected and ready for the
supply of the engine through the tube N, while the water, which is
disengaged from it, is collected at the bottom of the separator, and
is conducted through the tube T to the lowest vessel D, to be again
circulated through the boiler.
The pistons of the engine work on the axles of the hind wheels of the
carriage which bears the engine, by cranks, as in the locomotives on
the Manchester railway, so that the axle is kept in a constant state
of rotation while the engine is at work. The wheels placed on this
axle are not permanently fixed or keyed upon it, as in the Manchester
locomotives; but they are capable of turning upon it in the same
manner as ordinary carriage wheels. Immediately within these wheels
there are fixed upon the axles two projecting spokes or levers, which
revolve with the axle, and which take the position of two opposite
spokes of the wheel. These may be occasionally attached to the wheel
or detached from it; so that they are capable of compelling the wheels
to turn with the axle, or leaving the axle free to turn independent of
the wheel, or the wheel independent of the axle, at the pleasure of
the conductor. It is by these levers that the engine is made to propel
either or both of the wheels. If both pairs of spokes are thrown into
connexion with the wheels, the crank shaft or axle will cause both
wheels to turn with it, and in that case the operation of the carriage
is precisely the same as those of the locomotives already described
upon the Liverpool and Manchester line; but this is rarely found to be
necessary, since the adhesion of one wheel with the road is generally
sufficient to propel the carriage, and consequently only one pair of
these fixed levers are generally used, and the carriage propelled by
only one of the two hind wheels. The fore wheels of the carriage turn
upon a pivot similar to those of a four-wheeled coach. The position of
these wheels is changed at pleasure by a simple pinion and circular
rack, which is moved by the conductor, and in this manner the carriage
is guided with precision and facility.
The force of traction necessary to propel a carriage upon common
roads must vary with the variable quality of the road, and
consequently the propelling power, or the pressure upon the pistons of
the engine, must be susceptible of a corresponding variation; but a
still greater variation becomes necessary from the undulations and
hills which are upon all ordinary roads. This necessary change in the
intensity of the impelling power is obtained by restraining the steam
in the boiler by the throttle valve, as already described in the
locomotive engines on the railroad. This principle, however, is
carried much further in the present case. The steam in the boiler may
be at a pressure of from 100 to 200 lbs. on the square inch; while the
steam on the working piston may not exceed 30 or 40 lbs. on the inch.
Thus an immense increase of power is always at the command of the
conductor; so that when a hill is encountered, or a rough piece of
road, he is enabled to lay on power sufficient to meet the exigency of
the occasion.
The two difficulties which have been always apprehended in the
practical working of steam carriages upon common roads are, first, the
command of sufficient power for hills and rough pieces of road; and,
secondly, the apprehended insufficiency of the adhesion of the wheels
with the road to propel the carriage. The former of these difficulties
has been met by allowing steam of a very great pressure to be
constantly maintained in the boiler with perfect safety. As to the
second, all experiments tend to show that there is no ground for the
supposition that the adhesion of the wheels is in any case
insufficient for the purposes of propulsion. Mr. _Gurney_ states, that
he has succeeded in driving carriages thus propelled up considerable
hills on the turnpike roads about London. He made a journey to Barnet
with only one wheel attached to the axle, which was found sufficient
to propel the carriage up all the hills upon that road. The same
carriage, with only one propelling wheel, also went to Bath, and
surmounted all the hills between Cranford Bridge and Bath, going and
returning.
A double stroke of the piston produces one revolution of the
propelling wheels, and causes the carriage to move through a space
equal to the circumference of those wheels. It will therefore be
obvious, that the greater the diameter of the wheels, the better
adapted the carriage is for speed; and, on the other hand, wheels of
smaller diameter are better adapted for power. In fact, the propelling
power of an engine on the wheels will be in the inverse proportion to
their diameter. In carriages designed to carry great weights at a
moderate speed, smaller wheels will be used; while in those intended
for the transport of passengers at considerable velocities, wheels of
at least 5 feet in diameter are most advantageous.
Among the numerous popular prejudices to which this new invention has
given rise, one of the most mischievous in its effects, and most
glaring in its falsehood, is the notion that carriages thus propelled
are more injurious to roads than carriages drawn by horses. This error
has been most clearly and successfully exposed in the evidence taken
before the committee of the House of Commons upon steam carriages. It
is there fully demonstrated, not only that carriages thus propelled
did not wear a turnpike road more rapidly than those drawn by horses,
but that, on the other hand, the wear by the feet of horses is far
more rapid and destructive than any which could be produced by the
wheels of carriages. Steam carriages admit of having the tires of the
wheels broad, so as to act upon the road more in the manner of
rollers, and thereby to give consistency and firmness to the material
of which the road is composed. The driving wheels, being fully proved
not to slip upon the road, do not produce any effects more injurious
than the ordinary rolling wheels; consequently the wear occasioned by
a steam carriage upon a road is not more than that produced by a
carriage drawn by horses of an equivalent weight and the same or equal
tires; but the wear produced by the pounding and digging of horses'
feet in draught is many times greater than that produced by the wear
of any carriage. Those who still have doubts upon this subject, if
there be any such persons, will be fully satisfied by referring to the
evidence which accompanies the report of the committee of the House of
Commons, printed in October, 1831. In that report they will not only
find demonstrative evidence that the introduction of steam carriages
will materially contribute to the saving in the wear of turnpike
roads, but also that the practicability of working such carriages with
a great saving to the public--with great increase of speed and other
conveniences to the traveller--is fully established.
The weight of machinery necessary for steam carriages is sometimes
urged as an objection to their practical utility. Mr. _Gurney_ states,
that, by successive improvements in the details of the machinery, the
weight of his carriages, without losing any of the propelling power,
may be reduced to 35 cwt., exclusive of the load and fuel and water:
but thinks that it is possible to reduce the weight still further.
A steam carriage constructed by Mr. _Gurney_, weighing 35 cwt.,
working for 8 hours, is found, according to his statement, to do the
work of about 30 horses. He calculates that the weight of his
propelling carriage, which would be capable of drawing 18 persons,
would be equal to the weight of 4 horses; and the carriage in which
these persons would be drawn would have the same weight as a common
stage coach capable of carrying the same number of persons. Thus the
weight of the whole--the propelling carriage and the carriage for
passengers taken together--would be the same with the weight of a
common stage coach, with 4 horses inclusive.
(108.) There are two methods of applying locomotives upon common roads
to the transport of passengers or goods; the one is by causing the
locomotive to carry, and the other to draw the load; and different
projectors have adopted the one and the other method. Each is attended
with its advantages and disadvantages. If the same carriage transport
the engine and the load, the weight of the whole will be less in
proportion to the load carried; also a greater pressure may be
produced on the wheels by which the load is propelled. It is also
thought that a greater facility in turning and guiding the vehicle,
greater safety in descending the hills, and a saving in the original
cost, will be obtained. On the other hand, when the passengers are
placed in the same carriage with the engine, they are necessarily more
exposed to the noise of the machinery and to the heat of the boiler
and furnace. The danger of explosion is so slight, that, perhaps, it
scarcely deserves to be mentioned; but still _the apprehension_ of
danger on the part of the passengers, even though groundless, should
not be disregarded. This apprehension will be obviously removed or
diminished by transferring the passengers into a carriage separate
from the engine; but the greatest advantage of keeping the engine
separate from the passengers is the facility which it affords of
changing one engine for another in case of accident or derangement on
the road, in the same manner as horses are changed at the different
stages: or, if such an accident occur in a place where a new engine
cannot be procured, the load of passengers may be carried forward by
horses, until it is brought to some station where a locomotive may be
obtained. There is also an advantage arising from the circumstance
that when the engines are under repair, or in process of cleaning, the
carriages for passengers are not necessarily idle. Thus the same
number of carriages for passengers will not be required when the
engine is used to draw as when it is used to carry.
In case of a very powerful engine being used to carry great loads, it
would be quite impracticable to place the engine and loads on four
wheels, the pressure being such as no turnpike road could bear. In
this case it would be indispensably necessary to place a part of the
load at least upon separate carriages to be drawn by the engine.
In the comparison of carriages propelled by steam with carriages drawn
by horses, there is no respect in which the advantage of the former
is so apparent as the safety afforded to the passenger. Steam power is
under the most perfect control, and a carriage thus propelled is
capable of being guided with the most admirable precision. It is also
capable of being stopped almost suddenly, whatever be its speed; it is
capable of being turned within a space considerably less than that
which would be necessary for four-horse coaches. In turning sharp
corners, there is no danger, with the most ordinary care on the part
of the conductor. On the other hand, horse-power, as is well known, is
under very imperfect control, especially when horses are used, adapted
to that speed which at present is generally considered necessary for
the purpose of travelling. "The danger of being run away with and
overturned," says Mr. _Farey_, in his evidence before the House of
Commons, "is greatly diminished in a steam coach. It is very difficult
to control four such horses as can draw a heavy stage coach ten miles
an hour, in case they are frightened or choose to run away; and, for
such quick travelling, they must be kept in that state of courage that
they are always inclined to run away, particularly down hill, and at
sharp turns in the road. Steam power has very little corresponding
danger, being perfectly controllable, and capable of having its power
reversed to retard in going down hill. It must be carelessness that
would occasion the overturning of a steam carriage. The chance of
breaking down has been hitherto considerable, but it will not be more
than in stage coaches when the work is truly proportioned and properly
executed. The risk from explosion of the boiler is the only new cause
of danger, and that I consider not equivalent to the danger from
horses."
That the risk of accident from explosion is very slight indeed, if any
such risk exists, may be proved from the fact that the boilers used on
the Liverpool and Manchester railroad being much larger, and, in
proportion, inferior in strength to those of Mr. _Gurney_, and other
steam carriage projectors, have never yet been productive of any
injurious consequences by explosion, although they have frequently
burst. I have stood close to a locomotive on the railroad when the
boiler burst. The effect was that the water passed through the tubes
into the fire and extinguished it, but no other consequence ensued.
In fig. 65. is represented the appearance of a locomotive of Mr.
_Gurney's_ drawing after it a carriage for passengers.
[Illustration: Fig. 65.]
(109.) One of the greatest difficulties which locomotives upon a
turnpike road have to encounter is the ascent of very steep hills, for
it is agreed upon all hands that hills of very moderate inclinations
present no difficulty which may not be easily overcome, even in the
present state of our knowledge. The fact of Mr. _Gurney_ having
propelled his carriage up Old Highgate Hill, when the apparatus was in
a a much more imperfect state than that to which it has now attained,
establishes the mere question of the possibility of overcoming the
difficulty; but it remains still to be decided whether the
inconvenience caused by providing means of meeting the exigency of
very steep hills may not be greater than the advantage of being able
to surmount them can compensate for: and Mr. _Farey_, whose authority
upon subjects of this kind is entitled to the highest respect, thinks
that it is upon the whole more advantageous to provide, at very steep
hills, post horses to assist the steam carriage up them, than to incur
the inconvenience of providing the necessary power and strength of
machinery for occasions which at best but rarely occur. If the
question merely referred to the command of motive power, it appears
to me that Mr. _Gurney's_ boiler would be amply sufficient to supply
all that could be required for any hills which occur upon turnpike
roads; but it is not to be forgotten, that not merely an ample supply
of motive power, but also a strength and weight in the machinery
proportionate to the power to be exerted, is indispensably necessary.
The strength and weight necessary to ascend a very steep hill will be
considerably greater than that which is necessary for a level road, or
for hills of moderate inclinations; and it follows that if we ascend
those steep hills by the unaided power of the locomotive, we must load
the engine with all the weight of machinery requisite for such
emergencies, such additional weight being altogether unnecessary, and
therefore a serious impediment upon all other parts of the road,
inasmuch as it must exclude an equivalent weight of goods or
passengers, which might otherwise be transported, and thereby in fact
diminish proportionally the efficiency of the machine. It is right,
however, to observe, that this is a point upon which a difference of
opinion is entertained by persons equally competent to form a
judgment, and that some consider that it is practicable to construct
an engine without inconvenient weight which will ascend all the hills
which occur upon turnpike roads.
However this may be, the difficulty is one which the improved system
of roads in England renders of a comparatively trifling nature. If
horses were resorted to as the means of assistance up such hills as
the engine would be incapable of surmounting, such aid would not be
requisite more than twice or thrice upon the mail-coach road between
London and Holyhead; and the same may be said of the roads connecting
the greatest points of intercourse in the kingdom. Such hills as the
ascent at Pentonville upon the New Road, the ascent in St. James's
Street, the ascent from Waterloo Place to the County Fire Office, the
ascent at Highgate Archway, present no difficulty whatever. It is only
Old Highgate Hill, and hills of a similar kind, which would ever
require a supply of horses in aid of the engine. I therefore incline
to agree with Mr. _Farey_, that, at least for the present, it will be
more expedient to construct carriages adapted to surmount moderate
hills only, and to provide post horses in aid of the extreme
emergencies to which I have just alluded.
(110.) In the boiler to be used in the steam carriage projected by Mr.
_Walter Hancock_, the subdivision of the water is accomplished by
dividing a case or box by a number of thin plates of metal like a
galvanic battery, the water being allowed to flow between every
alternate pair of plates, at E, fig. 66., and the intermediate spaces
H forming the flue through which the flame and hot air are propelled.
[Illustration: Fig. 66.]
In fact, a number of thin plates of water are exposed on both sides to
the most intense action of flame and heated air; so that steam of a
high-pressure is produced in great abundance and with considerable
rapidity. The plates forming the boiler are bolted together by strong
iron ties, extending across the boiler, at right angles to the
plates, as represented in the figure. The distance between the plates
is two inches.
There are ten flat chambers of this kind for water, and intermediately
between them ten flues. Under the flues is the fire-place, or grate;
containing six square feet of fuel in vivid combustion. The chambers
are all filled to about two thirds of their depth with water, and the
other third is left for steam. The water-chambers, throughout the
whole series, communicate with each other both at top and bottom, and
are held together by two large bolts. By releasing these bolts, at any
time, the chambers fall asunder; and by screwing them up they may be
all made tight again. The water is supplied to the boiler by a
forcing-pump, and the steam issues from the centre of one of the flues
at the top.
These boilers are constructed to bear a pressure of 400 or 500 lbs. on
the square inch; but the average pressure of the steam on the safety
valve is from 60 to 100. There are 100 square feet of surface in
contact with the water exposed to the fire. The stages which such an
engine performs are eight miles, at the end of which a fresh supply of
fuel and water are taken in. It requires about two bushels of coke for
each stage.
The steam carriage of Mr. _Hancock_ differs from that of Mr. _Gurney_
in this,--that in the former the passengers and engine are all placed
on the same carriage. The boiler is placed behind the carriage; and
there is an engine-house between the boiler and the passengers, who
are placed on the fore part of the vehicle; so that all the machinery
is behind them. The carriages are adapted to carry 14 passengers, and
weigh, exclusive of their load, about 3-1/2 tons, the tires of the
wheels being about 3-1/2 inches in breadth. Mr. _Hancock_ states, that
the construction of his boiler is of such a nature, that, even in the
case of bursting, no danger is to be apprehended, nor any other
inconvenience than the stoppage of the carriage. He states that, while
travelling about 9 miles an hour, and working with a pressure of
about 100 lbs. on the square inch, loaded with 13 passengers, the
carriage was suddenly stopped. At first the cause of the accident was
not apparent; but, on opening one of the cocks of the boiler, it was
found that it contained neither steam nor water. Further examination
proved that the boiler had burst. On unscrewing the bolts, it was
found there were several large holes in the plates of the
water-chamber, through which the water had flowed on the fire; but
neither noise nor explosion, nor any dangerous consequences ensued.
This boiler has some obvious defects. It is evident that thin flat
plates are the form which, mechanically considered, is least
favourable to strength; nor does it appear that any material advantage
is gained to compensate for this by the magnitude of the surface
exposed to the action of the fire. It is a great defect that a part of
the surface of each of the plates is exposed to the action of the fire
while it is out of contact with the water; in fact, in the upper part
of the spaces marked E, fig. 66., steam only is contained. It has been
observed by engineers, and usually shown by experiment, that if steam
be heated on the surface of water it will be decomposed, and its
elasticity destroyed; this is not the only evil connected with the
arrangement, for on this part of the metal, nevertheless, the fire
acts,--with less intensity, it is true, than on that part which
contains the water,--but still with sufficient intensity to destroy
the metal. Mr. _Hancock_ appears to have attempted to remedy this
defect by occasionally inverting the position of the flat chambers,
placing that which at one time was at the bottom at the top, and _vice
versâ_. This may equalise the wear produced by the action of the fire
upon the metal out of contact with water, but still the wear on the
whole will not be less rapid. There appears to be no provision or
space for separating the steam from the water with which it is
charged; in fact, there are no means in this engine of discharging the
function of Mr. _Gurney's_ separator. This will be found to produce
considerable waste and loss of power in practice.
The bars upon which the fire rests are of solid metal; and such is the
intense heat to which they are subject, that, in an engine constantly
at work, it is unlikely that they will last, without being renewed,
more than about a week, if so much. The draft is maintained in this
engine by means of a revolving fan worked by the engine. This,
perhaps, is one of the greatest defects as compared with other
locomotives. The quantity of power requisite to work this bellows, and
of which the engine is robbed, is very great. This defect is so fatal,
that I consider it is quite impossible that the ingenious inventor can
persevere in the use of it. Mr. _Hancock_ has abandoned the use of the
cranks upon his working axle, and has substituted an endless chain and
rag-wheels. This also appears to me defective, and a source by which
considerable power is lost. On the other hand, however, the weakness
of the axle which is always produced by cranks is avoided.
(111.) Mr. _Nathaniel Ogle_ of Southampton obtained a patent for a
locomotive carriage, and worked it for some time experimentally; but
as his operations do not appear to have been continued, I suppose he
was unsuccessful in fulfilling those conditions, without which the
machine could not be worked with economy and profit. In his evidence
before a committee of the House of Commons, he has thus described his
contrivance:--
"The base of the boiler and the summit are composed of cross pieces,
cylindrical within and square without; there are holes bored through
these cross pieces, and inserted through the whole is an air tube. The
inner hole of the lower surface, and the under hole of the upper
surface, are rather larger than the other ones. Round the air tube is
placed a small cylinder, the collar of which fits round the larger
aperture on the inner surface of the lower frame, and the under
surface of the upper frame-work. These are both drawn together by
screws from the top; these cross pieces are united by connecting
pieces, the whole strongly bolted together; so that we obtain, in one
tenth of the space, and with one tenth of the weight, the same heating
surface and power as is now obtained in other and low-pressure
boilers, with incalculably greater safety. Our present experimental
boiler contains 250 superficial feet of heating surface in the space
of 3 feet 8 inches high, 3 feet long, and 2 feet 4 inches broad, and
weighs about 8 cwt. We supply the two cylinders with steam,
communicating by their pistons with a crank axle, to the ends of which
either one or both wheels are affixed, as may be required. One wheel
is found to be sufficient, except under very difficult circumstances,
and when the elevation is about one foot in six, to impel the vehicle
forward.
"The cylinders of which the boiler is composed are so small as to bear
a greater pressure than could be produced by the quantity of fire
beneath the boiler; and if any one of these cylinders should be
injured by violence, or any other way, it would become merely a safety
valve to the rest. We never, with the greatest pressure, burst, rent,
or injured our boiler; and it has not once required cleaning, after
having been in use twelve months."
(112.) Dr. _Church_ of Birmingham has obtained a succession of patents
for contrivances connected with a locomotive engine for stone roads;
and a company, consisting of a considerable number of individuals,
possessing sufficient capital, has been formed in Birmingham for
carrying into effect his designs, and working carriages on his
principle. The present boiler of Dr. _Church_ is formed of copper. The
water is contained between two sheets of copper, united together by
copper nails, in a manner resembling the way in which the cloth
forming the top of a mattress or cushion is united with the cloth
which forms the bottom of it, except that the nails or pins, which
bind the sheets of copper, are much closer together. The water, in
fact, seems to be "quilted" or "padded," in between two sheets of
thin copper. This double sheet of copper is formed into an oblong
rectangular box, the interior of which is the fire-place and ash-pit,
and over the end of which is the steam-chest. The great extent of
surface exposed to the immediate action of the fire causes steam to be
produced with great rapidity.
An obvious defect which such a boiler presents is the difficulty of
removing from it any deposite or incrustation, which may collect
between the sheets of copper so closely and intricately connected. Dr.
Church proposes to effect this, when it is required, by the use of an
acid, which will combine readily with the incrustation, and by which
the boiler may therefore be washed. This method of cleansing boilers
was recommended by Dr. Wollaston to Mr. Gurney, who informed me,
however, that he found that it was not practicable in the way in which
boilers must commonly be used.
I apprehend, also, that the spaces between the sheets of copper, in
Dr. Church's boiler just described, will hardly permit the steam
bubbles which will be formed to escape with sufficient facility into
the steam-chest; and being retained in that part of the boiler which
is exposed to the action of the fire, the metal will be liable to
receive an undue temperature.
I have, however, seen this engine working, and its performance was
very satisfactory.
(113.) Various other projects for steam carriages on common roads are
in various degrees of advancement, among which may be mentioned those
of Messrs. Maudslay and Field, Col. Macerone, and Mr. Russell of
Edinburgh; but our limits compel us to omit any detailed account of
them.
CHAPTER XII.
STEAM NAVIGATION.
Propulsion by paddle-wheels. -- Manner of driving them. -- Marine
Engine. -- Its form and arrangement. -- Proportion of its
cylinder. -- Injury to boilers by deposites and incrustation. --
Not effectually removed by _blowing out_. -- Mr. Samuel Hall's
condenser. -- Its advantages. -- Originally suggested by Watt. --
Hall's _steam saver_. -- Howard's vapour engine. -- Morgan's
paddle-wheels. -- Limits of steam navigation. -- Proportion of
tonnage to power. -- Average speed. -- Consumption of fuel. --
Iron steamers. -- American steam raft. -- Steam navigation to
India. -- By Egypt and the Red Sea to Bombay. -- By same route to
Calcutta. -- By Syria and the Euphrates to Bombay. -- Steam
communication with the United States from the west coast of
Ireland to St. John's, Halifax, and New York.
(114.) Among the various ways in which the steam engine has ministered
to the social progress of our race, none is more important and
interesting than the aid it has afforded to navigation. Before it lent
its giant powers to that art, locomotion over the waters of the deep
was attended with a degree of danger and uncertainty, which seemed so
necessary and so inevitable, that, as a common proverb, it became the
type and representative of everything else which was precarious and
perilous. The application, however, of steam to navigation has rescued
the mariner from much of the perils of the winds and waves; and even
in its actual state, apart from the improvements which it is still
likely to receive, it has rendered all voyages of moderate length as
safe and as regular as journeys over land. We are even now upon the
brink of such improvements as will probably so extend the powers of
the steam engine, as to render it available as the means of connecting
the most distant points of the earth.
The manner in which the steam engine is commonly applied to propel
vessels must be so familiar as to require but short explanation. A
pair of wheels, like common under-shot water-wheels, bearing on their
rims a number of flat boards called _paddle boards_, are placed one at
each side of the vessel, in such a position that when the vessel is
immersed to her ordinary depth the lowest paddle boards shall be
submerged. These wheels are fixed upon a shaft, which is made to
revolve by cranks placed upon it, in the same manner as the fly-wheel
of a common steam engine is turned. It is now the invariable custom to
place in steam vessels two engines, each of which works a crank: these
two cranks are placed at right angles to each other, in the same
manner as the cranks already described upon the working axles of
locomotive engines. When either crank is at its dead point, the other
is in full activity, so that the necessity for a fly wheel is
superseded. The engines may be either condensing or high-pressure
engines; but in Europe the low-pressure condensing engine has been
invariably used for nautical purposes. In the United States, where
steam navigation had its origin, and where it was, until a recent
period, much more extensively practised than in Europe, less objection
was felt to the use of high-pressure engines; and their limited bulk,
their small original cost, and simplicity of structure, strongly
recommended them, more especially for the purposes of river
navigation.
(_g._) The original type of nearly all the engines used in steam
navigation was that constructed at Soho by Watt and Bolton for Mr.
Fulton, and first used by him upon the Hudson river. This had the beam
below the piston-rod as in the English boat-engines, but the cylinder
above deck, as in the American. From this primitive form, the two
nations have diverged in opposite directions. The Americans,
navigating rivers, and having speed for their principal object, have
not hesitated to keep the cylinder above deck, and have lengthened the
stroke of the piston in order to make the power cut on a more
advantageous point of the wheel. Compactness has been gained by the
suppression of the working beam.
On the other hand, the English, having the safe navigation of stormy
seas as their more important object, have shortened the cylinder in
order that the piston-rod may work wholly under the deck, and the
arrangement of Fulton's working beam has been retained by them. In
this way there can be no doubt, that they have lost the power of
obtaining equal speed from a given expenditure of power, and those
conversant in the practice and theory of stowing ships may well doubt
whether security is not also sacrificed.--A. E.
The arrangement of the parts of the maritime engine differs in some
respects, from that of the land engine. Want of room renders greater
compactness necessary; and in order to diminish the height of the
machine, the working beam is transferred from above the cylinder to
below it. In fact, there are two beams, one at each side of the
engine, which are connected by a parallel motion with the piston, the
rods of the parallel motion extending from the lower part of the
engine to the top of the piston-rod. The working end of the beam is
connected with the crank by a connecting rod, presented upwards
instead of downwards, as in the land engine. The proportion of the
length and diameters of the cylinders differ from those of land
engines for a like reason: to save height, short cylinders with large
diameters are used. Thus, in an engine of 200 horse-power, the length
of the cylinder is sometimes 60 inches, and its diameter 53 inches:
the valves and the gearing which work them, the air-pump, condenser,
and other parts of the machine, do not differ materially from those
already described in the land engines.
(_h._) The action of machinery may be rendered more equable by using
two engines, each of half the power, instead of a single one. If one
of these be working with its maximum force when the other is changing
the direction of its motion, the result of their joint action will be
a force nearly constant. Such a combination was invented by Mr.
Francis Ogden, and has been used in several steam boats constructed
under his directions. It would however be far more valuable in other
cases, particularly where great uniformity in the velocity is
indispensable.
This method has now become almost universal in the engines used in the
English steam boats, each of which has usually two, both applied to
the same shaft, and therefore capable of being used singly or together
to turn the paddle wheels. In the American steam boats, although two
engines have been often applied, each usually acts upon no more than
one of the wheels. We can see no other good reason for this, than that
our engineers do not wish to be thought to copy Mr. Ogden.--A. E.
The nature of the work which the marine engine has to perform, is
such, that great regularity of action is neither necessary nor
possible. The agitation of the surface of the sea will cause the
immersion of the paddle-wheels to vary very much, and the resistance
to the engine will undergo a corresponding change: the governor, and
other parts of the apparatus already described, contrived for
imparting to the engine that extreme regularity which is indispensable
in its application to manufactures, are therefore here omitted; and
nothing is introduced except what is necessary to maintain the engine
in its full working power.
It is evident that it must be a matter of considerable importance to
reduce the space occupied by the machinery on board a vessel to the
least possible dimensions. The marine boilers, therefore, are
constructed so as to yield the necessary quantity of steam with the
smallest practical dimensions. With this view a much more extensive
surface in proportion to the size of the boiler is exposed to the
action of the fire. In fact, the flues which carry off the heated air
to the chimney are conducted through the boiler, so as to act upon
the water on every side in thin oblong shells, which traverse the
boiler backward and forward repeatedly, until finally they terminate
in the chimney. By this arrangement the original expense of the
boilers is very considerably increased; but, on the other hand, their
steam-producing power is also greatly augmented; and from experiments
lately made by Mr. Watt at Birmingham, it appears that they work with
an economy of fuel compared with common land boilers in the proportion
of about two to three. Thus they have the additional advantage of
saving the tonnage as well as the expense of one third of the fuel.
One of the most formidable difficulties which has been encountered in
applying the steam engine to the purposes of navigation has arisen
from the necessity of supplying the boiler with sea water, instead of
pure fresh water. This water (also used for the purpose of
condensation) being injected into the condenser and mixed with the
condensed steam, is conducted as feeding water into the boiler.
The salt contained in the sea water, not being evaporated, remains in
the boiler. In fact, it is separated from the water in the same manner
as by the process of distillation. As the evaporation in the boiler is
continued, the proportion of salt contained in the water is,
therefore, constantly increased, until a greater proportion is
accumulated than the water is capable of holding in solution; a
deposition of salt then commences, and is lodged in the cavities at
the bottom of the boiler. The continuance of this process, it is
evident, would at length fill the boiler with salt.
But besides this, under some circumstances, a deposition of lime[35]
is made, and a hard incrustation is formed on the inner surface of the
boiler. In some situations, also, sand and mud are received into the
boiler, being suspended in the water pumped in for feeding it. All
these substances, whether deposited in a loose form in the lower parts
of the boiler or collected in a crust on its inner surface, form
obstructions to the passage of heat from the fire to the water. The
crust thus formed is not unfrequently an inch or more in thickness,
and so hard that good chisels are broken in removing it. The heat more
or less intercepted by these substances collects in the metal of the
boiler, and raises it to a temperature far exceeding that of the water
within. It may even, if the incrustation be great, be sufficient to
render the boiler red-hot. These circumstances occasion the rapid wear
of the boiler, and endanger its safety by softening it.
[Footnote 35: Ten thousand grains of pure sea water contain muriate of
soda 220 grs., sulphate of soda 33 grs., muriate of magnesia 42 grs.,
and muriate of lime 8 grs.]
The remedy which has generally been adopted to remove or diminish
these injurious effects consists in allowing a stream of hot water
continually to flow from the boiler, and supplying from the feed-pipe
a corresponding portion of cold water. While the hot water which flows
from the boiler in this case contains, besides its just proportion of
salt, that portion which has been liberated from the water converted
into vapour, the cold water which is supplied through the feed-pipe
contains less than its just proportion of salt, since it is composed
of the natural sea water, mixed with the condensed steam, which latter
contains no salt. In this manner, the proportion of the salt in the
boiler may be prevented from accumulating; but this is attended with
considerable inconvenience and loss. It is evident that the discharge
of the hot water, and the introduction of so considerable a quantity
of cold water, entails upon the machine a great waste of fuel, and,
consequently, renders it necessary that the vessel should be supplied
with a much larger quantity of coals than are merely necessary for
propelling it. In long voyages, where this inconvenience is most felt,
this is a circumstance of obvious importance. But besides the waste of
fuel, the speed of the vessel is diminished by the rate of evaporation
in the boiler being checked by the constant stream of cold water
flowing into it. This process of discharging the water, which is
called _blowing out_, is only practised occasionally. In the Admiralty
steamers, the engineers are ordered to blow out every two hours. But
it is more usual to do so only once a day.
This method, however, of blowing out furnishes but a partial remedy
for the evils we have alluded to: a loose deposite will perhaps be
removed by such means, but an incrustation, more or less according to
the circumstances and quality of the water, will be formed; besides
which, the temptation to work the vessel with efficiency for the
moment influences the engine men to neglect blowing out; and it is
found that this class of persons can rarely be relied upon to resort
to this remedy with that constancy and regularity which are essential
for the due preservation of the boilers. The class of steam vessels
which, at present, are exposed to the greatest injury from these
causes are the sea-going steamers employed by the Admiralty; and we
find, by a report made by Messrs. Lloyd and Kingston to the Admiralty,
in August, 1834, that it is admitted that the method of blowing out
is, even when daily attended to, ineffectual. "The water in the
boiler," these gentlemen observe, "is kept from exceeding a certain
degree of saltness, by periodically blowing a portion of it into the
sea; but whatever care is taken, in long voyages especially, salt will
accumulate, and sometimes in great quantities and of great hardness,
so that it is with difficulty it can be removed. Boilers are thus
often injured as much in a few months as they would otherwise be in as
many years. The other evil necessarily resulting from this state of
things is, besides the rapid destruction of the boilers, a great waste
of fuel, occasioned by the difficulty with which the heat passes
through the incrustation on the inside, by the leaks which are thereby
caused, and by the practice of blowing out periodically, as before
mentioned, a considerable portion of the boiling water."
It would be impracticable to carry on board the vessel a sufficient
quantity of pure fresh water to work the engine exclusively by its
means. To accomplish this, it would be necessary to have a sufficient
supply of cold water to keep the condensing cistern cold, to supply
the jet in the condenser, and to have a reservoir in which the warm
water coming from the waste pipe of the cold cistern might be allowed
to cool. Engineers have therefore directed their attention to some
method by which the steam may be condensed without a jet, and after
condensation be preserved for the purpose of feeding the boiler. If
this could be accomplished, it would not be necessary to provide a
greater quantity of pure water than would be sufficient to make up the
small portion of waste which might proceed from leakage and from other
causes; and it is evident that this portion might always be readily
obtained by the distillation of sea water, which might be effected by
a small vessel exposed to the same fire which acts upon the boiler.
(115.) Mr. _Samuel Hall_ of _Basford_, near Nottingham, has taken out
patents for a new form of condenser, contrived for the attainment of
these ends, besides some other improvements in the engine.
The condenser of Mr. _Hall_ consists of a great number of narrow tubes
immersed in a cistern of cold water: the steam as it passes from the
cylinder, after having worked the piston, enters these tubes, and is
immediately condensed by their cold surfaces. It flows in the form of
water from their remote extremities, and is drawn off by the air-pump,
and conducted in the usual way to a cistern from which the boiler is
fed. In the marine engines constructed under Mr. _Hall_'s patents, the
tubes of the condenser being in an upright or vertical position, the
steam flows from the cylinder into the upper part of the condenser,
which is a low flat chamber, in the bottom of which is inserted the
upper extremities of the tubes, through which the steam passes
downwards, and as it passes is condensed. It flows thence into a
similar chamber below, from whence it is drawn off by the air-pump.
It is evident that at sea an unlimited supply of cold water may be
obtained to keep the condensing cistern cold, so that a perfect
condensation may always be effected by these tubes, if they be made
sufficiently small. The water formed by the condensed steam will be
pure distilled water; and if the boiler be originally filled with
water which does not hold in solution any earthy or other matter which
might be deposited or encrusted, it may be worked for any length of
time without injury. The small quantity of waste from leakage is
supplied in Mr. Hall's engine by a simple apparatus in which a
sufficient quantity of sea water may be distilled.
The following are the advantages, as stated by Mr. Hall, to be gained
by his condenser:--
1. A saving of fuel, amounting in some cases to so much as a third of
the ordinary consumption.
2. The preservation of the boilers from the destruction produced in
common engines by the corrosive action of sea or other impure water,
and by encrustations of earthy matter.
3. The saving of the time spent in cleaning the boilers.
4. A considerable increase of power, owing to the cleanness of the
boilers; the absence of injected water to be pumped out of a vacuum;
the greater perfection of the vacuum; the better preservation of the
piston and valves of the air-pump; and (by another contrivance of his)
the more perfect lubrication of the parts of the engine.
5. The water in the boiler being constantly maintained at the same
height by self-acting arrangement.
6. The size of a boiler exerting a given power, being much smaller
than the common kind, owing to its more perfect action.
Messrs. _Lloyd_ and _Kingston_ were employed by government to examine
and report the effects of Mr. Hall's boilers, and they stated in their
report, already referred to, that the result is so successful as to
leave nothing to be wished for. Among the advantages which they
enumerate are the increased durability of the engines; the prevention
of accidents through carelessness, or otherwise, arising from the
condenser and air-pump becoming choked with injection water; and the
additional security against the boilers being burnt in consequence of
the water being suffered to get too low. But the greatest advantages,
compared with which they consider all others to be of secondary
importance, are the increased durability of the boilers and the saving
of fuel.
About 16 engines, built either wholly upon Mr. Hall's principle or
having his condenser attached to them, have now (October, 1835) been
working in different parts of England, and on board different vessels
for various periods, from three years to three months; and it appears
from the concurrent testimony of the proprietors and managers of them,
that they are attended with all the advantages which the patentee
engaged for. The part of the contrivance the performance of which
would have appeared most doubtful would have been the maintenance of a
sufficiently good vacuum in the condenser, in the absence of the usual
method of condensation by the injection of cold water; nevertheless it
appears that a better vacuum is sustained in these engines than in the
ordinary engines which condense by jet. The barometer-gauge varies
from 29 to 29-1/2 inches, and in some cases comes up to 30 inches,
according to the state of the barometer: this is a vacuum very nearly
perfect, and indeed may be said to be so for all practical purposes.
The _Prince Llewellyn_ and the _Air_ steam packets, belonging to the
St. George Steam Packet Company, have worked such a pair of these
engines for about a year. The _City of London_ steam packet, the
property of the General Steam Navigation Company, has been furnished
with two fifty-horse engines, and has worked them during the same
period. In all cases the boilers have been found perfectly free from
scale or incrustation; and the deposite is either absolutely nothing
or very trifling, requiring the boiler to be swept about once in half
a year, and sometimes not so often. The trial which has been made of
these engines in the navy has proved satisfactory, so far as it has
been carried. The Lords of the Admiralty have lately ordered a pair of
seventy-horse engines to be constructed on this principle for a vessel
now (October, 1835) in process of construction;[36] and another vessel
in all respects similar, except having copper boilers, is likewise
ordered; so that a just comparison may be made. It would, however,
have been more fair if both vessels had been provided with iron
boilers, since copper does not receive incrustation as readily as
iron.
[Footnote 36: This order has, as Mr. Hall informs me, been given
without requiring any guarantee as to the performance of the engines.]
It would seem that the advantages of these boilers in the vessels of
the St. George Steam Packet Company were regarded by the directors as
sufficiently evident, since, after more than a year's experience, they
are about to place a pair of ninety-horse engines of this kind in a
new and powerful steamer called the _Hercules_.
Engines furnished with Mr. Hall's apparatus have not yet, so far as I
am informed, been tried with reference to the power exerted by the
consumption of a given quantity of fuel. The mere fact of a good
vacuum being sustained in the condenser cannot by itself be regarded
as a conclusive proof of the efficiency of the engine, without the
water or air introduced by a condensing jet. Mr. Hall, nevertheless,
uses as large an air-pump as that of an ordinary condensing engine,
and recommends even a larger one. For what purpose, it may be asked,
is such an appendage introduced? If there be nothing to be removed but
the condensed steam, a very small pump ought to be sufficient. It is
not wonderful that a good vacuum is sustained in the condenser, if the
power expended on the air-pump is employed in _pumping away
uncondensed steam_. Such a contrivance would be merely a deception,
giving an apparent but no real advantage to the engine.
Having mentioned these advantages, which are said to arise from Mr.
Hall's condenser, it is right to state that it is in fact a
reproduction of an early invention of Mr. Watt. There is in possession
of James Watt, esquire, a drawing of a condenser laid before
parliament in 1776, in which the same method of condensing without a
jet is proposed. Mr. Watt, however, finding that he could not procure
by that means so sudden or so perfect a vacuum as by injection,
abandoned it. I believe he also found that the tubes of the condenser
became furred with a deposite which impeded the process of
condensation. It would seem, however, that Mr. Hall has found means to
obviate these effects. It is right to add, that Mr. Hall, in his
specification, distinctly disclaims all claim to the method of
condensing by tubes without jet.
There is another part of Mr. Hall's contrivance which merits notice.
In all engines, a considerable quantity of steam is allowed to escape
from the safety valve. Whenever the vessel stops, the steam, which
would otherwise be taken from the boiler by the cylinders, passes out
through this valve into the atmosphere. Also, whenever the cylinders
work at under-power, and do not consume the steam as fast as it is
produced by the boiler, the surplus steam escapes through the valve.
Now, according to the principle of Mr. Hall's method, it is necessary
to save the water which thus escapes in vapour, since otherwise the
pure water of the boiler would be more rapidly wasted. Mr. Hall
accordingly places a safety valve of peculiar construction in
communication with a tube which leads to the condenser, so that
whenever, either by stopping the engine or diminishing its working
power, steam accumulates in the boiler, its increased pressure opens
the safety valve, and it passes through this pipe to the condenser,
where it is reconverted into water, and pumped off by the air-pump
into the cistern from which the boiler is fed.
The attainment of an object so advantageous as to extend the powers
of steam navigation, and to render the performance of voyages of any
length practicable, so far as the efficiency of the machinery is
concerned, has naturally stimulated the inventive genius of the
country. The preservation of the boiler by the prevention of deposite
and incrustation is an object of paramount importance; and its
attainment necessarily involves, to a certain degree, another
condition on which the extension of steam voyages must depend, viz.
the economy of fuel. In proportion as the economy of fuel is
increased, in the same proportion will the limit to which steam
navigation may be carried be extended.
(116.) A patent has been obtained by Mr. Thomas Howard of London for a
form of engine possessing much novelty and ingenuity, and having
pretensions to the attainment of a very extraordinary economy of fuel,
in addition to those advantages which have been already explained as
attending Mr. Hall's engines. In these engines, as in Mr. Hall's, the
steam is constantly reproduced from the same water, so that pure or
distilled water may be used; but Mr. Howard dispenses with the use of
a boiler altogether. The steam also with which he works is in a state
essentially different from the steam used in ordinary engines. In
these, the vapour is raised directly from the water in a boiling
state, and it contains as much water as it is capable of holding at
its temperature. Thus, at the temperature of 212°, a cubic foot of
steam used in common engines will contain about a cubic inch of water;
but in the contrivance of Mr. Howard, a considerable quantity of heat
is imparted to the steam before it passes into the cylinder in
addition to what is necessary to maintain it in the vaporous form.
A quantity of mercury is placed in a shallow wrought-iron vessel over
a coke fire, by which it is maintained at the temperature of from 400°
to 500°. The surface exposed to the fire is three fourths of a square
foot for each horse-power. The upper surface of the mercury is covered
by a very thin plate of iron, which rests in contact with it, and
which is so contrived as to present about four times as much surface
as that exposed beneath to the fire. Adjacent to this a vessel of
water is placed, kept heated nearly to the boiling-point, which
communicates by a nozzle and valve with the chamber or vessel
immediately above the mercury. At intervals corresponding to the
motion of the piston, a small quantity of water is injected from this
vessel, and thrown upon the plate of iron which rests upon the hot
mercury: from this it receives the heat necessary not only to convert
it into steam, but to expand that steam, and raise it to a temperature
above the temperature it would receive if raised in immediate contact
with water. In fact, the steam thus produced will have a temperature
not corresponding to its pressure, but considerably above that point,
and it will therefore be in circumstances under which it will part
with more or less of its heat, and allow its temperature to be lowered
without being even partially condensed, whereas steam used in the
ordinary steam engines must be more or less condensed by the slightest
diminution of its temperature. The quantity of liquid injected into
the steam chamber must be regulated by the power at which the engine
is intended to work. The fire is supplied with air by a blowing
machine, which is subject to exact regulation. The steam, produced in
the manner already explained, passes into a chamber which surrounds
the working cylinder; and this chamber itself is enclosed by another
space, through which the air from the furnace must pass before it
reaches the flue. In this way it imparts its redundant heat to the
steam which is about to work the cylinder, and raises it to a
temperature of about 400°; the pressure, however, not exceeding 25
lbs. per square inch. The arrangement of valves for the admission of
the steam to the cylinder is such as to cause the steam to act
expansively.
The vacuum on the opposite side of the piston is maintained by
condensation in the following manner:--The condenser is a copper
vessel placed in a cistern constantly supplied with cold water, and
the steam flows to it from the cylinder by an eduction pipe in the
usual way: a jet is admitted to it from an adjacent vessel, which,
before the engine commences work, is filled with distilled water; the
condensing water and condensed steam are pumped from the condenser by
air-pumps of the usual construction, but smaller, inasmuch as there is
no air to be withdrawn, as in common engines. The warm water thus
pumped out of the condenser is driven into a copper pipe or worm,
which is carried with many coils through a cistern of cold water, so
that when it arrives at the end of this pipe it is reduced to the
common temperature of the atmosphere. The pipe is then conducted into
the vessel of distilled water already mentioned, and the water flowing
from it continually replaces the water which flows into the condenser
through the condensing jet. The condensing water being purged of air,
a very small air-pump is sufficient; since it has only to exhaust the
condenser and tubes at starting, and to remove whatever air may enter
by casual leakage. The patentee states that the condensation takes
place as rapidly and as perfectly as in the best steam engine, and it
is evident that this method of condensation is applicable even where
the mercurial generator already described may not be employed. The
vessel from which the water is injected into the mercurial generator
is likewise fed by the air-pump connected with the condenser. There is
another pipe besides the copper worm already described, which is
carried from the hot well to this vessel, and the water is of course
returned through it without being cooled. This vessel is likewise
sufficiently exposed to the action of the fire to maintain it at a
temperature somewhat below the boiling point.
An apparatus of this construction was in the spring of the present
year (1835) placed in the Admiralty steamer called the COMET, in
connexion with a pair of 40-horse engines. The patentee states that
these engines were ill adapted to the contrivance; nevertheless, the
vessel was successfully worked in the Thames for 800 miles: she also
performed a voyage from Falmouth to Lisbon, but was prevented from
returning by an accident which occurred to the machinery near the
latter port. In this experimental voyage, the consumption of fuel is
stated never to have exceeded a third of her former consumption, when
worked by Bolton and Watt's engines; the former consumption of coals
being about 800 lbs. per hour, and the consumption with Mr. Howard's
engine being under 250 lbs. of coke per hour.
After this failure (which, however, was admitted to be one of accident
and not of principle) the government did not consider itself justified
in bestowing further time or incurring greater expense in trying this
engine. Mr. Howard, however, has himself built a new vessel, in which
he is about to place a pair of forty-horse engines. This vessel is now
(December, 1835) nearly ready, and will bring the question to issue by
a fair experiment. The advantages of the contrivance as enumerated by
the patentee are:--
_First_, The small space and weight occupied by the machinery, arising
from the absence of a boiler.
_Secondly_, The diminished consumption of fuel.
_Thirdly_, The reduced size of the flues.
_Fourthly_, The removal of the injurious effects arising from deposite
and incrustation.
_Fifthly_, The absence of smoke.
Some of these improvements, if realized, will be attended with
important advantages in steam navigation. Steamers of a given tonnage
and power will have more disposable space for lading and fuel, and in
short voyages may carry greater freight, or an increased number of
passengers; or by taking a larger quantity of fuel,[37] may make
greater runs than are now attainable; or, finally, with the same
tonnage and the same lading, they may be supplied with more powerful
machinery.
[Footnote 37: The fuel used in this form of engine is coke, and not
coal. A ton of coke occupies the same space as two tons of coal; the
saving of tonnage, therefore, by the increased economy of fuel will
not be in so great a proportion as the saving of fuel. A quantity of
fuel of equivalent power will occupy about half the present space, but
the displacement or immersion which it produces will be only one
fourth of its present effect.]
(117.) To obtain from the moving power its full amount of mechanical
effect in propelling the vessel, it would be necessary that its force
should propel, by constantly acting against the water in a horizontal
direction, and with a motion contrary to the course of the vessel. No
system of mechanical propellers has, however, yet been contrived
capable of perfectly attaining this end. Patents have been granted for
many ingenious mechanical combinations to impart to the propelling
surfaces such angles as appeared to the respective contrivers most
advantageous. In most of these, however, the mechanical complexity has
formed a fatal objection. No part of the machinery of a steam vessel
is so liable to become deranged at sea as the paddle-wheels; and,
therefore, such simplicity of construction as is compatible with those
repairs which are possible on such emergencies is quite essential for
safe practical use.
[Illustration: Fig. 67.]
The ordinary paddle-wheel, as I have already stated, is a wheel
revolving upon a shaft driven by the engine, and carrying upon its
circumference a number of flat boards, called paddle boards, which are
secured by nuts or braces in a fixed position; and that position is
such that the planes of the paddle boards diverge nearly from the
centre of the shaft on which the wheel turns. The consequence of this
arrangement is that each paddle board can only act in that direction
which is most advantageous for the propulsion of the vessel when it
arrives near the lowest point of the wheel. In figure 67. let o be
the shaft on which the common paddle wheel revolves; the position of
the paddle boards are represented at A, B, C, &c.; X, Y represents the
water line, the course of the vessel being supposed to be from X to Y;
the arrows represent the direction in which the paddle-wheel revolves.
The wheel is immersed to the depth of the lowest paddle board, since a
less degree of immersion would render a portion of the surface of each
paddle board mechanically useless. In the position A the whole force
of the paddle board is efficient for propelling the vessel; but, as
the paddle enters the water in the position H, its action upon the
water, not being horizontal, is only partially effective for
propulsion: a part of the force which drives the paddle is expended in
depressing the water, and the remainder in driving it contrary to the
course of the vessel, and, therefore, by its reaction producing a
certain propelling effect. The tendency, however, of the paddle
entering the water at H, is to form a hollow or trough, which the
water, by its ordinary property, has a continual tendency to fill up.
After passing the lowest point A, as the paddle approaches the
position B, where it emerges from the water, its action again becomes
oblique, a part only having a propelling effect, and the remainder
having a tendency to raise the water, and throw up a wave and spray
behind the paddle-wheel. It is evident that the more deeply the
paddle-wheel becomes immersed the greater will be the proportion of
the propelling power thus wasted in elevating and depressing the
water; and, if the wheel were immersed to its axis, the whole force of
the paddle boards, on entering and leaving the water, would be lost,
no part of it having a tendency to propel. If a still deeper immersion
takes place, the paddle boards above the axis would have a tendency to
retard the course of the vessel. When the vessel is, therefore, in
proper trim, the immersion should not exceed nor fall short of the
depth of the lowest paddle; but for various reasons it is impossible
in practice to maintain this fixed immersion: the agitation of the
surface of the sea, causing the vessel to roll, will necessarily
produce a great variation in the immersion of the paddle-wheels, one
becoming frequently immersed to its axle, while the other is raised
altogether out of the water. Also the draught of water of the vessel
is liable to change, by the variation in her cargo: this will
necessarily happen in steamers which take long voyages. At starting
they are heavily laden with fuel, which as they proceed is gradually
consumed, whereby the vessel is lightened; and it does not appear that
it is practicable to use sea water as ballast to restore the proper
degree of immersion.
(118.) Among the contrivances which have been proposed for remedying
these defects of the common paddle wheel by introducing paddle boards
capable of shifting their position as they revolve with the
circumference of the wheel, the only one which has been adopted to any
considerable extent in practice is that which is commonly known as
Morgan's Paddle Wheel. The original patent for this contrivance was
granted to Elijah Galloway, and sold by him to Mr. William Morgan.
Subsequently to the purchase some improvements in its structure and
arrangements were introduced, and it is now extensively adopted by
Government in the Admiralty steamers. It was first tried on board His
Majesty's steamer the CONFIANCE; and after several successful
experiments was ordered by the Lords of the Admiralty to be introduced
on board the FLAMER, the FIREBRAND, the COLUMBIA, the SPITFIRE, the
LIGHTNING, a large war steamer called the MEDEA,[38] the TARTARUS, the
BLAZER, &c. It has been tried by Government in several well-conducted
experiments, where two vessels of precisely the same model, supplied
with similar engines of equal power, and propelled, one by Morgan's
paddle-wheels, and the other by the common paddle-wheels; when it was
found that the advantage of the former, whether in smooth or in rough
water, was quite apparent. One of the commanders in these experiments
(Lieutenant Belson) states that the improvement in the speed of the
CONFIANCE, after being supplied with these wheels, was proportionately
greater in a sea way than in smooth water; that their action was not
impeded by the waves, since the variation of the velocity of the
engine did not exceed one or two revolutions per minute: the vessel's
way was never stopped, and there was no sensible increase of vibration
on the paddle boxes during the gale. Another commander reported that
on a comparison of the CONFIANCE and a similar and equally powerful
vessel, the CARRON, the CONFIANCE performed in fifty-four hours the
voyage which occupied the CARRON eighty-four hours in running.
Independently of the great saving of fuel effected (namely, ten
bushels per hour[39]), or the time saved in running the same
distance, other advantages have been secured by the modification in
question. On a comparison of the respective logs of the two vessels,
it appeared that the CONFIANCE had gained by the alteration in her
wheels an increase of speed amounting to 2 knots on 7 in smooth water,
and 2-1/2 knots on 4 to 4-1/2 knots in rough weather; that the action
of the paddles did not bring up the engine or retard their velocity in
a head sea; that in rolling their action assisted in righting the
vessel; and that the wear and strain, as well on the vessel as on the
engines, were materially reduced. With respect to the durability of
these wheels, the commander of the FLAMER reported in January, 1834,
that in six weeks of the most tempestuous weather they found them to
act remarkably well, without even a single float being shifted.[40]
[Footnote 38: This splendid ship is 860 tons burden, with engines of
220 horse power.]
[Footnote 39: See Report quoted in Mechanic's Magazine, vol. xxii. p.
275. This saving cannot amount to less than 40 per cent. upon the
whole consumption of fuel; it certainly is considerably beyond what I
should have conceived to be possible; but I have no reason to doubt
the accuracy of the Report. I estimate the former consumption at 10
pounds per horse power per hour, which on 220 horse-power would be
2200 pounds, of which 840 pounds = 10 bushels, would be about
4-10ths.]
[Footnote 40: See a more detailed account of these reports in the
Mechanic's Magazine, vol. xxii. p. 274., from which I have taken the
drawing of this paddle-wheel; and also see Report of the Committee of
the House of Commons on Steam Navigation to India, Evidence of William
Morgan, p. 95.]
This paddle-wheel is represented in figure 68. The contrivance may be
shortly stated to consist in causing the wheel which bears the paddles
to revolve on one centre, and the radial arms which move the paddles
to revolve on another centre. Let A B C D E F G H I be the polygonal
circumference of the paddle-wheel, formed of straight bars, securely
connected together at the extremities of the spokes or radii of the
wheel which turns on the shaft which is worked by the engine; the
centre of this wheel being at O. So far this wheel is similar to the
common paddle-wheel; but the paddle boards are not, as in the common
wheel, fixed at A B C, &c., so as to be always directed to the centre
O, but are so placed that they are capable of turning on axles which
are always horizontal, so that they can take any angle with respect to
the water which may be given to them. From the centres, or the line
joining the pivots on which these paddle boards turn, there proceed
short arms K, firmly fixed to the paddle boards at an angle of about
120°. On a motion given to this arm K, it will therefore give a
corresponding angular motion to the paddle board, so as to make it
turn on its pivots. At the extremities of the several arms marked K is
a pin or pivot, to which the extremities of the radial arms L are
severally attached, so that the angle between each radial arm L and
the short paddle arm K is capable of being changed by any motion
imparted to L; the radial arms L are connected at the other end with a
centre P, round which they are capable of revolving. Now since the
points A B C, &c., which are the pivots on which the paddle boards
turn, are moved in the circumference of a circle, of which the centre
is O, they are always at the same distance from that point;
consequently they will continually vary their distance from the other
centre P. Thus, when a paddle board arrives at that point of its
revolution at which the centre P lies precisely between it and the
centre O, its distance from P is less than in any other position. As
it departs from that point, its distance from the centre P gradually
increases until it arrives at the opposite point of its revolution,
where the centre O is exactly between it and the centre P; then the
distance of the paddle board from the centre P is greatest. This
constant change of distance between each paddle board and the centre P
is accommodated by the variation of the angle between the radial arm L
and the short paddle board arm K; as the paddle board approaches the
centre P this gradually diminishes; and as the distance of the paddle
board from P increases, the angle is likewise augmented. This change
in the magnitude of the angle, which thus accommodates the varying
position of the paddle board with respect to the centre P, will be
observed in the figure. The paddle board D is nearest to P; and it
will be observed that the angle contained between L and K is there
very acute; at E the angle between L and K increases, but is still
acute; at F it increases to a right angle; at G it becomes obtuse; and
at K, where it is most distant from the centre P, it becomes most
obtuse. It again diminishes at I, and becomes a right angle between A
and B. Now this continual shifting of the direction of the short arm K
is necessarily accompanied by an equivalent change of position in the
paddle board to which it is attached; and the position of the second
centre P is, or may be, so adjusted that this paddle board, as it
enters the water and emerges from it, shall be such as shall be most
advantageous for propelling the vessel, and therefore attended with
less of that vibration which arises chiefly from the alternate
depression and elevation of the water, owing to the oblique action of
the paddle boards.[41]
[Footnote 41: A paddle-wheel resembling this has lately been
constructed by Messrs. Seawards. It has been charged by Mr. Morgan as
being a colourable invasion of his patent, and the dispute has been
brought into the courts of law.]
(_i_) The relative value of the two wheels, namely, the common
paddle-wheel, and that of Morgan has been investigated by Professor
Barlow of the Military School, at Woolwich, and the results published
in a paper of much ability in the Philosophical Transactions for 1834.
By this paper it appears, that, when the paddles are not wholly
immersed, the wheel of Morgan has no important advantage over the
other, and only acquires one when the wheel wallows. But the most
important of his inferences is that the common paddle is least
efficient when in a vertical position, contrary to the usual opinion.
From this we have a right to infer that the search for a form of wheel
which shall always keep the paddle vertical is one whose success need
not be attended with any important consequence. The superior qualities
of Morgan's wheel when the paddles are deeply immersed is ascribed by
Barlow to the lessening of the shock sustained by the common
paddle-wheel when it strikes the water. This being the case, the
triple wheel of Stevens is probably superior to that of Morgan in its
efficiency, while it has the advantage of being far simpler and less
liable to be put out of order.--A. E.
(119.) To form an approximate estimate of the limit of the present
powers of steam navigation, it will be necessary to consider the
mutual relation of the capacity or tonnage of the vessel; the
magnitude, weight, and power of the machinery; the available stowage
for fuel; and the average speed attainable in all weathers, as well as
the general purposes to which the vessel is to be appropriated,
whether for the transport of goods and merchandise, or merely of
despatches and passengers. That portion of the capacity of the vessel
which is appropriated to the moving power consists of the space
occupied by the machinery and the space occupied by the fuel; the
magnitude of the latter will necessarily depend upon the length of the
voyage which the vessel must make without receiving a fresh supply of
coals. If the voyage be short, this space may be proportionally
limited, and a greater portion of room will be left for the machinery.
If, on the contrary, the voyage be longer, a greater stock of coals
will be necessary, and a less space will remain for the machinery.
More powerful vessels, therefore, in proportion to their tonnage, may
be used for short than for long voyages.
[Illustration: Pl. XII.]
Taking an average of fifty-one voyages made by the Admiralty steamers,
from Falmouth to Corfu and back during four years ending June, 1834,
it was found that the average rate of steaming, exclusive of
stoppages, was 7-1/4 miles per hour, taken in a direct line between
the places, and without allowing for the necessary deviations in the
course of the vessel. The vessels which performed this voyage varied
from 350 to 700 tons burthen by measurement, and were provided with
engines varying from 100 horse to 200 horse-power, with stowage for
coals varying from 80 to 240 tons. The proportion of the power to the
tonnage varied from 1 horse to 3 tons to 1 horse to 4 tons; thus, the
MESSENGER had a power of 200 horses, and measured 730 tons; the
FLAMER had a power of 120 horses, and measured 500 tons; the COLUMBIA
had 120 horses, and measured 360 tons.
In general, it may be assumed that for the shortest class of trips,
such as those of the Margate steamers, and the packets between
Liverpool or Holyhead and Dublin, the proportion of the power to the
tonnage should be that of 1 horse-power to every 2 tons by measure;
while for the longest voyages the proportion would be reduced to 1
horse to 4 tons, voyages of intermediate lengths having every variety
of intermediate proportion.
Steamers thus proportioned in their power and tonnage may then, on an
average of weathers, be expected to make 7-1/4 miles an hour while
steaming, which is equivalent to 174 miles per day of twenty-four
hours. But, in very long voyages, it rarely happens that a steamer can
work constantly without interruption. Besides stress of weather, in
which she must sometimes lie-to, she is liable to occasional
derangements of her machinery, and more especially of her paddles. In
almost every long voyage hitherto attempted, some time has been lost
in occasional repairs of this nature while at sea. We shall perhaps,
therefore, for long voyages, arrive at a more correct estimate of the
daily run of a steamer by taking it at 160 miles.[42]
[Footnote 42: The American reader will hardly be able to refrain from
a smile at this estimate of Dr. Lardner of the speed of steamboats,
founded upon the most improved practice of Europe at the close of the
year 1835. The boats on the Hudson River have for years past averaged
a speed of 15 miles per hour, and the Lexington, which was constructed
for a navigation part of which is performed in the open ocean, could
probably keep up a speed of the same amount, except in severe storms.
With boats, constructed on the principles of those which navigate Long
Island Sound and the Chesapeake, we should not fear to assume 12 miles
per hour, or, upwards of 280 miles per day as their average rate of
crossing the ocean.--A. E.]
By a series of carefully conducted experiments on the consumption of
coals, under marine boilers and common land boilers, which have been
lately made at the works of Mr. Watt, near Birmingham, it has been
proved that the consumption of fuel under marine boilers is less than
under land boilers, in the proportion of 2 to 3 very nearly. On the
other hand, I have ascertained from general observation throughout the
manufacturing districts in the North of England, that the average
consumption of coals under land boilers of all powers above the very
smallest class is at the rate of 15 pounds of coals per horse-power
per hour. From this result, the accuracy of which may be fully relied
upon, combined with the result of the experiments just mentioned at
Soho, we may conclude that the average consumption of marine boilers
will be at the rate of 10 lbs. of coal per horse power per hour. Mr.
Field, of the firm of Maudslay and Field, in his evidence before a
Select Committee of the House of Commons on Steam Navigation to India,
has stated from his observation, and from experiments made at
different periods, that the consumption is only 8 lbs. per horse-power
per hour. In the evidence of Mr. William Morgan, however, before the
same committee, the actual consumption of fuel on board the
Mediterranean packets is estimated at 16 cwt. per hour for engines of
200 horse power, and 8-1/4 cwt. for engines of 100 horse-power. From
my own observation, which has been rather extensive both with respect
to land and marine boilers, I feel assured that 10 lbs. per hour more
nearly represents the practical consumption than the lower estimate of
Mr. Field. We may then assume the daily consumption of coal by marine
boilers, allowing them to work upon an average for 22 hours, the
remainder of the time being left for casual stoppages, at 220 lbs. of
coal per horse-power, or very nearly 1 ton for every ten horses'
power. In short voyages, where there will be no stoppage, the daily
consumption will a little exceed this; but the distance traversed will
be proportionally greater.
When the proportion of the power to the tonnage remains unaltered,
the speed of the vessel does not materially change. We may therefore
assume that 10 pounds of coal per horse power will carry a sea-going
steamer adapted for long voyages 7-1/4 miles direct distance; and
therefore to carry her 100 miles will require 138 pounds, or the
1/16th part of a ton nearly. Now, the Mediterranean steamers are
capable of taking a quantity of fuel at the rate of 1-1/4 tons per
horse power; but the proportion of their power to their tonnage is
greater than that which would probably be adapted for longer runs. We
shall, therefore, perhaps be warranted in assuming that it is
practicable to construct a steamer capable of taking 1-1/2 tons of
fuel per horse-power. At the rate of consumption just mentioned, this
would be sufficient to carry her 2400 miles in average weather; but as
an allowance of fuel must always be made for emergencies, we cannot
suppose it possible for her to encounter this extreme run. Allowing,
then, spare fuel to the extent of a quarter of a ton per horse-power,
we should have as an extreme limit of a steamer's practicable voyage,
without receiving a relay of coals, a run of about 2000 miles.
(120.) This computation is founded upon results obtained from the use
chiefly of the North of England coal. It has, however, been stated in
evidence before the select committee above mentioned, that the
_Llangennech_ coal of Wales is considerably more powerful. Captain
Wilson, who commanded the HUGH LINDSAY steamer in India, has stated
that this coal is more powerful than Newcastle, in the proportion of 9
to 6-1/2.[43] Some of the commanders of the Mediterranean packets have
likewise stated that the strength of this coal is greater than that of
Newcastle in the proportion of 16 to 11.[44] So far then as relates to
this coal, the above estimate must be modified, by reducing the
consumption nearly in the proportion of 3 to 2.
[Footnote 43: Vide Report of Select Committee on Steam Navigation, p.
152.]
[Footnote 44: Ibid. p. 7.]
The class of vessels best fitted for undertaking long voyages,
without relays of coal, would be one from about 800 to 1000 tons
measurement furnished with engines from 200 to 250 horse-power.[45]
Such vessels could take a supply of from 300 to 400 tons of coals,
which being consumed at the rate of from 20 to 25 tons per day, would
last about fifteen days.
[Footnote 45: Engines in steam vessels generally work considerably
above their nominal power. The power, however, to which we uniformly
refer is the nominal power, or that power at which they would work
with steam of the ordinary pressure.]
Applying these results, however, to particular cases, it will be
necessary to remember that they are average calculations, and must be
subject to such modifications as the circumstances may suggest in the
particular instances: thus, if a voyage is contemplated under
circumstances in which an adverse wind generally prevails, less than
the average speed must be allowed, or, what is the same, a greater
consumption of fuel for a given distance. Against a strong head wind,
in which a sailing vessel would double-reef her top-sails, even a
powerful steamer cannot make more than from 2 to 3 miles an hour,
especially if she has a head sea to encounter.
(121.) In considering the general economy of fuel, it may be right to
state, that the results of experience obtained in the steam navigation
of our channels, and particularly in the case of the Post Office
packets on the Liverpool station, have clearly established the fact,
that by increasing the ratio of the power to the tonnage, an actual
saving of fuel in a given distance is effected, while at the same time
the speed of the vessel is increased. In the case of the Post Office
steamers called the DOLPHIN and the THETIS (Liverpool station,) the
power has been successively increased, and the speed proportionably
augmented; but the consumption of fuel per voyage between Liverpool
and Dublin has been diminished. This, at first view, appears
inconsistent with the known theory of the resistance of solids moving
through fluids; since this resistance increases in the same proportion
as the square of the speed. But this physical principle is founded on
the supposition that the immersed part of the floating body remains
the same. Now I have myself proved by experiments on canals, that when
the speed of the boat is increased beyond a certain limit, its draught
of water is rapidly diminished; and in the case of a large steam raft
constructed upon the river Hudson, it was found that when the speed
was raised to 20 miles an hour, the draught of water was diminished by
7 inches. I have therefore no doubt that the increased speed of
steamers is attended with a like effect; that, in fact, they rise out
of the water; so that, although the resistance is increased by reason
of their increased speed, it is diminished in a still greater
proportion by reason of their diminished immersion.
Meanwhile, whatever be the cause, it is quite certain that the
resistance in moving through the water must be diminished, because the
moving power is always in proportion to the quantity of coals
consumed, and at the same time in the proportion to the resistance
overcome. Since, then, the quantity of coals consumed in a given
distance is diminished while the speed is increased, the resistance
encountered throughout the same distance must be proportionally
diminished.
(122.) Increased facility in the extension and application of steam
navigation is expected to arise from the substitution of iron for
wood, in the construction of vessels. Hitherto iron steamers have been
chiefly confined to river navigation; but there appears no sufficient
reason why their use should be thus limited. For sea voyages they
offer many advantages; they are not half the weight of vessels of
equal tonnage constructed of wood; and, consequently, with the same
tonnage they will have less draught of water, and therefore less
resistance to the propelling power; or, with the same draught of water
and the same resistance, they will carry a proportionally heavier
cargo. The nature of their material renders them more stiff and
unyielding than timber; and they do not suffer that effect which is
called _hogging_, which arises from a slight alteration which takes
place in the figure of a timber vessel in rolling, accompanied by an
alternate opening and closing of the seams. Iron vessels have the
further advantage of being more proof against fracture upon rocks. If
a timber vessel strike, a plank is broken, and a chasm opened in her
many times greater than the point of rock which produces the
concussion. If an iron vessel strike she will either merely receive a
dinge, or be pierced by a hole equal in size to the point of rock
which she encounters. Some examples of the strength of iron vessels
was given by Mr. Macgregor Laird, in his evidence before the Committee
of the Commons on Steam Navigation, among which the following may be
mentioned:--An iron vessel, called the ALBURKAH, in one of their
experimental trials got aground, and lay upon her anchor: in a wooden
vessel the anchor would probably have pierced her bottom; in this
case, however, the bottom was only dinged. An iron vessel, built for
the Irish Inland Navigation Company, was being towed across Lough Derg
in a gale of wind, when the towing rope broke, and she was driven upon
rocks, on which she bumped for a considerable time without any injury.
A wooden vessel would in this case have gone to pieces. A further
advantage of iron vessels (which in warm climates is deserving of
consideration) is their greater coolness and perfect freedom from
vermin.
(123.) The greatest speed which has yet been attained upon water by
the application of steam has been accomplished in the case of a river
steamer of peculiar form, which has been constructed upon the river
Hudson. This boat, or rather raft, consisted of two hollow vessels
formed of thin sheet iron, somewhat in the shape of spindles or cigars
(from whence it was called the _cigar boat_.) In the thickest part
these floats were eight feet in diameter, tapering towards the ends,
and about 300 feet long: these floats or buoys, being placed parallel
to each other, having a distance of more than 16 feet between them,
supported a deck or raft 300 feet long, and 32 feet wide. A
paddle-wheel 30 feet in diameter and 16 feet broad revolved between
the spindles, impelled by a steam engine placed upon the deck. This
vessel drew about 30 inches of water, and attained a speed of from 20
to 25 miles an hour: she ran upon a bank in the river Hudson, and was
lost. The projector is now employed in constructing another vessel of
still larger dimensions. It is evident that such a structure is
altogether unfitted for sea navigation. In the case of a wide
navigable river, however, such as the Hudson, it will no doubt be
attended with the advantage of greater expedition.
(124.) Several projects for the extension of steam navigation to
voyages of considerable length have lately been entertained both by
the public and by the legislature, and have imparted to every attempt
to improve steam navigation increased interest. A committee of the
House of Commons collected evidence and made a report in the last
session in favour of an experiment to establish a line of steam
communication between Great Britain and India. Two routes have been
suggested by the committee, each being a continuation of the line of
Admiralty steam packets already established to Malta and the Ionian
Isles. One of the routes proposed is through Egypt, the Red Sea, and
across the Indian Ocean to Bombay, or some of the other Presidencies;
the other across the north part of Syria to the banks of the
Euphrates, by that river to the Persian Gulf, and from thence to
Bombay. Each of these routes will be attended with peculiar
difficulties, and in both a long sea voyage will be encountered.
In the route by the Red Sea, it is proposed to establish steamers
between Malta and Alexandria (860 miles). A steamer of 400 tons
burthen and 100 horse-power would perform this voyage, upon an average
of all weathers incident to the situation, in from 5 to 6 days,
consuming 10 tons of coal per day. But it is probable that it might be
found more advantageous to establish a higher ratio between the power
and the tonnage. From Alexandria, the transit might be effected by
land across the Isthmus to Suez--a journey of from 4 to 5 days--by
caravan and camels; or the transit might be made either by land or
water from Alexandria to Cairo, a distance of 173 miles; and from
Cairo to Suez, 93 miles, across the desert, in about 5 days. At Suez
would be a station for steamers, and the Red Sea would be traversed in
3 runs or more. If necessary, stations for coals might be established
at Cosseir, Judda, Mocha, and finally at Socatra--an island
immediately beyond the mouth of the Red Sea, in the Indian Ocean: the
run from Suez to Cosseir would be 300 miles--somewhat more than twice
the distance from Liverpool to Dublin. From Cosseir to Judda, 450
miles; from Judda to Mocha, 517 miles; and from Mocha to Socatra, 632
miles. It is evident that all this would, without difficulty, in the
most unfavourable weather, fall within the present powers of steam
navigation. If the terminus of the passage be Bombay, the run from
Socatra to Bombay will be 1200 miles, which would be, upon an average
of weather, about 8 days' steaming. The whole passage from Alexandria
to Bombay, allowing 3 days for delay between Suez and Bombay, would be
26 days: the time from Bombay to Malta would therefore be about 33
days; and adding 14 days to this for the transit from Malta to
England, we should have a total of 47 days from London to Bombay, or
about 7 weeks.
If the terminus proposed were Calcutta, the course from Socatra would
be 1250 miles south-east to the Maldives, where a station for coals
would be established. This distance would be equal to that from
Socatra to Bombay. From the Maldives, a run of 400 miles would reach
the southern point of Ceylon, called the Point de Galle, which is the
best harbour (Bombay excepted) in British India: from the Point de
Galle, a run of 600 miles will reach Madras; and from Madras to
Calcutta would be a run of about 600 miles. The voyage from London to
Calcutta would be performed in about 60 days.
At a certain season of the year there exists a powerful physical
opponent to the transit from India to Suez: from the middle of June
until the end of September, the south-west monsoon blows with unabated
force across the Indian Ocean, and more particularly between Socatra
and Bombay. This wind is so violent as to leave it barely possible for
the most powerful steam packet to make head against it, and the voyage
could not be accomplished without serious wear and tear upon the
vessels during these months--if indeed it would be practicable at all
for any continuance in that season. The attention of parliament has
therefore been directed to another line of communication, not liable
to this difficulty: it is proposed to establish a line of steamers
from Bombay through the Persian Gulf to the Euphrates. The run from
Bombay to a place called Muscat, on the southern shore of the Gulf,
would be 840 miles in a north-west direction, and therefore not
opposed to the south-west monsoon. From Muscat to Bassidore, a point
upon the northern coast of the strait at the mouth of the Persian
Gulf, would be a run of 255 miles; from Bassidore to Bushire, another
point on the eastern coast of the Persian Gulf, would be a run of 300
miles; and from Bushire to the mouth of the Euphrates, would be 120
miles. It is evident that the longest of these runs would offer no
more difficulty than the passage from Malta to Alexandria. From
Bussora near the mouth of the Euphrates, to Bir, a town upon its left
bank near Aleppo, would be 1143 miles, throughout which there are no
physical obstacles to the river navigation which may not be overcome.
Some difficulties arise from the wild and savage character of the
tribes who occupy its banks. It is, however, thought that by proper
measures, and securing the co-operation of the Pacha of Egypt, any
serious obstruction from this cause may be removed. From Bir, by
Aleppo, to Scanderoon, a port upon the Mediterranean, opposite Cyprus,
is a land journey, said to be attended with some difficulty but not of
great length; and from Scandaroon to Malta is about the same distance
as between the latter place and Alexandria. It is calculated that the
time from London to Bombay by the Euphrates--supposing the passage to
be successfully established--would be a few days shorter than by Egypt
and the Red Sea.
Whichever of these courses may be adopted, it is clear that the
difficulties, so far as the powers of the steam engine are concerned,
lie in the one case between Socatra and Bombay, or between Socatra and
the Maldives, and in the other case between Bombay and Muscat. Even
the run from Malta to Alexandria or Scandaroon is liable to objection,
from the liability of the boiler to deposite and incrustation, unless
some effectual method be taken to remove this source of injury. If,
however, the contrivance of Mr. Hall, or of Mr. Howard, or any other
expedient for a like object, be successful, the difficulty will then
be limited to the necessary supply of coals for so long a voyage.
This, however, has already been encountered and overcome on four
several voyages by the HUGH LINDSAY steamer from Bombay to Suez: that
vessel encountered a still longer run on these several trips, by
going, not to Socatra but to Aden, a point on the coast of Arabia near
the Straits of Babel Mandeb, being a run of 1641 miles, which she
performed in 10 days and 19 hours. The entire distance from Bombay to
Suez was in one case performed in 16 days and 16 hours; and under the
most unfavourable circumstances, in 23 days. The average was 21 days
for each trip.
(125.) Another projected line of steam communication, which offers
circumstances of equal interest to the people of these countries and
the United States, is that which is proposed to be established between
London and New York. On the completion of the London and Liverpool
railroad, Dublin will be connected with London, by a continuous line
of steam transport. It is proposed to continue this line by a railroad
from Dublin to some point on the western coast of Ireland; among
others, the harbour of VALENTIA has been mentioned. The nearest point
of the western continent is St. John's, Newfoundland, the distance of
which from Valentia is 1900 miles; the distance from St. John's to New
York is about 1200 miles, Halifax (Nova Scotia) being a convenient
intermediate station. The distance from Valentia to St. John's comes
very near the point which we have already assigned as the probable
present limit of steam navigation. The Atlantic Ocean also offers a
formidable opponent in the westerly winds which almost constantly
prevail in it. These winds are, in fact, the reaction of the trades,
which blow near the equator in a contrary direction, and are produced
by those portions of the equatorial atmosphere which, rushing down the
northern latitudes, carry with them the velocity from west to east
proper to the equator. Besides this difficulty, St. John's and Halifax
are both inaccessible, by reason of the climate, during certain months
of the year. Should these causes prevent this project from being
realized, another course may be adopted. We may proceed from the
southern point of Ireland or England to the Azores, a distance of
about 1800 miles: from the Azores to New York would be a distance of
about 2000 miles, or from the Azores to St. John's would be 1600
miles.[46]
[Footnote 46: A treatise on the Steam Engine is not the place to enter
into discussion on the causes of the several constant, periodic, and
prevailing winds, otherwise we should feel it our duty to correct the
opinion adopted by Dr. Lardner from the older authorities, in relation
to the course of the westerly winds. These winds are, in the South
Atlantic, and in both South and North Pacific, constant winds. In the
North Atlantic between the latitudes, of 35° and 45°, and, therefore,
in the track of the vessels which navigate between the United States
and Great Britain, they are the most frequent prevailing winds, except
in the months of April and October. They are certainly not the
reaction of the trade winds, which is in a well known zone, to the
south of the region in which these westerly winds prevail, under the
name of the Horse latitudes of our navigations, and the Grassy sea of
the Spaniards. Those who wish to study the true theory of these winds
will find it in Daniell's learned and ingenious work, "On Atmospheric
Phenomena," or in the analysis of that work in the American Quarterly
Review.]
(_k_) While the inhabitants of Great Britain are discussing the
project of the communication with New York, by means of the stations
described by Dr. Lardner, those of the United States appear to be
seriously occupied in carrying into effect a direct communication from
New York to Liverpool. At the speed which has been given to the
American steam boats, this presents no greater difficulties than the
voyage from the Azores to New York, would, to one having the speed of
no more than 7-1/4 miles per hour. As this attempt is beyond the limit
of individual enterprise, there is, at the present moment, an
application before the Legislature of the State of New York for a
charter to carry this project into effect. It will be difficult to
estimate the results of this enterprise, which will bring the old and
new world within 12 or 15 days voyage of each other.--A. E.[47]
[Footnote 47: Dr. Lardner appears purposely to have omitted any detail
of the history of Steam Navigation. It would be an invidious task on
the part of a mere editor to attempt to supply what he has thought
proper to avoid. We therefore merely refer to this subject for the
purpose of expressing the hope, that this silence is an earnest that
the writers of Europe are about to abandon the claims they have set up
for their countrymen to the merit of introducing the successful
practice of Steam navigation, and that the respective services of
Fitch, Evans, Fulton, and the elder Stevens will soon be universally
acknowledged.--A. E.]
CHAPTER XII.
GENERAL ECONOMY OF STEAM POWER.
Mechanical efficacy of steam -- proportional to the quantity of
water evaporated, and to the fuel consumed -- Independent of the
pressure. -- Its mechanical efficacy by condensation alone. -- By
condensation and expansion combined -- by direct pressure and
expansion -- by direct pressure and condensation -- by direct
pressure, condensation, and expansion. -- The power of engines.
-- The duty of engines. -- Meaning of horse power. -- To compute
the power of an engine. -- Of the power of boilers. -- The
structure of the grate-bars. -- Quantity of water and steam room.
-- Fire surface and flue surface. -- Dimensions of steam pipes.
-- Velocity of piston. -- Economy of fuel. -- Cornish duty
reports.
(130.) Having explained in the preceding chapters the most important
circumstances connected with the principal varieties of steam engines,
it remains now to explain some matters of detail connected with the
power, efficiency, and economy of these machines, which, though
perhaps less striking and attractive than the subjects which have
hitherto engaged us, are still not undeserving of attention.
It has been shown in the first chapter, that water exposed to the
ordinary atmospheric pressure (the amount of which may be expressed by
a column of 30 inches of mercury) will pass from the liquid into the
vaporous state when it arrives at the temperature of 212°; and the
vapour thus produced from it will have an elastic force equal to that
of the atmosphere. If the water, however, to which heat is applied, be
submitted to a greater or less pressure than that of the atmosphere,
it will boil at a greater or less temperature, and will always produce
steam of an elastic force equal to the pressure under which it boils.
Now it is a fact as remarkable as it is important, that to convert a
given weight of water into vapour will require the same quantity of
heat, under whatever pressure, and at whatever temperature the water
may boil. Let us suppose a tube, the base of which is equal to a
square foot, in which a piston fits air-tight and steam-tight.
Immediately under the piston, let a cubic inch of water be placed,
which will be spread in a thin layer over the bottom of the tube. Let
the piston be counterbalanced by a weight (acting over a pulley) which
will be equivalent to the weight of the piston, so that it shall be
free to ascend by the application of any pressure below it. Now let
the flame of a lamp be applied at the bottom of the tube: the water
under the piston being affected by no pressure from above, except that
of the atmosphere acting upon the piston, will boil at the temperature
of 212°, and by the continued application of the lamp it will at
length be converted into steam. The steam into which the cubic inch of
water is converted will expand into the magnitude of a cubic foot,
exerting an elastic force equal to the atmospheric pressure;
consequently the piston will be raised one foot above its first
position in the tube, and the cubic foot beneath it will be completely
filled with steam. Let us suppose, that to produce this effect
required the lamp to be applied to the tube for the space of fifteen
minutes.
The water being again supposed at its original temperature, and the
piston in its first position, let a weight be placed upon the piston
equal to the pressure of the atmosphere, so that the water beneath the
piston will be pressed down by double the atmospheric pressure. If the
lamp be once more applied, the water will, as before, be converted
into vapour; but the piston will now be raised to the height of only
six inches[48] from the bottom, the steam expanding into only half
its former bulk. The temperature at which the water would commence to
be converted into vapour, instead of being 212°, would be 250°; but
the time elapsed between the moment of the first application of the
lamp, and the complete conversion of the water into steam, will still
be fifteen minutes.
[Footnote 48: Strictly speaking, the height to which the piston would
be raised would not diminish in so great a proportion as the pressure
is increased, because the increase of pressure being necessarily
accompanied by an increase of temperature, a corresponding expansion
would be produced. Therefore there will be a slight increase in the
total mechanical effect of the steam. The difference, however, is not
very important in practice, and it is usual to consider the density of
steam as proportional to the pressure.]
Again, if the piston be loaded with a weight equal to double the
atmospheric pressure, the water will be pressed down by the force of
three atmospheres. If the lamp be applied as before, the water would
be converted into steam in the same time; but the piston will now be
raised only four inches above its first position, and the steam will
consequently be three times as dense as when the piston was pressed
down only by the atmosphere.
From these and similar experiments we infer:--
_First_, That the elastic pressure of steam is equal to the mechanical
pressure under which the water producing the steam has been boiled.
_Secondly_, That the bulk which steam fills is diminished in the same
proportion as the pressure of the steam is increased; or, in other
words, that the density of steam is always in the same proportion as
its pressure.
_Thirdly_, That the same quantity of heat is sufficient to convert the
same weight of water into steam, whatever be the pressure under which
the water is boiled, or whatever be the density and pressure of the
steam produced.
_Fourthly_, That the same quantity of water being converted into
steam, produces the same mechanical effect, whatever be the pressure
or the density of the steam. Thus, in the first case, the weight of
one atmosphere was raised a foot high; in the second case, the weight
of two atmospheres was raised through half a foot; and, in the third
case, the weight of three atmospheres was raised through the third of
a foot; the weight raised being in every case increased in the same
proportion as the height through which it is elevated is diminished.
Every increase of the weight is, therefore, compensated by a
proportionate diminution of the height through which it is raised, and
the mechanical effect is consequently the same.
_Fifthly_, That the same quantity of heat or fuel is necessary and
sufficient to produce the same mechanical effect, whatever be the
pressure of the steam which it produces.
If steam be used to raise a piston against the atmospheric pressure
only, although a definite physical force will be exerted by it, and a
mechanical effect produced, yet under such circumstances it will exert
no directly useful efficiency; but after the piston has been raised,
and the tube beneath it filled with steam balancing the atmosphere
above it, a useful effect to the same amount may be obtained by
cooling the tube, and thereby reconverting the steam into water. The
piston will thus be urged downwards by the unresisted force of the
atmosphere, and any chain or rod attached to it will be drawn
downwards with a corresponding force. If the area of the piston be, as
already supposed, equal to the magnitude of one square foot, the
atmospheric pressure upon it, being 15 pounds for each square inch,
will amount to 144 times 15 pounds, or 2160 pounds. By drawing down a
chain or rope acting over a pulley, the piston would in its descent
(omitting the consideration of friction, &c.) raise a weight of 2160
pounds a foot high. Since 2160 pounds are nearly equal to one ton, it
may, for the sake of round numbers be stated thus:--
"_A cubic inch of water, being converted into steam, will, by the
condensation of that steam, raise a ton weight a foot high._" Such is
the way in which the force of steam is rendered practically available
in the atmospheric engine.
(131.) The method by which steam is used in the single-acting steam
engine of Watt is, in all respects, similar, except that the piston,
instead of being urged downwards by the force of the atmosphere, is
pressed by steam of a force equal to the atmospheric pressure. It is
evident, however, that this does not alter the mechanical result.
We have stated that a considerable increase of power, from a given
quantity of steam, was produced by cutting off the steam after the
piston had made a part of its descent, and allowing the remainder of
the descent to be produced by the expansive force of the steam already
admitted. We shall now more fully explain the principle on which this
increase of power depends.
Let A B (_fig. 69._,) as before, represent a tube, the bottom of which
is equal to a square foot, and let P be a piston in it, resting upon a
cubic inch of water spread over the bottom; and let w be an empty
vessel, the weight of which exactly counterpoises the piston. By the
application of the lamp, the water will be converted into steam of the
atmospheric pressure, and the piston will be raised from P to P´,
through the height of one foot, the space in the tube beneath it being
filled with steam, and the vessel w will have descended through one
foot. Let half a ton of water be now poured into the vessel W; its
weight will draw the piston P´ upwards, so that the steam below it
will expand into a larger space. When the piston P´ was only balanced
by the empty vessel W, it was pressed downwards by the whole weight of
the atmosphere above, which amounts to about one ton: now, however,
half of this pressure is balanced by the half ton of water poured into
the vessel W; consequently the effective downward pressure on the
piston P´ will be only half a ton, or half its former amount. The
piston will therefore rise, until the pressure of the steam below it
is diminished to the same extent. By what has been already explained,
this will take place when the steam is allowed to expand into double
its former bulk; consequently, when the piston has risen to P´´, one
foot higher, or two feet from the bottom of the tube, the steam will
then exactly balance the downward pressure on the piston, and the
latter will remain stationary; the vessel W, with the half ton of
water it contains, will have descended one foot lower, or two feet
below its first position. Let the steam now be cooled and reconverted
into water, and at the same time let another half ton of water be
supplied to the vessel W; the pressure below the piston being entirely
removed, the atmospheric pressure will act above it with undiminished
force; and this force, amounting to one ton, will draw up the vessel
W, with its contents. When the piston descends, as it will do, to the
bottom of the tube, the ton of water contained in the vessel W will be
raised through two perpendicular feet.[49]
[Footnote 49: Strictly speaking, the quantity of water supposed in
these cases to be placed in the vessel W would just _balance_ the
atmospheric pressure. A slight preponderance must therefore be given
to the piston, to produce the motion.]
Now, in this process it will be observed that the quantity of steam
consumed is not more than in the former case, viz. the vapour produced
by boiling one cubic inch of water. Let us consider, however, the
mechanical effect which has resulted from it; half a ton of water has
been allowed to descend through one foot, while a ton has been raised
through two feet: deducting the force lost by the descent of half a
ton through one foot from the force obtained by the ascent of one ton
through the two feet, we obtain for the whole mechanical effect one
ton and a half raised through one foot; for it is evident that half a
ton has been raised from the lowest point to which the vessel W
descended one foot above that point, and one ton has been raised
through the other foot, which is equivalent to one ton and a half
through one foot.
Comparing this with the effect produced in the first case, where the
steam was condensed without causing its expansion, it will be evident
that there is an increase of 50 per cent. upon the whole mechanical
effect produced.
But this is not the limit of the increase of power by expansion.
Instead of condensing the steam when the piston had arrived at P´´,
let a further quantity of water amounting to one sixth of a ton be
poured into the vessel W, in addition to the half ton which it
previously contained; the effective pressure on the piston P´´, being
only half a ton, will be overbalanced by the preponderating weight in
the vessel w, and the piston will consequently ascend. It will become
stationary when the steam by expansion loses a quantity of force equal
to the additional weight which the vessel W has received: now, that
vessel, having successively received a half and a sixth of a ton will
contain two thirds of a ton; consequently the effective downward
pressure on the piston will be only a third of a ton, and the steam to
balance this must expand into three times the space it occupied when
equal, to the atmospheric pressure. It must therefore ascend to P´´´,
three feet above the bottom of the tube. If the steam in the tube be
now condensed, and at the same time one third of a ton of water be
supplied to the vessel W, so as to make its total contents amount to
one ton, the piston will descend, being urged downwards by the
unresisted atmospheric pressure, and the ton of water contained in the
vessel W will be raised through three perpendicular feet.
In this case, as in the former, the total quantity of steam consumed
is that of one cubic inch of water; but the mechanical effect it
produces is still further increased. To calculate its amount, we must
consider that half a ton of water has fallen through two feet, which
is equivalent to a ton falling through one foot, besides which the
sixth part of a ton has fallen through one foot. The total loss,
therefore, by the fall of water has been one ton and one sixth through
one foot, while the force gained by the ascent of water has been one
ton raised through three feet, which is equivalent to three tons
through one foot. If, then, from three tons we deduct one and one
sixth, the remainder will be one ton and five sixths raised through
one foot; this effect being above 80 per cent. more than that which is
produced in the first case, where the steam was not allowed to expand.
To carry the inquiry one step further: Let us suppose that, upon the
arrival of the piston at P´´´, a further addition of water to the
amount of one twelfth of a ton be added to it: this, with the water it
already contained, would make the total contents three fourths of a
ton; consequently, the effective pressure upon the piston would now be
reduced to one fourth of the atmospheric pressure. The atmospheric
steam would balance this when expanded into four times its original
volume: consequently, the piston would come to a state of rest at
P´´´´, four feet above the bottom of the tube, and the vessel W would
consequently have descended through four perpendicular feet. If the
steam in the tube be now condensed as in the former cases, and at the
same time a quarter of a ton of water be added to the vessel W, the
piston will descend to the bottom of the tube, and the ton of water in
the vessel W will be raised through four perpendicular feet. To
estimate the mechanical effect thus produced, we have, as before, to
deduct the total force lost by the fall of water from the force gained
by its elevation: the water has fallen in three distinct portions:
first, half a ton has fallen through three perpendicular feet, which
is equivalent to one ton and a half through one foot; secondly, one
sixth of a ton has fallen through two perpendicular feet, which is
equivalent to one third of a ton through one foot; and thirdly, one
twelfth of a ton has fallen through one foot: these added together
will be equivalent to one ton and eleven twelfths through one foot.
One ton has been raised through four feet, which is equivalent to four
tons through one foot: deducting from this the force lost by the
descent, the surplus gained will be two tons and one twelfth through
one foot, being about 108 per cent. more than the force resulting from
the condensation of steam without expansion.
To the increase of mechanical effect to be produced in this way, there
is no theoretical limit. According to the manner in which we have here
explained it, to produce the greatest possible effect by a given
extent of expansion, it would be necessary to supply the water or
other counterpoise to the vessel W, not in separate masses, as we
have here supposed, but continuously, so as to produce a regular
motion of the piston upwards.
Such is the principle on which the advantages of the expansive engine
of Watt and Hornblower depend, explained so far as it can be without
the aid of the language and reasoning of analysis.[50]
[Footnote 50: A strict investigation of this important property, as
well as of the other consequences of the quality of expansion, would
require more abstruse mathematical processes than would be consistent
with the nature of this work.]
(132.) We have here, however, only considered the mechanical effect
produced by the condensation of steam. Let us now examine its direct
action.
Let the piston P be supposed to be connected by a rod with a load or
resistance which it is intended to raise, and let the load placed upon
it be supposed to amount to one ton, the total pressure on the piston
will then be two tons; one due to the atmospheric pressure, and the
other to the amount of the load. Upon applying heat to the water,
steam will be produced; and when the water has been completely
evaporated, the piston will rise to the height of six inches from the
bottom of the tube. The total mechanical effect thus produced will be
one ton weight raised through six perpendicular inches, which is
equivalent to half a ton raised through one foot.
Again, let the load upon the piston be two tons; this will produce a
total pressure upon the water below it amounting to three tons,
including the atmospheric pressure. The water, when converted into
vapour under this pressure, will raise the piston and its load through
four perpendicular inches: the useful mechanical effect will then be
two tons raised through the third of a foot, which is equivalent to
two thirds of a ton raised one foot. In the same manner, if the piston
were loaded with three tons, the mechanical effect would be equivalent
to three fourths of a ton raised through one foot, and so on.
It appears therefore from this reasoning, that when the direct force
of steam of greater pressure than the atmosphere is used without
condensation, the total mechanical effect is always less than that
produced by the condensation of atmospheric steam without expansion;
but that the greater the pressure under which the steam is produced,
the less will be the difference between these effects. In general, the
proportion of the mechanical effect of high-pressure steam to the
effect produced by the condensation of atmospheric steam, will be as
the number of atmospheres expressing the pressure of the steam to the
same number increased by one. Thus, if steam be produced under the
pressure of six atmospheres, the proportion of its effect to that of
the condensation of atmospheric steam will be as six to seven.
(133.) Another method of applying the power of steam mechanically is,
to combine its direct action with condensation but without expansion.
The piston being, as before, loaded with one ton, the evaporation of
the water will raise it through six perpendicular inches, and the
result so far will be equivalent to a ton raised half a foot; but if
the piston-rod be supposed also to act by a chain or cord over a
wheel, so as to pull a weight up, the steam which has just raised the
ton weight through six inches, may be condensed, and the piston will
descend with a force of one ton into the vacuum thus produced, and
another ton may be thus raised through half a foot. The total
mechanical power thus yielded by the steam, adding to its direct
action its effect by condensation, will then be one ton raised through
one foot, being an effect exactly equal to that obtained by the
condensation of atmospheric steam.
If the piston be loaded with two tons, its direct action will, as we
have shown, raise these two tons through four inches, which is
equivalent to two thirds of a ton raised a foot. By condensing this
steam a ton weight may be raised in the same manner, by the descent
of the piston through a third of a foot, which is equivalent to the
third of a ton raised through one foot.
By pursuing like reasoning, it will appear that, if the direct force
of high-pressure steam be combined with the indirect force produced by
its condensation, the total mechanical effect will be precisely equal
to the mechanical effect by the mere condensation of atmospheric
steam.
(134.) In applying the principle of expansion to the direct action of
high-pressure steam, advantages are gained analogous to those already
explained with reference to the method of condensation.
Let the piston be supposed to be loaded with three tons: the
evaporation of the water beneath it will raise this weight, including
the atmospheric pressure, through three perpendicular inches. Let one
ton be now removed, and the remaining two tons will be raised, by the
expansion of the steam, through another perpendicular inch. Let the
second ton be now removed, and the piston loaded with the remaining
ton will rise, by the expansion of the steam, to the height of six
inches from the bottom. These consequences follow immediately from the
principle that steam will expand in proportion as the pressure upon it
is diminished, observing that in this case the atmospheric pressure,
amounting to one ton, must always be added to the load. In this
process three separate effects are produced: one ton is raised through
three inches, which is equivalent to a quarter of a ton raised through
one foot; another ton is raised through four inches, which is
equivalent to a third of a ton through a foot, and the third ton is
raised through six inches, which is equivalent to half a ton raised
through a foot. The total of these effects amounts to one and
one-twelfth of a ton raised through one foot, while the same load,
raised by the high-pressure steam without expansion, would be
equivalent to only half a ton raised through one foot.
Again, let the load placed upon the piston be five tons: the
evaporation of the water will raise this through the sixth part of a
foot; if one ton be now removed, the other four tons will be raised to
a height above the bottom of the tube equal to a fifth part of a foot;
another ton being removed, the remaining three will be raised to a
height from the bottom equal to a fourth of a foot; and so on, the
last ton being raised through half a foot. To estimate the total
mechanical effect thus produced, we are to consider that the several
tons raised from their first position are raised through the sixth,
fifth, fourth, third, and half of a perpendicular foot, giving a total
effect equal to the sixth, fifth, fourth, third, and half of a ton
severally raised through one foot; these, therefore, added together,
will give a total of nineteen twentieths of a ton raised through one
foot.
In general, the expansive force applied to the direct action of
high-pressure steam, therefore, will increase its effect according to
the same law, and subject to the same principles as were shown with
respect to the method of condensation accompanied with expansion.
The expansive action of high-pressure steam may be accompanied with
condensation, so as considerably to increase the mechanical effect
produced; for, after the weights with which the piston is loaded have
been successively raised to the extent permitted by the elastic force
of the steam, and are removed from the piston, the steam will expand
until it balances the atmospheric pressure. It may afterwards be made
further to expand, by adding weights to the counterpoise W in the
manner already explained; and, the steam being subsequently condensed,
all the effects will be produced upon the descent of the piston which
we have before noticed. It is evident that by this means the
mechanical effect admits of very considerable increase.
(135.) We have hitherto considered the piston to be resisted by the
atmospheric pressure above it; but, as is shown in the preceding
chapters, in the modern steam engines, the atmosphere is expelled from
the interior of the machine by allowing the steam to pass freely
through all its cavities in the first instance, and to escape at some
convenient aperture, which, opening outwards, will effectually
prevent the subsequent re-admission of air. The piston-rod and other
parts which pass from the external atmosphere to the interior of the
machine, are likewise so constructed and so supplied with oil or other
lubricating matter that neither the escape of steam nor the entrance
of air is permitted. We are therefore now to consider the effect of
the action of steam against the piston P, when subjected to a
resistance which may be less in amount, to any extent, than the
atmospheric pressure.
In such machines the steam always acts both directly by its power, and
indirectly by its condensation. In calculating its effects, excluding
friction, &c., we have therefore only to estimate its total force upon
the piston, and to deduct the force of the uncondensed vapour which
will resist the motion of the piston.
Supposing, then, the total force exerted upon the piston, after
deducting the resistance from the uncondensed vapour, to be one ton,
and the length of the cylinder to be one foot, each motion of the
piston from end to end of the cylinder will produce a mechanical force
equivalent to a ton weight raised one foot high. If in this case the
magnitude of the piston be equivalent to one square foot, the pressure
of the steam will be equal to that of the atmosphere, and the quantity
of water in the form of steam which the cylinder will contain will be
a cubic inch, while the quantity of steam in it will be a cubic foot.
In proportion as the area of the piston is enlarged the pressure of
the steam will be diminished, if the moving force is required to
remain the same; but with every diminution of pressure the density of
the steam will be diminished in the same proportion, and the cylinder
will still contain the same quantity of water in the form of vapour.
In this way steam may be used, as a mechanical agent, with a pressure
to almost any extent less than that of the atmosphere, and at
temperatures considerably lower than 212°. To obtain the same
mechanical force, it is only necessary to enlarge the piston in the
same proportion as the pressure of the steam is diminished.
By a due attention to this circumstance, the expansive power of steam,
both in its direct action and by condensation, may be used with very
much increased advantage; and such is the principle on which the
benefits derived from Woolf's contrivances depend. If steam of a
high-pressure, say of three or four atmospheres, be admitted to the
piston, and allowed to impel it through a very small portion of the
descent, it may then be cut off and its expansion may be allowed to
act upon the piston until the pressure of the steam is diminished
considerably below the atmospheric pressure; the steam may then be
condensed and a vacuum produced, and the process repeated.
In the double-acting engines, commonly used in manufactures and in
navigation, and still more in the high-pressure engines used for
locomotion, the advantageous application of the principle of expansion
appears to have been hitherto attended with difficulties; for,
notwithstanding the benefits which unquestionably attend it in the
economy of fuel, it has not been generally resorted to. To derive from
this principle full advantage, it would be necessary that the varying
power of the expanding steam should encounter a corresponding, or a
nearly corresponding, variation in the resistance: this requisite may
be attained, in engines applied to the purpose of raising water, by
many obvious expedients; but when they have, as in manufactures, to
encounter a nearly uniform resistance, or, in navigation and
locomotion, a very irregular resistance, the due application of
expansion is difficult, if indeed it be practicable.
We have seen that the mechanical effect produced by steam when the
principle of expansion is not used, is always proportional to the
quantity of water contained in the steam, and is likewise in the same
proportion so long as a given degree of expansion is used. It is
apparent, therefore, that the mechanical power which is or ought to
be exerted by an engine is in the direct proportion of the quantity of
water evaporated. It has also been shown that the quantity of water
evaporated, whatever be the pressure of the steam, will be in the
direct proportion of the quantity of heat received from the fuel, and
therefore in the direct proportion of the quantity of fuel itself, so
long as the same proportion of its heat is imparted to the water.
(136.) The POWER of an engine is a term which has been used to express
the rate at which it is able to raise a given load, or overcome a
given resistance. The DUTY of an engine is another term, which has
been adopted to express the load which may be raised a given
perpendicular height, by the combustion of a given quantity of fuel.
When steam engines were first introduced, they were commonly applied
to work pumps or mills which had been previously wrought by horses. It
was, therefore, convenient, and indeed necessary, in the first
instance, to be able to express the performance of these machines by
reference to the effects of animal power, to which manufacturers,
miners, and others, had been long accustomed. When an engine,
therefore, was capable of performing the same work in a given time, as
any given number of horses of average strength usually performed, it
was said to be an engine of so many _horses' power_. This term was
long used with much vagueness and uncertainty: at length, as the use
of steam engines became more extended, it was apparent that confusion
and inconvenience would ensue, if some fixed and definite meaning were
not assigned to it, so that the engineers and others should clearly
understand each other in expressing the powers of these machines. The
term _horse-power_ had so long been in use, that it was obviously
convenient to retain it. It was only necessary to agree upon some
standard by which it might be defined. The performance of a horse of
average strength, working for eight hours a day, was, therefore,
selected as a standard or unit of steam-engine power. Smeaton
estimated the amount of mechanical effect which the animal could
produce at 22,916 pounds, raised one foot per minute; Desaguiliers
makes it 27,500 pounds, raised through the same height. Messrs. Bolton
and Watt caused experiments to be made with the strong horses used in
the breweries in London; and from the result of these they assigned
33,000 pounds raised one foot per minute, as the value of a horse's
power: this is, accordingly, the estimate now generally adopted; and,
when an engine is said to be of so many horses' power, it is meant
that, when in good working order and properly managed, it is capable
of overcoming a resistance equivalent to so many times 33,000 pounds
raised one foot per minute. Thus, an engine of ten-horse-power would
be capable of raising 330,000 pounds one foot per minute.
As the same quantity of water converted into steam will always produce
the same mechanical effect, whatever be the density of the steam
produced from it, and at whatever rate the evaporation may proceed, it
is evident that the _power_ of a steam engine will depend on two
circumstances: first, the rate at which the boiler with its appendages
is capable of evaporating water; and, secondly, the rate at which the
engine is capable of consuming the steam by its work. We shall
consider these two circumstances separately.
The rate at which the boiler produces steam will depend upon the rate
at which heat can be transmitted from the fire to the water which it
contains. Now this heat is transmitted in two ways: either by the
direct action of the fire radiating heat against the surface of the
boiler; or by the flame, and heated air which escapes from the fire,
passing through the flues, as already explained. The surface of the
boiler exposed to the direct radiation of the fire is technically
called _fire surface_; and that which takes heat from the flame and
air, on its way to the chimney, is called _flue surface_. Of these the
most efficient in the generation of steam is the former. In stationary
boilers, used for condensing engines, where magnitude and weight are
matters of little importance, it has been found that the greatest
effect has been produced in general by allowing four and a half square
feet of fire surface, and four and a half square feet of flue surface,
for every horse-power. By means of this quantity of fire and flue
surface, a cubic foot of water per hour may be evaporated.
It has been already shown that the total power exerted by a cubic inch
of water, converted into steam, will be equivalent to 2160 pounds
raised one foot. A cubic foot of water consists of 1728 cubic inches,
and the power produced by its evaporation will therefore be found by
multiplying 2160 by 1728; the product, 3,732,480, expresses the number
of pounds' weight which the evaporation of a cubic foot of water would
raise one foot high, supposing that its entire mechanical force were
rendered available: but to suppose this in practice, would be to
suppose the machine, through the medium of which it is worked, moved
without any power being expended upon its own parts. It would be, in
fact, supposing all its moving parts to be free from friction and
other causes of resistance. To form a practical estimate, then, of the
real quantity of available mechanical power obtained from the
evaporation of a given quantity of water, it will be necessary to
inquire what quantity of this power is intercepted by the engine
through which it is transmitted. In different forms of steam
engine--indeed, we may say in every individual steam engine--the
amount thus lost is different; nevertheless, an approximate estimate
may be obtained, sufficiently exact to form the basis of a general
conclusion.
Let us consider, then, severally, the means by which mechanical power
is intercepted by the engine.
_First_, The steam must flow from the boiler into the cylinder to work
the piston; it passes necessarily through pipes more or less
contracted, and is, therefore, subject to friction as well as cooling
in its passage.
_Second_, Force is lost by the radiation of heat from the cylinder
and its appendages.
_Third_, The friction of the piston in the cylinder must be overcome.
_Fourth_, Loss of steam takes place by leakage.
_Fifth_, Force is expended in expelling the steam after having worked
the piston.
_Sixth_, Force is required to open and close the several valves, to
pump up the water for condensation, and to overcome the friction of
the various axes.
_Seventh_, Force is expended upon working the air-pump.
In engines which do not condense the steam, and which, therefore, work
with steam of high-pressure, some of these sources of waste are
absent, but others are of increased amount. If we suppose the total
effective force of the water evaporated per hour in the boiler to be
expressed by 1000, it is calculated that the waste in a high-pressure
engine will be expressed by the number 392; or, in other words, taking
the whole undiminished force obtained by evaporation as expressed by
10, very nearly 4 of these parts will be consumed in moving the
engine, and the other 6 only will be available.
In a single-acting engine which condenses the steam, taking, as
before, 1000 to express the total mechanical power of the water
evaporated in the boiler, 402 will express the part of this consumed
in moving the engine, and 598, therefore, will express the portion of
the power practically available; or, taking round numbers, we shall
have the same result as in the non-condensing engine, viz. the whole
force of the water evaporated being expressed by 10, 4 will express
the waste, and 6 the available part.
In a double-acting engine the available part of the power bears a
somewhat greater proportion to the whole. Taking, as before, 1000 to
express the whole force of the water evaporated, 368 will express the
proportion of that force expended on the engine, and 632 the
proportion which is available for work.
In general, then, taking round numbers, we may consider that the
mechanical force of four tenths of the water evaporated in the boiler
is intercepted by the engine, and the other six tenths are available
as a moving force. In this calculation, however, the resistance
produced in the condensing engine by the uncondensed steam is not
taken into account: the amount of this force will depend upon the
temperature at which the water is maintained in the condenser. If this
water be kept at the temperature of 120°, the vapour arising from it
will have a pressure expressed by three inches seven tenths of
mercury; if we suppose the pressure of steam in the boiler to be
measured by 37 inches of mercury, then the resistance from the
uncondensed steam will amount to one tenth of the whole power of the
boiler; this, added to the four tenths already accounted for, would
show a waste amounting to half the whole power of the boiler, and
consequently only half the water evaporated would be available as a
moving power.
If the temperature of the condenser be kept down to 100°, then the
pressure of uncondensed steam will be expressed by two inches of
mercury, and the loss of power consequent upon it would amount to a
proportionally less fraction of the whole power.
The following example will illustrate the method of estimating the
effective power of an engine.
In a double-acting engine, in good working condition, the total power
of steam in the boiler being expressed by 1000, the proportion
intercepted by the engine, exclusive of the resistance of the
uncondensed steam, will be 368, and the effective part 632. Now,
suppose the pressure of steam in the boiler to be measured by a column
of 35 inches of mercury; the thousandth part of this will be seven two
hundredths of an inch of mercury, and 632 of these parts will express
the effective portion of the power. By multiplying seven two
hundredths by 632, we obtain 22 nearly. Now, suppose the temperature
in the condenser is 1200, the pressure of steam corresponding to that
temperature will be measured by 3-7/10 inches of mercury. Subtracting
this from 22, there will remain 18-3/10 inches of mercury, as the
effective moving force upon the piston; this will be equivalent to
about 7 lbs. on each circular inch.
If the diameter of the piston then be 24 inches, its surface will
consist of a number of circular inches expressed by the square of 24,
or 24 × 24 = 576; and, as upon each of these circular inches there is
an effective pressure of 7 lbs., we shall find the total pressure in
pounds by multiplying 576 by 7, which gives 4032 lbs.
We shall find the space through which this force works per minute, by
knowing the length of the cylinder and the number of strokes per
minute. Suppose the length of the cylinder to be 5 feet, and the
number of strokes per minute 21-1/2. In each stroke[51] the piston
will, therefore, move through 10 feet, and in one minute it will move
through 215 feet. The moving force, therefore, is 4032 lbs. moved
through 215 feet per minute, which is equivalent to 215 times 4032
lbs., or 866,880 lbs., raised one foot per minute.
[Footnote 51: By a stroke of the piston is meant its motion from one
end of the cylinder and back again.]
For every 33,000 lbs. contained in this, the engine has a horse-power.
To find the horse-power, then, of the engine, we have only to divide
866,880 by 33,000; the quotient is 26 nearly, and, therefore, the
engine is one of 26 horse power.
Let it be required to determine the quantity of water which a boiler
must evaporate per hour, for each horse-power of the engine which it
works.
It has been already explained that one horse-power expresses 33,000
lbs. raised one foot high per minute, or, 1,980,000 lbs. raised one
foot high per hour. The quantity of water necessary to produce this
mechanical effect by evaporation, will be found by considering that a
cubic inch of water, being evaporated, will produce a mechanical
force equivalent to 2160 lbs. raised a foot high. If we divide
1,980,000, therefore, by 2160, we shall find the number of cubic
inches of water which must be evaporated per hour, in order to produce
the mechanical effect expressed by one horse-power; the result of this
division will be 916,6, which is therefore the number of cubic inches
of water per hour, whose evaporation is equivalent to one horse-power.
But it has been shown that, for every 6 cubic inches of water
evaporated in the boiler which are available as a moving power, there
will be 4 cubic inches intercepted by the engine. To find, then, the
quantity of waste corresponding to 916 cubic inches of water, it will
be necessary to divide that number by 6, and to multiply the result by
4: this process will give 610 as the number of cubic inches of water
wasted. The total quantity of water, therefore, which must be
evaporated per hour, to produce the effect of one horse-power, will be
found by adding 610 to 916, which gives 1526.
This result, however, being calculated upon a supposition of a degree
of efficiency in the engines which is, perhaps, somewhat above their
average state, it has been customary with engineers to allow a cubic
foot of water per hour for each horse-power, a cubic foot being 1728
cubic inches, or above 11 per cent. more than the above estimate.
(137.) It has been stated, that to evaporate a cubic foot of water per
hour requires 9 square feet of surface exposed to the action of the
fire and heated air. This, therefore, is the quantity of surface
necessary for each horse-power, and we shall find the total quantity
of fire and flue surface necessary for a boiler of a given power, by
multiplying the number of horses in the power by 9; the product will
express, in square feet, the quantity of boiler surface which must be
exposed to the fire, one half of this being fire surface and the other
half flue surface.
Since the supply of heat to the boiler must be proportionate to the
quantity of fuel maintained in combustion, and the quantity of that
fuel must depend on the extent of grate surface, it is clear that a
determinate proportion must exist between the power of the boiler, and
the extent of grating in the fire place. The quantity of oxygen which
combines with the fuel varies with the quality of that fuel; for
different kinds of coal it varies from two to three pounds for each
pound of coal.
We shall take it an average of 2-1/2 pounds. Now 2-1/2 pounds of
oxygen will measure 30 cubic feet; also 5 cubic feet of atmospheric
air contain 1 cubic foot of oxygen; and consequently 150 cubic feet of
atmospheric air will be necessary for the combustion of 1 pound of
average coals. At least one third of the air, which passes through a
fire, escapes uncombined into the chimney. We must, therefore, allow
220 cubic feet of atmospheric air to pass through the grate-bars for
every pound of fuel which is consumed. Now since land boilers will
consume 15 pounds, and marine boilers 10 pounds, per hour per
horse-power, it follows that the spaces between the grate-bars, and
the extent of grate surface, must be sufficient to allow 3000 cubic
feet of air per hour in land boilers, and 2000 cubic feet in marine
boilers, to pass through them for each horse-power, or, what is the
same, for each foot of water converted into steam per hour. The
quantity of grate surface necessary for this does not seem to be
ascertained with precision; but, perhaps, we may take as an
approximate estimate for land boilers one square foot of grate surface
per horse-power, and for marine boilers two thirds of a square foot,
the spaces between the grate-bars being equal to their breadth.
It is evident that the capacity of a boiler for water and steam must
have a determinate relation to the power of the engine it is intended
to supply. For each horse-power of the engine, it has been shown that
a cubic foot of water must pass from the boiler in the form of steam
per hour. Now, it is evident that the steam could not be supplied of a
uniform force, if the quantity of steam contained at any moment in
the boiler were not considerably greater than the contents of the
cylinder. For example, if the volume of steam in the boiler were
precisely equal to the capacity of the cylinder, then one measure of
the cylinder would for the moment cause the steam to expand into
double its bulk and to lose half its force, supposing it to pass
freely from the boiler to the cylinder. In the same manner, if the
volume of steam contained in the boiler were twice the contents of the
cylinder, the steam would for a moment lose a third of its force, and
so on. It is clear, therefore, that the space allotted to steam in the
boiler must be so many times greater than the magnitude of the
cylinder, that the abstraction of a cylinder full of steam from it
shall cause a very trifling diminution of its force.
In the same manner, we may perceive the necessity of maintaining a
large proportion between the total quantity of water in the boiler,
and the quantity supplied in the form of steam to the cylinder. If,
for example (taking as before an extreme case,) the quantity of water
in the boiler were only equal to the quantity supplied in the form of
steam to the cylinder in a minute, it would be necessary that the
contents of the boiler should be replaced by cold water once in each
minute: and, under such circumstances, it is evident that the action
of the heat upon the water would be quite unmanageable. But,
independent of this, the quantity of water must be sufficient to fill
the boiler above the point at which the flue surface terminates,
otherwise the heat of the fuel would act upon the part of the boiler
containing steam and not water; and, steam receiving heat sluggishly,
the metal of the boiler would be gradually destroyed by undue
temperature.
The total quantity of space for water and steam in boilers is subject
to considerable variation in proportion to their power. Small boilers
require a greater proportion of steam and water-room, or a greater
capacity of boiler, in proportion, than large ones; and the same
applies to their fire surface and flue surface.
The general experience of engineers has led to the conclusion, that a
low-pressure boiler of the common kind requires ten cubic feet of
water-room, and ten cubic feet of steam-room in the boiler, for every
cubic foot which the engine consumes per hour, or, what is the
same, for each horse-power of the engine. Thus, an engine of
ten-horse-power, according to this rule, would require a boiler having
the capacity of 200 cubic feet which should be constantly kept half
filled with water. There are, however, different estimates of this.
Some engineers hold that a boiler should have twenty-five cubic feet
of capacity for each horse-power of the engine, while others reduce
the steam so low as eight cubic feet.
In a table of the capacities of boilers of different powers, and the
feed of water necessary to be maintained in them, Mr. Tredgold assigns
to a boiler of five-horse-power fourteen cubic feet of water per
horse-power; for one of ten-horse power, twelve and a half cubic feet;
and, for one of forty-horse, eleven cubic feet.
For engines of greater power it is generally found advantageous to
have two or more boilers of small power, instead of one of large
power. This method is almost invariably adopted on board steam boats,
and has the advantage of securing the continuance of the working of
the engine, in case of one of the boilers being deranged. It is also
found convenient to keep an excess of power in the boilers, above the
wants of the engine. Thus, an engine of sixty-horse-power may be
advantageously supplied with two forty-horse boilers, and an engine of
eighty-horse-power with two fifty-horse boilers, and so on.
(138.) The pressure of steam in the cylinder of an engine is always
less than the pressure of steam in the boiler, owing to the
obstructions which it encounters in its passage through the steam
pipes and valves. The difference between these pressures will depend
upon the form and magnitude of the passages: the straighter and wider
they are, the less the difference will be; if they are contracted and
subject to bends, especially to angular inflexions, the steam will be
considerably diminished in its pressure before it reaches the
cylinder. The throttle valve placed in the steam pipe may also be so
managed as to diminish the pressure of steam in the cylinder to any
extent: this effect, which is well understood by practical engineers,
is called _wire-drawing_ the steam. By such means it is evidently
possible for the steam in the boiler to have any degree of
high-pressure while the engine is worked at any degree of low
pressure. Since, however, the pressure of the steam in the cylinder is
a material element in the performance of the engine, the magnitude,
position, and shape of the steam pipe and of the valves are a matter
of considerable practical importance. But theory furnishes us with
little more than very general principles to guide us. One practical
rule which has been adopted is, to make the diameter of the steam pipe
about one fifth of that of the cylinder: by this means the area of the
transverse action of the pipe will be one twenty-fifth part of the
superficial magnitude of the piston; and, since the same quantity of
steam per minute must flow through this pipe as through the cylinder,
it follows that the velocity of the steam, in passing through the
steam pipe, will be twenty-five times the velocity of the piston.
(139.) Another rule which has been adopted is, to allow a square inch
of magnitude, in the section of the steam pipe, for each horse-power
of the engine.
The result of this and all similar rules is, that the steam should
always pass through the steam pipe with the same velocity, whatever be
the power of the engine.
In engines of the same power, the piston will have very different
velocities in the cylinder, according to the effective pressure of the
steam, and the proportions and capacity of the cylinder. It is clear
from what has been already explained, that, when the power is the
same, the same actual quantity of water, in the form of steam, must
pass through the cylinder per minute: but, if the steam be used with a
considerable pressure, being in a condensed state, the same weight of
it will occupy a less space; and consequently the cylinders of
high-pressure engines are smaller than those of the same power in
low-pressure engines: the magnitude of the cylinder and the piston
therefore, as well as the velocity of the latter, will depend first
upon the pressure of the steam.
But with steam of a given pressure, the velocity of the piston will be
different: with a given capacity of cylinder, and a given pressure of
steam, the power of the engine will determine the number of strokes
per minute. But the actual velocity of the piston will depend, in that
case, on the proportion which the diameter of the cylinder bears to
its length; the greater the diameter of the piston is with respect to
its length, the less will be its velocity. In case of stationary
engines used on land, that proportion of the diameter of the cylinder
to its length is selected, which is thought to contribute to the most
efficient performance of the machine. According to some engineers, the
length of the cylinder should be twice its diameter;[52] others make
the length equal to two diameters and a half; but there are
circumstances in which considerations of practical convenience render
it necessary to depart from these proportions. In marine engines,
where great length of cylinder would be inadmissible, and where, on
the other hand, considerable power is required, cylinders of short
stroke and great diameter are used. In these engines the length of the
stroke is often not greater than the diameter of the piston, and
sometimes even less.
[Footnote 52: This is the proportion under which the cylinder with a
given capacity will present the least possible surface to the cooling
effect of the atmosphere.]
The actual velocity which has been found to have the best practical
effect, for the piston in low-pressure engines, is about 200 feet per
minute. This, however, is subject to some variation.
(140.) A given weight or measure of fuel burnt under the boiler of an
engine is capable of producing a mechanical effect through the means
of that engine, which, when expressed in an equivalent number of
pounds' weight lifted a foot high, is called the _duty_ of the engine.
If all the heat developed in the combustion of the fuel could be
imparted to the water in the boiler, and could be rendered
instrumental in producing its evaporation; and if, besides, the steam
thus produced could be all rendered mechanically available at the
working point; then the duty of the engine would be the entire
undiminished effect of the heat of combustion; but it is evident that
this can never practically be the case. In the first place, the heat
developed by the combustion can never be wholly imparted to the water
in the boiler: some part of it will necessarily escape without
reaching the boiler at all; another portion will be consumed in
heating the metal of the boiler, and in supplying the loss by
radiation from its surface; another portion will be abstracted by the
various sources of the waste and leakage of steam; another portion
will be abstracted by the reaction of the condensed steam; and another
portion of the power will be consumed in overcoming the friction and
resistance of the engine itself. It is apparent that all these sources
of waste will vary according to the circumstances and conditions of
the machine, and according to the form and construction of the
furnace, flues, boilers, &c. The duty, therefore, of different engines
will be different; and when such machines are compared, with a view to
ascertain their economy of fuel, it has been found necessary carefully
to register and to compare the fuel consumed with the weight or
resistance overcome. In engines applied to manufactures generally or
navigation, it is not easy to measure the amount of resistance which
the engine encounters, but when the engine is applied to the pumping
of water, its performance is more easily determined.
In the year 1811, several of the proprietors of the mines in
Cornwall, suspecting that some of their engines might not be doing a
duty adequate to their consumption of fuel, came to a determination to
establish a uniform method of testing the performance of their
engines. For this purpose a counter was attached to each engine, to
register the number of strokes of the piston. All the engines were put
under the superintendence of Messrs. Thomas and John Lean, engineers;
and the different proprietors of the mines, as well as their directing
engineers, respectively pledged themselves to give every facility and
assistance in their power for the attainment of so desirable an end.
Messrs. Lean were directed to publish a monthly report of the
performance of each engine, specifying the name of the mine, the size
of the cylinder, the load upon the engine, the length of the stroke,
the number of pump lifts, the depth of the lift, the diameter of the
pumps, the time worked, the consumption of coals, the load on the
pump, and, finally, the duty of the engine, or the number of pounds
lifted one foot high by a bushel of coals. The publication of these
monthly reports commenced in August, 1811, and have been regularly
continued to the present time.
The favourable effect which these reports have produced upon the
vigilance of the several engineers, and the emulation they have
excited, both among engine-makers and those to whom the working of the
machines are intrusted, are rendered conspicuous in the improvement
which has gradually taken place in the performance of the engines, up
to the present time. In a report published in December, 1826, the
highest duty was that of an engine at Wheal Hope mine in Cornwall. By
the consumption of one bushel of coals, this engine raised 46,838,246
pounds a foot high, or, in round numbers, forty-seven millions of
pounds.
In a report published in the course of the present year (1835) it was
announced that a steam engine, erected at a copper mine near St.
Anstell, in Cornwall, had raised by its average work 95 millions of
pounds 1 foot high, with a bushel of coals. This enormous mechanical
effect having given rise to some doubts as to the correctness of the
experiments on which the report was founded, it was agreed that
another trial should be made in the presence of a number of competent
and disinterested witnesses. This trial accordingly took place a short
time since, and was witnessed by a number of the most experienced
mining engineers and agents: the result was, that for every bushel of
coal consumed under the boiler the engine raised 125-1/2 millions of
pounds weight one foot high.
(141.) It may not be uninteresting to illustrate the amount of
mechanical virtue, which is thus proved to reside in coals, in a more
familiar manner.
Since a bushel of coal weighs 84 lbs. and can lift 56,027 tons a foot
high, it follows that a pound of coal would raise 667 tons the same
height; and that an ounce of coal would raise 42 tons one foot high,
or it would raise 18 lbs. a mile high.
Since a force of 18 lbs. is capable of drawing 2 tons upon a railway,
it follows that an ounce of coal possesses mechanical virtue
sufficient to draw 2 tons a mile, or 1 ton 2 miles, upon a level
railway.[53]
[Footnote 53: The actual consumption of coal upon railways is in
practice about eight ounces per ton per mile. It is, therefore, worked
with sixteen times less effect than in the engine above mentioned.]
The circumference of the earth measures 25,000 miles. If it were
begirt by an iron railway, a load of one ton would be drawn round it
in six weeks by the amount of mechanical power which resides in the
third part of a ton of coals.
The great pyramid of Egypt stands upon a base measuring 700 feet each
way, and is 500 feet high; its weight being 12,760,000,000 lbs. To
construct it, cost the labour of 100,000 men for 20 years. Its
materials would be raised from the ground to their present position by
the combustion of 479 tons of coals.
The weight of metal in the Menai bridge is 4,000,000 lbs., and its
height above the level of the water is 120 feet: its mass might be
lifted from the level of the water to its present position by the
combustion of 4 bushels of coals.[54]
[Footnote 54: Some of these examples were given by Sir John Herschel,
in his Preliminary Discourse on Natural Philosophy; but since that
work was written an increased power has been obtained from coals, in
the proportion of 7 to 12-1/2.]
The enormous consumption of coals in the arts and manufactures, and in
steam navigation, has of late years excited the fears of some persons
as to the possibility of the exhaustion of our mines. These
apprehensions, however, may be allayed by the assurance received from
the highest mining and geological authorities, that, estimating the
present demand from our coal mines at 16 millions of tons annually,
the coal fields of Northumberland and Durham alone are sufficient to
supply it for 1700 years, and after the expiration of that time the
great coal basin of South Wales will be sufficient to supply the same
demand for 2000 years longer.
But, in speculations like these, the probable, if not certain,
progress of improvement and discovery ought not to be overlooked; and
we may safely pronounce that, long before a minute fraction of such a
period of time shall have rolled over, other and more powerful
mechanical agents will altogether supersede the use of coal.
Philosophy already directs her finger at sources of inexhaustible
power in the phenomena of electricity and magnetism. The alternate
decomposition and recomposition of water, by magnetism and
electricity, has too close an analogy to the alternate processes of
vaporisation and condensation, not to occur at once to every mind: the
development of the gases from solid matter by the operation of the
chemical affinities, and their subsequent condensation into the liquid
form, has already been essayed as a source of power. In a word, the
general state of physical science, at the present moment, the vigour,
activity, and sagacity with which researches in it are prosecuted in
every civilized country, the increasing consideration in which
scientific men are held, and the personal honours and rewards which
begin to be conferred upon them, all justify the expectation that we
are on the eve of mechanical discoveries still greater than any which
have yet appeared; and that the steam engine itself, with the gigantic
powers conferred upon it by the immortal Watt, will dwindle into
insignificance in comparison with the hidden powers of nature still to
be revealed; and that the day will come when that machine, which is
now extending the blessings of civilisation to the most remote skirts
of the globe, will cease to have existence except in the page of
history.
CHAPTER XIX.
PLAIN RULES FOR RAILWAY SPECULATORS.
(142.) For some time after the completion of the Liverpool and
Manchester railway, doubts were entertained of its ultimate success as
a commercial speculation; and, even still, after several years'
continuance, some persons are found, sceptical by temperament, who
have not acquired full confidence in the permanency of its advantages.
The possibility of sustaining a system of regular transport upon it,
with the unheard of speed effected at the commencement of the
undertaking, was, for a long period, questioned by a considerable
portion even of the scientific world; and, after that possibility was
established, by the regular performance of some years, the
practicability of permanently profitable work, at that rate of speed,
was still doubted by many, and altogether denied by some. The numerous
difficulties to be encountered, and the enormous expense of
locomotive power, have been fully admitted by the directors in their
semi-annual reports. Persons interested in canals and other rival
establishments, and others constitutionally doubtful of everything,
attributed the dividends to the indirect proceedings of the managers,
and asserted that when they appeared to be sharing their profit's,
they, in reality, were sharing their capital. This delusion, however,
could not long continue, and the payment of a steady semi-annual
dividend of 4-1/2 per cent. since the opening of the railway, together
with the commencement of a reserved fund of a considerable amount,
with a premium of above 100 per cent. on the original shares, has
brought conviction to understandings impenetrable to general
reasoning; and the tide of opinion, which, for a time, had turned
against railways, has now, by the usual reaction, set in so violently
in their favour, that it becomes the duty of those who professionally
devote themselves to such inquiries, to restrain and keep within
moderate bounds the public ardour, rather than to stimulate it.
The projects for the construction of great lines of internal
communication which have been announced would require, if realized, a
very large amount of capital. Considering that the estimated capital
is invariably less than the amount actually required, we shall not,
perhaps, overrate the extent of the projected investments if we
estimate them at fifty millions. The magnitude of this amount has
created alarm in the minds of some persons, lest a change of
investment so extensive should produce a serious commercial shock. It
should, however, be considered that, even if all the projected
undertakings should be ultimately carried into execution, a long
period must elapse, perhaps not less than fifteen or twenty years,
before they can be all completed: the capital will be required, not
suddenly, but by small instalments, at distant intervals of time. Even
if it were true, therefore, that, to sustain their enterprises, an
equivalent amount of capital must be withdrawn from other investments,
the transfer would lake place by such slow degrees as to create no
serious inconvenience. But, in fact, it is not probable that any
transfer of capital whatever will be necessary. Trade and
manufacturers are at the present moment in a highly flourishing
condition; and the annual accumulation of capital in the country is so
great, that the difficulty will probably be, not to find capital to
meet investments, but to find suitable investments for the increasing
capital. In Manchester alone, it is said that the annual increment on
capital is no less than three millions. In fifteen years, therefore,
this mart alone would be sufficient to supply all the funds necessary
for the completion of all the proposed railroads, without withdrawing
capital from any other investment.
The facilities which these Joint Stock Companies offer for the
investment of capital, even of the smallest amount, the temptations
which the prospect of large profits hold out, and the low interest
obtained on national stock of every description, have attracted a vast
body of capitalists, small and great, who have subscribed to these
undertakings with the real intention of investment. But, on the other
hand, there is a very extensive body of speculators who engage in them
upon a large scale, without the most distant intention, and, indeed,
without the ability, of paying up the amount of their shares. The loss
which the latter class of persons may sustain would, probably, excite
little commiseration, were it not for the consequences which must
result to the former, should a revolution take place, and the market
be inundated with the shares of these gambling speculators, who buy
only to sell again. Effects would be produced which must be ruinous to
a large proportion of the _bonâ fide_ subscribers. It may, therefore,
be attended with some advantage to persons who really intend to make
permanent investments of this nature, to state, in succinct and
intelligible terms, the principal circumstances on which the
efficiency and economy of railroads depend, so as to enable them, in
some measure, to form a probable conjecture of the prospective
advantages which the various projects hold out. In doing this, we
shall endeavour, as much as possible, to confine our statements to
simple facts and results, which can neither be denied nor disputed,
leaving, for the most part, the inferences to which they lead to be
deduced from them by others.
It may be premised, that persons proposing to engage in any railroad
speculation should obtain _first_ a table of _gradients_; that is, an
account of all the acclivities upon the line from terminus to
terminus, stating how many feet in a mile each incline rises or falls,
and its length. _Secondly_, it would also be advantageous to have a
statement of the lengths of the radii of the different _curves_ as
well as the lengths of the curves themselves. _Thirdly_, an account of
the actual intercourse which has taken place, for a given time, upon
the turnpike road connecting the proposed termini, stating the number
of coaches licensed, and the average number of passengers they carry;
also as near an account of the transport of merchandise as may be
obtained. The latter, however, is of less moment. An approximate
estimate may be made of the intercourse in passengers, by allowing for
each coach, upon each trip, half its licensed complement of load.
_Fourthly_, the water communication by canal or otherwise between the
places; and the amount of tonnage transported by it. With the
information thus obtained, the following succinct maxims will be found
useful:--
I.
No railroad can be profitably worked without a large intercourse of
passengers. Goods, merchandise, agricultural produce, &c., ought to be
regarded as of secondary importance.
II.
A probable estimate of the number of passengers to be expected upon a
projected line of railroad may be made by increasing the average
number of passengers for the last three years, by the common road, in
a twofold proportion.
The average number of passengers daily between Liverpool and
Manchester, before the formation of the railway, was about 450; the
present average number is above 1300. A short railroad of about five
miles is constructed between Dublin and Kingstown: on which the
average number of passengers daily between those places has increased
in nearly the same proportion.
III.
Passengers can be profitably transported by canal, at a speed not
exceeding nine miles an hour, exclusive of delays at locks, at the
rate of one penny per head per mile. The average fares charged upon
the Manchester railway are at the rate of 1-84/100_d._ per head per
mile, the average speed being twenty miles an hour.
To transport passengers at the rate of ten miles an hour on a railway,
would cost very little less than the greater speed of twenty miles an
hour, so that a railroad could not enter into competition on equal
terms with a canal by equalising the speed.
The canal between Kendal and Preston measures 57 miles: passengers are
transported upon it between these places at the average speed of a
mile in 6-1/2 minutes, or 9-1/4 miles an hour nearly, exclusive of
delays at locks. The fare charged is at the average rate of a penny a
mile. There are eight locks, rising 9 feet each, and a tunnel 400
yards long, through which the boat is tracked by hand; the tunnel
requires 5 minutes, and the locks from 25 to 28 minutes, in
descending, and 45 to 48 minutes in ascending.
Similar boats are worked on the Forth and Clyde, and the Union canals
in Scotland, and on the Paisley and Johnstone canal at nearly the
same fares.
IV.
At the fare of 1-84/100_d._ per head per mile, the profit on the
Manchester railroad is 100 per cent. on the disbursements for
passengers.
V.
Goods can be profitably transported by canal at a lower tonnage than
by railroads; the speed on the canal (for goods) being, however, but
one-fifth of the speed on the railroad.
VI.
Goods are transported on the Liverpool and Manchester railroad at
three-pence three farthings per ton per mile, with a profit of about
40 per cent. upon the disbursement, having the competition of a canal
between its termini.
VII.
A long railroad can be worked with greater relative economy than a
short one.
VIII.
Steam engines work with the greatest efficiency and economy, when the
resistance they have to overcome is perfectly uniform and invariable.
IX.
The variation of resistance on railroads depends, first, on
acclivities, secondly, on curves.
By curves are meant the changes of direction of the road to the right
or to the left. The direction of a railroad cannot be changed suddenly
by an angle, but must be effected gradually by a curve. Supposing the
curve to be (as it generally is) the arc of a circle, the radius of
the curve is the distance of the centre of the circle from the curve.
This radius is an important element in the estimate of the road.
X.
The more nearly a railroad approaches to an absolute level, and
perfect straightness, the more profitably will it be worked.
XI.
The total amount of mechanical power necessary to transfer a given
load from one extremity of a railroad to another is a matter of easy
and exact calculation, when the gradients and curves are known; and
the merits of different lines may be compared together in this
respect: but it is not the only test of their efficiency which must be
applied.
XII.
A railroad having gradients exceeding seventeen feet in a mile will
require more mechanical power to work it than it would were it level;
and the more of these excessive gradients there are upon it, and the
more steep they are, the greater will be this disadvantage.
XIII.
Although a railroad having no gradients exceeding seventeen feet in a
mile does not require more mechanical power than a level, yet the
mechanical power which it requires will not be so advantageously
expended, and, therefore, it will not be so economical.
XIV.
A railroad which has gradients above thirty feet in a mile will
require such gradients to be worked by assistant locomotive engines,
which will be attended with a waste of power, and an increase of
expenditure, more or less, according to the number and length of such
gradients.
XV.
A very long inclined plane cannot be worked by an assistant locomotive
without a wasteful expense. Gradients exceeding seventeen feet per
mile must, therefore, be short.
XVI.
Gradients exceeding fifty feet in a mile cannot be profitably worked
except by stationary engines and ropes, an expedient attended with so
many objections as to be scarcely compatible with a large intercourse
of passengers.
XVII.
Steep gradients, provided they descend from the extremities of a line,
are admissible provided they be short.
It is evident that in this case the inclined planes will help at
starting to put the trains in motion, at the time when, in general,
there would be the greatest strain upon the moving power; and, in
approaching the terminus, the momentum would be sufficient to carry
the train to the top of the plane, if its length were not great, since
it must, at all events, come to a stop at the extremity.
XVIII.
The effect of gradients in increasing the resistance during the
ascent may be estimated by considering that a gradient of seventeen
feet in a mile doubles the resistance of the level, thirty-four feet
in a mile triples it, and eight and a half feet in a mile adds one
half its amount, and so on.
XIX.
With the speed now attainable on railways, curves should be avoided
with radii shorter than a mile. Expedients may diminish the
resistance, but, through the negligence of engine drivers, they must
always be attended with danger. Curves are not objectionable near the
extremities of a line.
XX.
The worst position for a curve is the foot of an inclined plane,
because of the velocity which the trains acquire in the descent, and
the occasional impracticability of checking them.
XXI.
In proportion as the speed of locomotives is increased by the
improvements they are likely to receive, the objections and dangers
incident to curves will be increased.
XXII.
The difficulty which attends the use of long tunnels arises from the
destruction of the vital air which is produced by the combustion in
the furnaces of the engines. Tunnels on a level should, therefore, be
from twenty-five to thirty feet high, and should be ventilated by
shafts or other contrivances.
XXIII.
The transition from light to darkness, the sensation of humidity, and
the change in summer from a warm atmosphere to a cold one, will always
form an objection to long tunnels on lines of railroad intended for a
large intercourse of passengers.
XXIV.
All the objections to a tunnel are aggravated when it happens to be
upon an acclivity. The destruction of vital air in ascending it will
be increased in exactly the same proportion as the moving power is
increased. Thus, if it ascend 17 feet in a mile, the destruction of
vital air will be twice as great as on a level; if it ascend 34 feet
in a mile, it will be three times as great; 51 feet in a mile, four
times as great, and so on.
XXV.
If by an overruling necessity a tunnel is constructed on an acclivity,
its magnitude and means of ventilation should be greater than on a
level, in the same proportion as the resistance produced by the
acclivity is greater than the resistance upon a level.
XXVI.
Tunnels should be ventilated by shafts at intervals of not more than
200 yards.
XXVII.
While a train is passing through a tunnel, no beneficial ventilation
can be obtained from shafts. The engine will leave behind it the
impure air which it produces, and the passengers will be enveloped in
it before it has time to ascend the shafts. Sufficient magnitude,
however, may be given to the tunnel to prevent any injurious
consequences from this cause. A disagreeable and inconvenient odour
will be experienced.
XXVIII.
Tunnels on a level, the length of which do not exceed a third of a
mile, will probably not be objectionable. Tunnels of equal length upon
acclivities would be more objectionable.
I may observe generally that we have as yet little or no experience of
the effect of tunnels on lines of railroad worked by locomotive
engines, where there is a large intercourse of passengers. On the
Leicester and Swannington railroad, there is a tunnel of about a mile
long, on a part of the road which is nearly level; it is ventilated by
eight shafts, and I have frequently passed through it with a
locomotive engine. Even when shut up in a close carriage the annoyance
is very great, and such as would never be tolerated on a line of road
having a large intercourse in passengers. This railroad is chiefly
used to take coals from some collieries near Swannington, and there is
no intercourse in passengers upon it, except of the labouring classes
from the adjacent villages: the engines burn coal, and not coke; and
they consequently produce smoke, which is more disagreeable than the
gases which result from the combustion of coke. This tunnel also is of
small calibre.
On the Leeds and Selby railroad there is a tunnel, on a part which is
nearly level, the length of which is 700 yards, width 22 feet, height
17. It is ventilated by three shafts of about 10 feet diameter and 60
feet high. There is an intercourse of passengers amounting to four
hundred per day, upon this road, and, generally speaking, they do not
object to go through the tunnel with a locomotive engine. The fuel is
coke.
(_l_) In order to show the present state of railroad transportation in
the United States, and enable our readers to compare it with the
opinions and facts adduced by Dr. Lardner, we take the latest accounts
from the Charleston and Hamburgh Railroad. The engines drag a train of
cars which carry a load of 130 tons, and perform the distance (240
miles) in three days, travelling only by day-light. With these loads
they mount planes having inclinations of 37 feet per mile. The same
engines are capable of carrying passengers at the rate of 40 miles per
hour, and often perform 30, but their average speed is limited by
regulation to 20 miles per hour.
This railroad is remarkable for being the largest which has yet been
constructed, and is besides an object of just pride, in as much as it
was commenced at a time, when according to Dr. Lardner, the subject
was but imperfectly understood even in Europe, and all its
arrangements are due to native talent and skill, unassisted by
previous discoveries in Europe.--A. E.
INDEX.
A.
Atmospheric air, elastic force of, 23
Atmospheric pressure rendered available as a mechanic agent
by Denis Papin, 48
Atmospheric engine, first conception of by Newcomen, 61.
Description of, 63.
Advantage of over that of Savery, 69
B.
Barometer, the, 21
Barometer gauge, the, 123
Belidor, 133
Birmingham and London railroad, probable advantages to be derived
from, 206
Black, Dr., his doctrine of latent heat, 76
Blasco de Garay, his contrivance to propel vessels, 42
Blinkensop, Mr., constructs a locomotive engine, 161
Boiler, methods for showing the level of water in the, 118.
Its power and proportions, 297
Bolton, Matthew, his connexion with Watt, 88
---- and Watt, Messrs., immense expenditure of, in bringing their
engines into use, 91
Booth, Mr., his method of using tubes to conduct heated air through
locomotive boilers, 176.
His report to the directors of the Liverpool and Manchester railway
on the apparent discrepancies of Messrs. Walker and Rastrick's
estimate of locomotive power, 189
Braithwaite and Ericsson, Messrs., their "Novelty" described, 175
Branca, Giovanni, his machine for propelling a wheel by a blast of
steam, 45
Brewster, Dr., 79
Brunton, Mr., his improved furnace described, 130
C.
Canals, transport on, 208.
Experiments with boats on, 209,
Comparison of with railroads, 210
Cartwright, Rev. Mr., description of his improvements in the steam
engine, 142
Cawley, John, 61
"Century of Inventions" by the Marquis of Worcester, 46
Chapman, Messrs., obtain a patent for working a locomotive by means
of a chain, 162
Church, Dr., his steam carriage, 239
Cohesion, attraction of, 32
Condensation of solids, 28
Condensation by jet, accidental discovery of, 65
Cornwall, reports of duty of steam engines in, 303
Cotton, processes in the culture of, 18
Cylinder, its proportions, 300
D.
D valve, description of the, 113
Damper, the, 126
Duty of a steam engine, 291
Duty, reports of, in Cornwall, 303
E.
Eccentric; description of the, 111
Edelcrantz, the Chevalier, 127
F.
Farey, Mr., his statement respecting the variations in the work
of different steam engines, 133
Fluids, property of, 21
Fly-wheel, introduction of the, 104
Four-way cock, description of the, 115
Fuel, table of the consumption of, in different locomotives, 180
G.
Governor, description of the, 105
Guericke, Otto, inventor of the air-pump, 70
Gurney, Mr., his steam carriage, 216
H.
Hackworth, Mr., description of his engine, the "Sanspareil," 173
Hall, Mr. Samuel, his patent steam engine, 248.
Its advantages for navigation, 249.
Its successful application, 250
Hamilton, Duke of, 88
Hancock, Mr. Walter, his steam carriage, 235
Heat, phenomena of, 29
Hero of Alexandria, description of his machine, 41
Hopper, the, or apparatus for supplying the fire-place with coals, 131
Hornblower, Mr., his double-cylinder engine, 134
Horse power and steam power, comparison between, 202
Horse power of an engine, 291.
Method of calculating it, 293
Howard, Mr. Thomas, his patent steam engine, 253.
Its advantages in navigation, 256
Huskisson, Mr., 154
I.
Inclined planes, their injurious effects on railroads, 194.
Methods proposed to remedy these, 194
India, steam communication with, 271
K.
Kendal and Preston canal, speed of boats on, 209
L.
Leeds and Selby railroad, 317
Leicester and Swannington railroad, 317
Leupold, his "Theatrum Machinarum," 116.
His engine described, 147
Liquids converted into vapour by the application of heat, 27.
Difference of temperatures of, 35
Liverpool and Manchester railroad, effects of the introduction
of steam transport on, 152.
Want of experience in the construction of the engines, 154.
Proceedings of the directors, 167.
Premium offered by them for the best engine, 169.
Experiments made on, 183.
Passengers the chief source of profit to the proprietors, 204
Liverpool and London, supposed advantages from the connexion of
these places by railroad, 206
Llangennech coal, its economy, 267
Locomotive engines, description of the "Rocket," 171.
The "Sanspareil," 173.
The "Novelty," 175.
Mr. Booth's method of using tubes to conduct heated air through
boilers, 177.
Mr. Stephenson's method of subdividing the flue, 179.
Amount of fuel consumed in, 180.
Progressive improvement of, 180.
Description of an improved form of engine, 181.
Circumstances on which their efficiency depends, 183.
Experiments with, on Liverpool and Manchester railroad, 184.
Defects of, 186.
Improvement in the method of tubing, 188.
Proposed methods for working them on levels and inclined planes, 194.
Extraordinary speed and power of, 204.
Their introduction on turnpike roads, 213
Locomotive power, expense of, 188
Locomotive boilers, improved form of, 177
M.
Machines, definition of, 19
Manufactures, motions required in, 19
Morgan, Mr., his patent paddle-wheel, 259
Morland, Sir Samuel, his application of steam to raise water, 47
Motion, a primary agent in the cultivation of cotton, 18.
Variety of, 19
Murray, Mr., description of his suggested slide valve, 113
N.
Newcomen, Thomas, and John Cawley, turn their attention to the
practicability of applying steam engines to the drainage of
mines, 61
Newcomen, Thomas, his construction of the atmospheric engine, 63
"Novelty," description of the, 175
O.
Ogle, Mr., his steam carriage, 239
Oldham, Mr., his modification of the self-regulating furnace, 132
P.
Paddle-wheel, the common one, 257. Mr. Morgan's patent one, 259
Papin, Denis, his contrivance, by which atmospheric pressure is
rendered available as a mechanical agent, 48.
Description of his steam engine, 71
Parallel motion, description of the, 95
Piston, its velocity, 302
Post-office steam packets, their speed, 268
Potter, Humphrey, his contrivance for working the valves, 67
Power of a steam engine, how estimated, 291
R.
Railroads, first introduction of locomotives on, 151.
Important effects to be expected from their adoption, 155.
Imaginary difficulty respecting the progression of carriages on, 160.
Various methods resorted to, to remedy this supposed difficulty, 161.
One of these methods described, 162.
Comparative estimate of the expenses of locomotive and stationary
engines, 168.
Difficulties arising from changes of level, 192.
Their great extension, 206.
Comparison of, with turnpike roads, 213.
Inclined planes on, 194
Railway speculators, plain rules for, 307
Roads, their resistance to draft, 213.
Compared with railroads, 213
Robinson, Dr., 73
Roebuck, Dr., assistance rendered by him to Watt, 87.
His embarrassments, 88
"Rocket," description of the, 171
S.
Savery, Thomas, obtains a patent for an engine to raise water, 49.
His discovery of the principle of condensation, 49.
Constructs the first engine brought into operation, 50.
Description of, 51.
Inefficiency of, 57.
Great consumption of fuel necessary in his engines, 60.
Different purposes to which he proposed to apply the steam engine, 61.
Limited power of his engine, 69
"Sanspareil", description of the, 173
Smeaton turns his attention to the details of the atmospheric engines, 73
Solids converted into liquids by the application of heat, 27
Solomon De Caus, description of the apparatus of, 43
Somerset, Edward, Marquis of Worcester, invention of the steam engine
ascribed to him, 45.
Description of his contrivance, 45.
Similar to Savery's, 46. His "Century of Inventions," 46
Steam carriages, Mr. Gurney's, 216.
Mr. Hancock's, 235.
Mr. Ogle's, 238.
Dr. Church's, 239
Steam, its properties described, 30.
Its mechanical power in proportion to the water evaporated, 277.
Its volume, 279.
Its quantity of heat, 279.
Its power in respect of fuel, 280.
Its expansive action, how advantageous, 280.
Combination of expansion with condensation, 285.
High-pressure, its expansive action, 288.
Examples illustrative of its mechanical force, 305
Steam engine, first mover in, 19.
Physical effects connected with, 20.
Claims to the invention of, 38.
Efficacy of, as a mechanical agent, 39.
First brought into operation by Savery, 50.
Its inefficiency, 58.
First proposed to be applied to the drainage of mines, 61.
Accidental discovery of condensation by jet, 65.
Further improvements by Humphrey Potter and Beighton, 67, 68.
Description of Papin's engine, 71.
First experiments of Watt and subsequent improvements, 73.
Dr. Black's theory of latent heat, 76.
Watt's method of condensation, 76.
Further improvements of Watt, 77.
Description of Watt's single-acting engine, 80.
The cold water pump, 86.
The hot water pump, 86.
Erection of a specimen engine at Soho, and gradual demand for them, 89.
The single-acting engine inapplicable to manufactures, 91.
The double-acting engine, 92.
Invention of the parallel motion, 95.
Introduction of the rotatory motion, 100.
The fly-wheel, 104.
The governor, 105.
The throttle valve, 105.
The eccentric, 111.
The D valve, 113.
The four-way cock, 115.
Methods for ascertaining the level of water in the boiler, 118.
The engine made to feed its own boiler, 120.
Waste of water prevented, 121.
The steam gauge, 122.
Barometer gauge, 123.
The damper, 125.
Methods proposed for preventing the waste of fuel, 128.
Mr. Brunton's furnace described, 130.
Mr. Oldham's modification of the self-regulating furnace, 132.
Improvements by Hornblower and Woolf, 134.
Description of the improvements of Mr. Cartwright, 142.
High-pressure engines, 145.
Leupold's engine described, 147.
Construction of the first high-pressure engine by Messrs. Trevithick
and Vivian, 148.
First application of the steam engine to propel carriages on
railroads, 151.
How applied to navigation, 242.
Marine engine; its form and arrangement, 243.
Mr. Howard's patent engine described, 253.
Mr. Hall's engine described, 248
Steam gauge, the, 122
Steam navigation, incredulity which existed respecting, 159.
The limit of its present powers, 264
Steam vessels, their average speed, 265.
Their average consumption of fuel, 265.
Proportion of their power to their tonnage, 266.
Speed of post-office packets, 268.
Iron steam vessels, 269.
American vessel called the "Cigar Boat," its great speed, 270
Stephenson's, Mr., description of an engine constructed by him, 164.
Premium awarded to this engine by the Liverpool and Manchester
Railway Directors, 170.
His method of dividing the flues, 179
Stephenson and Lock, Messrs., appointed by the Directors of the
Liverpool and Manchester Railroad to make reports on the merits
of various railroads, 167
Sun and planet wheels, 101
T.
Thermometer, description of, 24
Throttle valve, use of, 104
Traction, force of, on a railroad, 192
Tredgold,70
Trevithick and Vivian, Messrs., construct the first high-pressure
engine used in this country, 148
U.
United States, steam communication with, 274
V.
Vacuum, production of, by experiment, 37
Vapour, elastic, force of, 35
Valves, Watt's method of working the, 109
W.
Walker and Rastrick, Messrs., apparent discrepancy of their estimated
expense of locomotive power, 189
Washborough takes out a patent for Watt's invention of the rotatory
motion, 100
Water, sea, injurious to marine boilers, 245.
How remedied by blowing out, 246
Watt, James, important discoveries of, 39.
His acquaintance with Dr. Robinson and first experiments on the steam
engine, 73.
His subsequent improvements, 75.
His method of condensation, 76.
His first introduction of the air-pump into the steam engine, 77.
Further improvements, 78.
His difficulties, 78.
Description of his single-acting engine, 80.
His introduction to Dr. Roebuck, 88.
Erects his first engine on the estate of the Duke of Hamilton, 88.
After further improvements, obtains a patent for this engine, in
conjunction with Roebuck, 88.
His difficulties owing to Dr. Roebuck's failure, and subsequent
connexion with Bolton, 88.
Obtains an extension of his patent, 89.
Ingenious invention of, to determine the rate of remuneration he
should receive, 89.
His invention of the parallel motion, 95.
His method for producing a rotatory motion anticipated by Washborough,
who takes out a patent for it, 101.
His contrivance of the governor, 104.
His method of working the valves, 109.
His suggestion of the D valve, 113
Wood, Mr. Nicholas, 168
Woolf, Mr., his improvements in the steam engine, 134.
Obtains a patent for the double-cylinder engine, 137
* * * * *
JUST PUBLISHED, IN ONE VOLUME, 8VO.
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BEING
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BY OLINTHUS GREGORY, LL. D., F. R. A. S.
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SECOND EDITION, CORRECTED AND IMPROVED.
_Extract of a Letter from_ WALTER R. JOHNSON, _Professor of Mechanics
and Natural Philosophy in the Franklin Institute_.
"This treatise is intended and admirably calculated to supply the
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_Letter from_ ALBERT B. DOD, _Professor of Mathematics in the College
of New Jersey_.
"MESSRS. CAREY & HART,
"Gentlemen,--I am glad to learn that you have published an
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Men.' I have for some time been acquainted with this work, and I
esteem it highly. It contains the best digest, within my
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found highly useful as a book of reference.
"With much respect,
"Your obedient servant,
"ALBERT B. DOD,
"Professor of Mathematics in the College of New Jersey.
"_Princeton, Nov. 11, 1834._"
_Extract of a Letter from_ EDWARD H. COURTENAY, _Professor of
Mathematics in the University of Pennsylvania_.
"The design of the author--that of furnishing a valuable
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unable, from the want of time and previous preparation, to
investigate mathematical principles--appears to have been very
successfully attained in the present volume. The information
which it affords in various branches of the pure and mixed
Mathematics embraces a great variety of subjects, is arranged
conveniently, and is in general conveyed in accurate and concise
terms. To THE ENGINEER, THE ARCHITECT, THE MECHANIC--indeed to
all for whom _results_ are chiefly necessary--the work will
doubtless form a very valuable acquisition."
_Letter from_ CHARLES DAVIES, _Professor of Mathematics in the
Military Academy, West Point_.
"MILITARY ACADEMY, West Point, May 14th, 1835.
"_To Messrs. E. L. Carey & A. Hart_,--
"The 'Mathematics for Practical Men,' by Dr. Gregory, which you
have recently published, is a work that cannot fail to be
extensively useful.
"It embraces, within a comparatively small compass, all the rules
and formulas for mathematical computation, and all the practical
results of mechanical philosophy. It is, indeed, a collection of
the useful results of science and the interesting facts which
have been developed by experience. It may safely be said, that no
work, of the same extent, contains so much information, with the
rules for applying it to practical purposes.
"I have the honour to be,
"With great respect,
"Your obedient servant,
"CHARLES DAVIES,
"_Professor of Mathematics_."
_Extract from a Letter from_ J. A. MILLER, _Professor of Mathematics
in Mount St. Mary's College, Emmettsburg, Md._
"Since the London edition of Gregory's Mathematics for Practical
Men appeared in this country, it has been much used in this
institution. The accuracy of its definitions, its beautiful
systematic arrangement, the many simplified and facilitated
methods which it proposes, and its highly practical character,
must recommend it strongly to public patronage, as one of the
very best works which have lately issued from the press. I have
examined your edition of this valuable work sufficiently to say
with confidence that it is very accurately printed."
* * * * *
[Transcriber's note: Only obvious printer's errors have been corrected
(e.g.: 3 s instead of 2, etc.). The author's spelling has been
maintained and inconsistencies have not been standardised.
The advertisement page has been move from the front to the end of the
book.
Other corrections made:
--Page x: "the wealth of rations" has been replaced by "the wealth of
nations".
--Page 17: "Pressure of Rarified Air." has been replaced by "Pressure
of Rarefied Air."
--Page 98: "This beautiful contrivance, which is incontestibly" has
been replaced by "This beautiful contrivance, which is incontestably".
--Page 100: "the working beam no longer used" has been replaced by
"the working beam is no longer used".
--Page 223: "is attended with peliar difficulty" has been replaced by
"is attended with peculiar difficulty".
--Page 271: "The projecter is now employed in" has been replaced by
"The projector is now employed in".]
End of the Project Gutenberg EBook of The Steam Engine Familiarly Explained
and Illustrated, by Dionysius Lardner
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