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Title: Wonders of physical science
Author: E. E. Fournier d'Albe
Release date: April 27, 2026 [eBook #78559]
Language: English
Original publication: London: Macmillan and Co., Limited, 1910
Other information and formats: www.gutenberg.org/ebooks/78559
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*** START OF THE PROJECT GUTENBERG EBOOK WONDERS OF PHYSICAL SCIENCE ***
[Illustration]
[Illustration: Frontispiece]
WONDERS OF PHYSICAL SCIENCE
=Readable Books in Natural Knowledge=
Globe 8vo.
WONDERS OF PHYSICAL SCIENCE. By
E. E. FOURNIER, B.Sc.
TILLERS OF THE GROUND. By Marion
I. NEWBIGIN, D.Sc.
THREADS IN THE WEB OF LIFE. By
Professor J. ARTHUR THOMSON, M.A., and
MARGARET R. THOMSON.
_And others to follow._
MACMILLAN AND CO., LTD., LONDON.
=Readable Books in Natural Knowledge=
WONDERS OF PHYSICAL SCIENCE
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MACMILLAN AND CO., Limited
LONDON • BOMBAY • CALCUTTA
MELBOURNE
THE MACMILLAN COMPANY
NEW YORK • BOSTON • CHICAGO
ATLANTA • SAN FRANCISCO
THE MACMILLAN CO. OF CANADA, Ltd.
TORONTO
[Illustration]
WONDERS OF
PHYSICAL SCIENCE
BY
E. E. FOURNIER, B.Sc.
ASSOCIATE OF ROYAL COLLEGE OF SCIENCE, LONDON
To try things oft, and never to give over, doth wonders.--BACON.
MACMILLAN AND CO., LIMITED
ST. MARTIN’S STREET, LONDON
1910
PUBLISHERS’ NOTE
SO much attention is now given to the practical and systematic study
of science in schools that the valuable influence of descriptive
scientific literature is apt to be overlooked. An intimate knowledge
of the simplest fact in Nature can be obtained only by personal
observation or experiment in the open air or the laboratory, but broad
views of scientific thought and progress are secured best from books in
which the methods and results of investigation are stated in language
which is simple without being childish.
Books intended to promote interest in science must differ completely
from laboratory guides, text-books, or works of reference. They should
aim at exalting the scientific spirit which leads men to devote their
lives to the advancement of natural knowledge, and at showing how the
human race eventually reaps the benefit of such research. Inspiration
rather than information should be the keynote; and the execution should
awaken in the reader not only appreciation of the scientific method of
study and spirit of self-sacrifice, but also a desire to emulate the
lives of men whose labours have brought the knowledge of Nature to its
present position.
These are the objects of the series of Readable Books in Natural
Knowledge to which the present volume belongs. Each volume will
endeavour to stimulate interest in the studies with which it is
concerned, and to present natural phenomena and laws broadly and
attractively. It is hoped that the books will provide the reading
matter urgently required in connection with the science work in
schools and will appeal also to a wide circle of other readers. The
series should be of service in directing attention to the nobility of
scientific ideals and the ultimate value of results obtained by careful
and faithful work.
CONTENTS
CHAP. PAGE
1. ARCHIMEDES 1
2. THE WISE MEN OF ALEXANDRIA 11
3. ARABIAN DAYS 22
4. DR. GILBERT OF COLCHESTER 34
5. GALILEO 50
6. THE BAROMETER 65
7. THE AIR-PUMP 80
8. THE INVENTOR OF THE STEAM-ENGINE 88
9. ELECTRIC SPARKS 100
10. THE ELECTRIC CURRENT 113
11. THE ELECTRIC TELEGRAPH 124
12. THE TELEPHONE 136
13. ELECTRIC LIGHT 149
14. MICHAEL FARADAY 159
15. TELEGRAPHING WITHOUT WIRES 170
16. THE NEW RAYS 178
17. AIR-SHIPS AND FLYING MACHINES 190
CHAPTER I
ARCHIMEDES
THE land of Italy is shaped like a boot, and at the toe of
the boot lies a three-cornered island called Sicily. It was famous
in ancient times for its cities, temples, and palaces; for its
fruit-gardens and cornfields; and for its great volcano called Etna,
from the top of which a cloud of steam and dust ascends, and sometimes
a fiery stream of molten rock flows down.
Sicily has often been shaken by earthquakes, but the worst of all was
the earthquake which destroyed the town of Messina at Christmas 1908.
The greatest city of Sicily was called Syracuse. It is now a town of
25,000 inhabitants, built on an island joined to the mainland by a
bridge. But in ancient times it had ten times as many people, and its
buildings covered the heights and cliffs opposite, and extended along
the banks of the river, up to where the papyrus plants grow, which the
ancients made into paper.
In that great city dwelt a man called Archimedes. He was a rich and
wise man, a friend and relative of the king, and had he chosen, he
could have spent all his days in the pursuit of pleasure, going to the
races and matches and theatres, and dining every day at the king’s
table. But his mind was given to the pursuit of science and of truth.
His great delight lay in studying the laws of Nature, and finding
out the secrets of her working. He believed that everything happens
according to some law or principle, and that if once he could discover
the law he could rule the world.
Archimedes was born 300 years before Christ. When he was young, war was
raging in Sicily between the Greeks and Romans and the people of the
city of Carthage in Africa, each wishing to possess the beautiful and
fertile island. The king of Syracuse joined the Romans, and as they
proved the strongest, he was able to reign in peace for fifty years.
During that time the trade and commerce of the city flourished. The
king built many ships, some of them larger than any seen before, and
these ships sailed to Egypt and Greece, Africa and Spain, and along the
coasts of France and Italy, exchanging grain and fruit for purple and
gems, Arabian horses, and amber from the shores of the Baltic.
Archimedes spent much of his time in the harbour and in the
shipbuilding yards, watching the sailors and shipwrights at their
work, and helping them with his new inventions. He saw them lift heavy
weights, and taught them how to do it without great exertion. They
already knew how to use a lever, or crowbar, for lifting heavy stones.
They pushed the end of the lever under the stone, and rested the lever
or bar on another stone close to it.
[Illustration: Fig. 1.--A Lever to move the Earth.]
In that way they could exert a great force. Archimedes measured the
force they could exert in this way, and found it was increased when
the bar was made longer and the distance between the stone and the
support was made less. When the distance between the support and the
hand was five times the distance between the support and the stone,
Archimedes found that the force of the hand was multiplied by five,
and one man could lift a stone which it would take five men to lift
without a lever. “Now,” he said, “if you make the lever long enough
there is no weight too heavy for us to lift.” He told the king about
this discovery, and said to him, “If you gave me some place on which
to stand, I could move the whole earth.”
[Illustration: Fig. 2.--The Endless Screw.]
Archimedes knew, of course, that the matter was not quite so simple
as that. He knew that the lever required to move the earth would have
to be exceedingly stout and strong, as thick, indeed, as the earth
itself. For, otherwise, it would not stand the strain. Besides, he
knew he would also require a fixed support against which to rest the
lever. And the place on which he was to stand would have also to be
firmly fixed, so as to allow him to exert the force required. We know
now that the earth floats freely in space, and that there is no fixed
place anywhere. Therefore we cannot, after all these 2200 years, try
the great experiment suggested by Archimedes.
But many other things which Archimedes proposed are now used every day.
One of these is the Endless Screw, by means of which Archimedes is said
to have drawn a ship, with cargo and crew, along the sand as easily as
if it were floating in water, to the great delight of his friend the
king.
Not long after this the king showed Archimedes a great mark of favour
and confidence. This is how it came about.
[Illustration: Fig. 3.--Archimedes’ Screw can be used to raise Water.]
The king had given one of his goldsmiths a certain weight of gold,
and instructed him to make it into a golden crown which he wanted to
present to one of the temples. After a few weeks the goldsmith came
to the palace and brought the crown to the king. The king weighed the
crown, and found it was the same weight as the gold he had given out.
But somebody told him that the goldsmith had mixed a quantity of silver
with the gold, and kept the remaining gold for himself.
The king was a just man, and did not want to accuse the goldsmith of
the crime without having a proof of his guilt. He therefore sent for
Archimedes and gave him the crown, asking him to find out whether any
silver had been mixed with the gold, or whether it was made of pure
gold.
Archimedes was long puzzled over this problem. He weighed the crown,
and found the weight was correct. It also looked like pure gold, so
that the amount of silver could not be very much. Archimedes made
two blocks, one of silver and the other of gold, of exactly the same
size, and found that the block of gold weighed nearly twice as much as
the block of silver. “Now,” he said to himself, “if I could melt up
the crown and make it into a square block, and make another block of
exactly the same size of pure gold, both should weigh the same if the
crown is pure gold, but if it is not, the block made from the crown
should be lighter.” He wished for a moment to melt up the crown, but it
was beautifully made, and he thought it a pity to throw away the fine
work. If he could only find out the exact bulk of the crown, he thought
he could determine easily if it was the proper weight which it should
be if made of pure gold. The problem then was how to find out the total
bulk of the crown without melting it up into a block.
Archimedes was in the habit of following up a problem steadily until he
had solved it. He allowed nothing to come in the way. Sometimes he was
found tracing lines and circles in the ashes of the grate, working out
some problem. Sometimes, even, when he had had his bath, and his slaves
had rubbed him with oil to make his skin smooth, he traced diagrams
with his fingers on the layer of oil on his skin. He could think of
nothing but the problem he was trying to solve.
The problem of the crown puzzled him greatly; but one day the solution
flashed upon him just as he was having his bath. He was taking a bath
in a kind of large cup of his own size, as the Greeks used to do. The
cup stood on a pedestal, with a sink round it to let the water flow
off. When he got in, the cup was full of water to the brim, therefore
it overflowed as he was getting in. He dipped himself entirely under
water, head and all, and when he got out again the water was no longer
up to the brim, but considerably below it. _The amount of water which
had flowed out was exactly the bulk of his own body_. The truth came
upon Archimedes like a flash. He had found a way of determining the
bulk of his own body without melting himself down into a block, and why
should he not find the bulk of the crown in the same manner?
He was so excited that he forgot to dry and clothe himself, but ran
home just as he was, calling out, “I have found it! I have found it!”
He took a big jug full of water to the brim, and carefully let down the
crown into it, holding it by a thread. A quantity of water overflowed,
and when the crown was taken out the water was on a lower level. He
took a measuring glass, and carefully measured how much water was
required to bring the water up to the brim again. That quantity of
water had the same bulk as the crown.
He then measured out a quantity of gold and a quantity of silver, both
quantities weighing exactly the same as the crown. These quantities
he melted up into blocks. In this manner he got three things weighing
exactly the same: a block of gold, a block of silver, and the crown
which the goldsmith had made. He then dipped the gold block into
the jug filled to the brim. It displaced a certain amount of water.
Again, he dipped the silver block into the jug. It displaced nearly
twice as much water. Finally, he dipped the crown into the full jug.
It displaced more water than the gold block, but less water than the
silver block. And so it was proved that _the crown contained a
quantity of silver_.
Archimedes next made some other blocks, containing gold mixed with
various proportions of silver, but always of the same weight as
the crown. In the end he obtained a block of silver and gold which
displaced exactly as much water as the crown. He went to the king and
told him exactly how much silver was in the crown, and how much gold
the goldsmith had stolen.
The king thanked his clever friend. He sent for the goldsmith and told
him he had found him out. The goldsmith confessed his guilt, and the
king made him restore the gold and kept him in prison until he had been
justly punished for his crime.
Archimedes continued his experiments with all sorts of bodies and
substances.
[Illustration: Fig. 4.--An Archimedes Cup.]
He weighed things in air and in water, and found that when
they were suspended in water by a thread, and the thread was attached
to one arm of a balance, the weight necessary to counterbalance it
was less than when the body hangs in air. The difference in the
counterbalancing weight is equal to the weight of the water which the
body displaces. This may be proved by a simple experiment.
A glass beaker with a spout, like that in Fig. 4, has water poured
into it until the water just runs out of the spout. The beaker with
the water in it is placed in one pan of a balance, and weights are put
in the other pan until the balance is even. A block of wood is then
weighed and floated on the water in the cup. Some of the water flows
out of the spout and may be caught in another beaker. But when the cup
is weighed with the floating block it is found that the weight is
exactly the same as before. The wood exerts no additional weight, since
its weight is equal to that of the water which flowed over, as may be
proved by weighing the water caught in the small beaker. Archimedes put
this rule into a few words as follows: “Every body loses as much weight
in water as the displaced water weighs.” This is called the Principle
of Archimedes.
[Illustration: Fig. 5.--Ancient. War-Engine for throwing Rocks.]
When, after fifty years’ peace, the Romans made war on Syracuse,
Archimedes took charge of the defence of the city. He made great
war-engines which threw rocks upon the attacking army, and sank the
enemy’s ships in the harbour. Marcellus, the Roman general, could not
help admiring the genius of the great Syracusan engineer, and when he
captured the city at last he ordered his soldiers to spare Archimedes.
One of them came upon the great inventor just as he was working out a
problem with a stick in the sand on the floor. When the soldier asked
his name, he told him to wait till he had solved the problem, and not
to tread upon his circles. Thereupon the soldier killed him.
Marcellus was greatly grieved, and tried to atone for this by showing
great kindness to the relatives.
Archimedes was buried near the city. On his grave was put a monument
in the shape of a cylinder enclosing a ball or sphere. It was one of
his proudest achievements to find that a ball enclosed in a cylinder of
the same height fills up just two-thirds of its bulk. No more suitable
device could have been placed on the grave of one of the greatest men
of science that ever lived.
CHAPTER II
THE WISE MEN OF ALEXANDRIA
WHEN Alexander the Great overcame the Egyptians, he sailed
along the African coast past the mouths of the river Nile until he came
to a place which seemed to him suitable for building a great city. It
was a strip of land on the sea-shore with a great lake behind it and
an island in front. “Here,” said the mighty conqueror, “shall be my
capital city, to be called Alexandria after my name.” So he sent for
his great architect Dinocrates, and told him to plan the streets and
palaces of the new royal city. Thousands of slaves were soon set to
work. The streets were made straight and wide, and canals were dug to
join the lake to the open sea and to the Nile, so that great merchant
vessels would find shelter from the storms.
The island was joined to the city by a causeway a mile long, and a
lighthouse 400 feet high was built on the island. The lighthouse was
in the shape of a round tower. On the top of the tower a bright fire
was kept constantly burning, so that the smoke by day and the flame by
night should guide ships safely into the harbour. This tower was the
first lighthouse ever built, and is said to have cost a quarter of a
million pounds (£250,000).
Within fifty years of its foundation Alexandria became one of the
foremost centres of commerce. The second of the Egyptian kings founded
the famous Library of Alexandria, and thus secured for the city a place
in the front rank of learning, which it kept for six hundred years.
Many of the great works of Greek writers were saved from destruction
by being preserved in Alexandria. The third king compelled every
stranger who passed through Alexandria with a book in his possession to
leave a copy of it at the Library. He also built a Museum, which was an
institution closely resembling what we should now call a University.
It had dwelling-rooms for professors, it had lecture halls, and a
great dining-room. The University grew and flourished, and attracted
clever young men from all the shores of the Mediterranean Sea. One of
these students was Euclid, who wrote the books on Geometry. There were
also poets, critics, and historians. Many books from all nations were
translated into Greek, including the Old Testament, at which seventy
scribes are said to have worked.
It was an Alexandrian astronomer named Aristarchus, who about 270 B.C.
first determined the distance of the sun from the earth in comparison
with that of the moon. He knew that the moon was a round ball, and
that its various shapes or phases were produced by the sun shining
upon it at different angles. He carefully watched the growing sickle
of the moon from day to day, waiting for the time when the moon should
appear to be exactly a half-circle. When this is the case an imaginary
line drawn from the eye of the observer to the centre of the moon is
at right angles to a similar line drawn from the centre of the moon
to the sun, as shown in the picture (Fig. 6). At the instant when the
sun’s light was seen to be shining squarely on the moon, that is just
before sunset, Aristarchus pointed one leg of an instrument like a
pair of compasses to the moon, and the other leg to the sun. The angle
between the two directions was found to be not quite a right angle. It
measured, in fact, 87 degrees (Fig. 6), whereas a right angle contains
90 degrees.
[Illustration: Fig. 6.--Comparing the Distances of the Sun and Moon.]
Now, if a triangle be drawn like that in Fig. 6, in which the angle
at M is a right angle, and that at E is 87 degrees, this triangle
illustrates the results of the observations. The moon’s distance is
represented in the triangle by the line EM, and the sun’s distance by
SM. It does not matter how large or small the triangle is drawn; for,
if the lines are inclined at the proper angles, the line SM will always
be the same number of times longer than EM.
Aristarchus concluded from his observations that the sun was eighteen
or nineteen times farther away than the moon. In reality, it is about
four hundred times that distance, but considering that the ancients
had very imperfect instruments, and that it is almost impossible to
decide the exact instant when the moon is just half-full, it is not
remarkable that the result obtained was far too small. As the result
of other observations, Aristarchus found that 720 suns, placed edge to
edge, would just circle the sky, and this is very close to the truth.
Men of science honour him for his ingenious methods of measuring the
sun’s distance and size, and for his painstaking observations, though
the results were not perfect.
Another famous astronomer of Alexandria, who was also the custodian of
the Library, was the first to measure the size of the earth. He was in
the habit of sailing up the Nile, and found that the farther he sailed
the more new stars appeared in the south, while the northern stars
disappeared gradually. This fact convinced him that the earth, like the
moon, is a round globe, and he thought that if he could go very much
farther in the same direction towards the south, he would eventually
circle the earth, and arrive at Alexandria again from the north. But
to measure the circle of the earth it was not necessary to travel
all round it. It was sufficient to find the fraction of the circle
traversed by travelling a certain distance, and this could be done by
measuring how much higher the southern stars appeared in the sky after
travelling a certain distance. Since the stars are very much farther
away than the sun, the difference in their height could not be due to
the distance travelled, but only to the roundness of the earth.
The astronomer sailed from Alexandria as far as the Falls of Aswan, and
carefully measured the distance he had travelled, which he found to be
520 miles. He found that at midday the sun stood about 7 degrees higher
in the sky at Aswan than at Alexandria. This angle was just about
1-50th of a whole circle. The astronomer concluded that if he continued
to travel south for fifty times that distance he would travel round the
globe, and in doing so he would cover the distance of 26,000 miles. The
real circumference of the earth is 23,700 miles, so that the learned
Alexandrian was not far wrong. But he would have been even more correct
had he known that Aswan is not really due south of Alexandria, but a
little east of south. This made his line too long.
The name of this great astronomer was Eratosthenes. His end was a sad
one. He lost the sight of his eyes and his powers of observation.
He starved himself to death, saying that life without the means of
pursuing his studies was not worth living.
Another remarkable man who lived in those days at Alexandria was Heron.
He was employed at the Museum, lecturing on Mechanics, Optics, and
the principles of Surveying.
[Illustration: Fig. 7.--Heron’s Steam-Engine.]
He invented a number of contrivances and machines which were the wonder
of his age. The people of Babylon had invented clocks driven by water.
[Illustration: Fig. 8.--The Jumping Ball.]
Heron improved these by letting the water drop through a small hole
bored in a precious stone of great hardness, so that the water should
not be able to enlarge the hole and to make the clock run faster. The
water dropped into a vessel containing a little boat. The boat had a
mast which was graduated, so that as the water rose, one graduation
after another appeared above the edge of the vessel.
These graduations indicated the hours. He also made the boat turn a
wheel, which at each hour caused a number of balls to fall into a
silver goblet. The number of balls indicated the number of hours, so
that the clock struck the hour very much like our clocks of to-day.
Heron is famous for having invented a kind of steam-engine, consisting
of a hollow ball with two nozzles pointed in opposite direction (Fig.
7). The steam passed into the ball through the arms by which it was
suspended, and the steam hitting against the air outside caused the
ball to spin round. In another form of the apparatus (Fig. 8) a ball
was kept jumping up and down by the steam issuing from a vertical pipe.
Heron found that when a narrow tube is put into water, and the upper
end is stopped with the finger while still under water, then on lifting
the tube out of the water with the stopped end uppermost, the water
does not flow out at the open end below until the finger is removed.
He, therefore, constructed a vessel (Fig. 9) which became known as the
“Vestal’s Goblet”; it was filled by plunging it into water, stopping
the upper opening with the finger, and lifting it out. The water
escaped in a spray through the small holes below on removing the finger.
[Illustration: Fig. 9.--The Vestal’s Goblet.]
[Illustration: Fig. 10.--The Double Goblet.]
In another form of the instrument there were two compartments which
were filled in the same manner with wine and water respectively. It was
a custom among the Greeks to mix wine with water, and offer the same
mixture to the gods in the temples.
Heron also made a self-feeding wick for oil lamps, which were then in
common use. The oil was burnt in an open vessel (Fig. 11), and as it
burnt away the wick was consumed also, but Heron made the oil itself
turn up the wick. He made a plate of wood float on the oil; as the oil
burnt away the plate of wood gradually sank, and in doing so it moved a
cog wheel. The cog wheel in turning moved a straight rack, or toothed
bar of wood, at the end of which the wick was fastened. In this way
the wick was moved on as the oil was consumed, and the lamp was kept
steadily burning.
The most important instrument which Heron improved was the Surveyor’s
Level. Egypt depends for its fertility upon the floods of the Nile, as
there is very little rain in Egypt.
[Illustration: Fig. 11.--Heron’s Self-Feeding Wick for Lamps.]
From the oldest times the Egyptians had to see that they could
distribute the land after the flood in the same way as it was
distributed before, each man getting his own land back. This was found
to be very difficult, as many landmarks were destroyed by the flood.
The art of Surveying was practised at a very early date in Egypt, and
there is no doubt that the science of Geometry was born in Egypt,
as it was necessary in order to be able to determine the extent and
boundaries of land.
[Illustration: Fig. 12.--Modern form of Heron’s Level.]
An important problem in connection with Surveying was to find when
two points are on the same level. This is done nowadays with a
spirit-level if the points are close together, or with an instrument
called a theodolite if the points are far apart. But a theodolite
requires a telescope, and the Alexandrians lived long before the
telescope was invented. Heron, however, got over the difficulty by
constructing a long box filled with water and provided with openings,
in the form of a cross, at each end. Above the crosses were glass tubes
communicating with each other, in which the water was kept at the same
level, and when that was the case the two crosses were exactly at the
same level, and any object seen through both crosses together was at
the same level as the crosses themselves. Heron probably used rods very
similar to the black and white rods used by surveyors at the present
day for measuring distances and levels.
Heron also invented a mechanical stone crusher, an organ driven by
water power, a counting machine to show the distance travelled by a
ship or a chariot, and an arrangement of mirrors which he called a
spy-glass.
Many of the works of Heron have been preserved to us. They are
written in Greek, which was the language used in Alexandria since its
foundation. Heron was always careful to make his reasoning quite clear
to his pupils, and he insisted that they must not believe anything
without proof.
“It is necessary,” he said, “that those who wish to become acquainted
with mechanical art should know what causes are at work in every
motion. It is important that nothing should be put before students
without proof, and that nothing should remain doubtful for them. In our
presentation every problem is to find a solution. We therefore recall
various principles taught by the ancients which are connected with our
subject.”
CHAPTER III
ARABIAN DAYS
TO the east of the Red Sea there is a land called Arabia.
Part of it is a desert over which a wind, called the simoom, blows
its suffocating blasts. Another part is fertile mountain land, where
the red anemone blooms. The third part adjoins the Indian Ocean and
the Gulf of Persia. This part is rich and green, and planted with
date-palms and coffee shrubs.
From Arabia comes the Arab horse, small and grey, but beautiful to
behold. It can go for forty-eight hours without a drink. The Arabs ride
it without a saddle or stirrup, and only guide it by the pressure of
the knee and a kind word. Its greased hoofs traverse the Arabian desert
from end to end.
Sometimes the Arabs ride on a white ass, mounted on a side saddle. They
also ride the camel with a single hump, and from it good milk and wool
are obtained. It can travel eighty miles per day and goes for a week
without water; but it never learns to know or love its master. Jackals,
panthers, and hyenas prowl about the desert, which is also infested
with ants, scorpions, and locusts, as well as a horrible kind of spider
with double pincers. The Arabs are called the sons of Ishmael. They
are traders and workers in leather. They are calm and dignified in
appearance, and have never been conquered.
Thirteen centuries ago these Arabs were divided into a great number
of tribes who made war upon each other. Then Mohammed arose, and
proclaimed that he was a prophet sent by God to teach the Arab
people, and through them to convert all the world to a new religion.
He suffered much persecution at first, but in the end he overcame all
his enemies and ruled the whole of Arabia. He destroyed all idols, and
taught his people to give up alcoholic drinks, gambling, and usury.
His successors, who were called Khalifs, went forth to conquer the
world. Those who would not be converted to the Mohammedan religion
they forced to pay heavy taxes, those who resisted by force of arms
they slaughtered without mercy, but those who became Mohammedans they
rewarded with a share of their booty and power of government. They
captured Damascus and Jerusalem, they conquered Egypt and North Africa,
and besieged Constantinople. In the end they conquered Spain in the
west and Persia in the east, so that their Empire stretched from the
Atlantic Ocean to India.
The Arabs destroyed many kingdoms, but they also knew how to build.
They founded great cities, the most glorious of which was Bagdad on the
river Tigris, where Harun-al-Rashid reigned and Queen Scheherazade told
her tales of goblins, battles, and treasures, and Aladdin worked his
wonderful lamp. They also founded Cairo, within sight of the Egyptian
pyramids, and made it the capital of Egypt after they had pillaged
Alexandria. But Mecca in Arabia, where Mohammed first preached his new
doctrine, remains the centre of the Mohammedan religion to this day.
[Illustration: Fig. 13.--View of Mecca in the Seventeenth Century.]
When the Roman Empire was destroyed by the barbarians from the north,
it was the Arabs, strange to say, who took up the cultivation of
science where it had been left by the Greeks and Romans. The Khalifs
who built Bagdad, Cairo, and Cordova in Spain, loved to surround
themselves with learned men from all parts of the world. An Arabian
writer says, “To Cordova came from all parts of the world students
eager to cultivate poetry, to study the sciences, or to be instructed
in divinity or law, so that it became the meeting-place of the eminent
in all matters, the abode of the learned, and the place of resort for
the studious. Its interior was always filled with the eminent and the
noble of all countries, its literary men and scholars were continually
vying with each other to gain renown, and its precincts never ceased to
be the arena of the distinguished, the recourse of readers, the halting
place of the noble, and the repository of the true and virtuous.
Cordova was to Spain what the head is to the body, or what the breast
is to the loin.”
The sciences generally cultivated among the Arabs were Chemistry and
Optics. In chemistry they discovered a number of new metals and acids.
They knew how to distil a liquid and crystallise a solid, and how to
mingle various metals to form new alloys with variable properties.
They were excellent makers of sword-blades, and their scimitars or
curved swords were dreaded by all the soldiers of Christendom.
[Illustration: Fig. 14.--Cordova after the Expulsion of the Moors.]
The science of Optics was originally cultivated by a native of
Mesopotamia named Alhazen, who died in the year 1038 A.D.
Alhazen was called to Egypt by one of the Khalifs who had heard that
he had thought out plans for regulating the flow of the Nile in such a
manner that each year there should be plenty of water for inundations.
Alhazen went to Egypt and inspected the land, but found that his
plan was not suitable. The Khalif was angry, and would have punished
Alhazen, but the latter pretended to be mad, and managed to hide
himself until the Khalif died. He then reappeared and became famous as
a scholar and instructor of youth, copying old manuscripts and writing
books on Astronomy, Mathematics, and Optics.
Alhazen’s greatest work was done on the subject of the laws which
govern the production of light. He proved, first of all, that light
travels along straight lines. “If the light of the sun,” he says, “or
the light of the moon, or the light of fire, enters a dark room through
a narrow slit, and dust is in the room, or dust is made to fly in it,
the light entering through the slit is made clearly visible in the air
mixed with dust; it is also visible on the floor or on the opposite
wall of the room. And it is found that the light travels through the
slit to the floor or to the opposite wall along straight lines. And
when a straight rod is held along that visible light, it is found that
the light travels along the straight rod.
[Illustration: Fig. 15.--Light travels in Straight Lines.]
But if there is no dust in the room, and the light appears on the floor
or on the opposite wall, and a straight rod is held between the slit
and the patch of light, or between both a thread is stretched, and
a body is brought between the patch and the slit, the light becomes
visible on the opaque body and disappears from the place at which it
was visible. If then the opaque body is moved to and fro in the space
indicated by the rod, the light always remains visible on the opaque
body. It is, therefore, clear that the light proceeds along straight
lines from the slit to the place where it is visible.”
That light travels in straight lines is only true so long as it travels
through air or through the same kind of substance. When a looking-glass
is held in the path of the rays in a dusty room, it is seen that a beam
of light starts from the looking-glass in a direction quite different
from that of the original beam. This fact did not escape the notice of
Alhazen, and he carefully observed the direction taken by the reflected
beam in various circumstances. He had read the books written by the
learned Greeks, and he knew from them that the reflected beam makes
the same angle with the mirror as the original beam does. To this
observation he added another, and a very important one. He found that
when a flat surface, such as a piece of paper or cardboard, is held
against the mirror so that it touches the original and the reflected
beam, that surface or sheet is always upright on the mirror, and stands
at right angles to its surface. This law, discovered by Alhazen,
explains how we see trees reflected in a river or a pond pointing
straight down, and not slanting to the right or to the left. If we
had a big fiat surface which we could make to stand vertically on the
water, that surface, if made to pass, through the tree, would also pass
through its reflection in the water.
[Illustration: Fig. 16--Effect of Refraction of Light by Water.]
Most of us have observed that when a kitten sees its image in a
looking-glass for the first time, it takes it for another kitten, but
after a few attempts to make friends with the supposed companion it
finds out that the image is only an illusion. This observation was
explained by Alhazen, by pointing out that the eye always perceives
objects in the direction in which the beam of light enters the eye.
He investigated another curious illustration of this. When a coin is
placed at the bottom of a dish, and we move away from the dish until
the rim just hides the coin, the latter is, of course, invisible. But
on filling the dish with water without changing our position, we find
that the coin becomes visible again, being apparently raised, although
it really remains at the bottom of the dish as before.
Alhazen proved that this is another case of illusion due to the bending
of the beam of light. He showed this bending of the beam of light in
a very ingenious manner. He took a big glass vessel, and filled it
with water containing a few drops of milk. He took it into a dark room
where he had already studied the reflection of light. He found that
when the water in the glass vessel was placed in the beam of light,
the beam appeared broken at the surface of the water, bending suddenly
down more towards the floor, and making it shorter. The beam was made
visible in the air by the suspended particles of dust, and in the water
by the suspended particles of milk. Alhazen showed that the beam under
water always makes a greater angle with the surface than does the
original beam, but he did not succeed in finding out the exact relation
between the two angles, nor was that relation discovered until several
centuries afterwards.
Like a true Arab, Alhazen was very fond of studying illusions of all
kinds. But there was one appearance which was before his time not
considered an illusion, but a reality. The sun and moon appear much
larger when they are rising or setting than they do when they are high
up in the sky. Alhazen, however, showed that the apparent size is in
reality the same in both cases. He held a coin at arm’s length so that
it just covered the rising moon. He again held the same coin at the
same distance from his eye between himself and the moon when she was
up in the sky. He found that the coin again just covered the moon, but
that if the moon were seen reflected in a distant mirror so that it was
apparently on the horizon it again appeared larger. This, he said, was
simply owing to our habit of judging things on the earth by comparison
with earthly objects.
The Arabs, whether in Bagdad, Damascus, Cairo, or Cordova, were well
acquainted with the principle of Archimedes, and they knew that a body
in water weighs less than in air, the difference being the weight of
the water displaced by the body. But they were first to use the idea
of what is now known as specific gravity, and they explained that the
specific gravity of a body is obtained by dividing the weight of the
body by the weight of the same volume of water. This definition is used
even now, and tables of specific gravity are found in all books dealing
with the properties of various materials.
Before the destruction of the great Arabian Empire, which put an end
to the cultivation of science by the Arabs, and transferred their work
to the new nations of Europe, they had reached the summit of fame in
the whole world. Their last achievement was one which gave rise to many
remarkable developments. It is described in a book called _The Book
of the Balance of Wisdom_. The Arabian author says in this book that
even air must have some weight, and that the true weight of a body
cannot be the weight as measured in air, because air, like water, must
somewhat reduce its weight. This was the observation upon which later
the great inventions of the barometer and the air-pump were based.
CHAPTER IV
DR. GILBERT OF COLCHESTER
THE ancients were in the habit of consulting books written
by old masters whenever they were puzzled by an unknown power or
force of nature. Instead of making experiments they went to their
bookshelves, and read what the wise men of long ago had written on the
same or similar subjects. They thought that they could not surpass
the old masters in wisdom, and that what the old masters did not know
was not worth knowing. This habit accounts for the slow progress made
in science in Europe during the Middle Ages, that is, from about the
sixth to the fifteenth centuries. But when Columbus discovered America,
and broke through the traditions of the ancients by his courageous
exploration of the unknown, a new era began to dawn.
One of the heralds of this new era was Dr. William Gilbert. He was
born in Colchester, in Essex, in the year 1540, and was the son of
the Recorder of that city. He was educated at Cambridge, and became a
medical doctor at the age of twenty-nine. He was appointed physician
to Queen Elizabeth, and was elected president of the Royal College of
Physicians in the year 1600.
It was Dr. Gilbert who founded the science of Magnetism and the science
of Electricity, collecting all that was known on these subjects, and
making many new and valuable experiments of his own. All these facts
he expounded in a great book which was published in London in the
year 1600. It is called, _On the Magnet and Magnetic Bodies, and on
the Great Magnet, the Earth_. In his introduction he lays down new
principles to guide scientific work in the future, in words somewhat as
follows:--
[Illustration: Fig. 17.--Dr. William Gilbert, who founded the Sciences
of Magnetism and Electricity.]
“In the discovery of secrets and in the investigation of the hidden
causes of things, clear proofs are afforded by trustworthy experiments
rather than by probable guesses and opinions of ordinary professors and
philosophers. In order, therefore, that the noble substance of that
great magnet, the earth, hitherto quite unknown, and the exalted powers
of this globe of ours may be better understood, I shall first of all
deal with common magnets, stones, and iron materials, and with magnetic
bodies, and with the near parts of the earth, which we can reach with
our hands and perceive with our senses. After that I shall proceed to
show my new magnetic experiments, and so I shall penetrate for the
first time into the innermost parts of the earth.
“After I had seen and thoroughly examined many of those things which
have been obtained from mountain heights and ocean depths, or from
deep caves and hidden mines, I applied much prolonged labour on
investigating magnetic forces, which surpass in wonder all other
things about us. This labour has not been idle or unfruitful, since
daily, during my experimenting, new and unexpected properties came
to light. In this manner my knowledge has increased so much through
actual observation, that I felt able to explore the interior parts of
our globe, and explain its substance upon magnetic principles, and to
reveal to mankind the earth, our common mother, and point it out, as
if with the finger, by real demonstration and by experiments appealing
to the senses.
“As geometry ascends from very small and very easy principles to the
greatest and most difficult, so our magnetic doctrine and science
sets forth in convenient order the things which are less obscure.
From these have come to light others that are more remarkable, and at
length in due order are opened the concealed and most secret things of
the globe of the earth, and the causes are made known of those things
which either through the ignorance of the ancients, or the neglect of
moderns, have remained unrecognised and overlooked.” Dr. Gilbert then
proceeds to ask why he should expose himself to the violent attacks and
criticisms of men, calling themselves learned though unacquainted with
the facts:--
“Why should I, in so vast an ocean of books, by which the minds of men
are troubled and fatigued, by which the world and men, without reason,
are intoxicated and puffed up, books written by people who profess to
be philosophers, physicians, mathematicians, and astrologers, but who
yet despise and neglect men of learning,--why should I, I say, add
anything to this disturbed republic of books and expose this noble
science, which seems new and incredible by reason of so many things
hitherto unknown, to be torn to pieces by those who are either sworn
to the opinions of other men, or are little better than idiots? To
you alone, honest and true men of science, who seek knowledge, not
from books only, but also from things themselves, do I address these
magnetic principles and this new sort of philosophy. If any disagree
with my opinion, let them at least take note of the experiments and
discoveries which have been worked out and demonstrated by me, with
many pains and vigils and expenses. Let them rejoice in these, and
employ them to better use if they are able.”
The remarks by Dr. Gilbert as to the careful way in which experiments
should be made, the plain words in which they should be described, and
the attention that should be paid to the work of others, are as true
to-day as they were in his time. He said:--
“Whoever wishes to try the same experiments let him handle the
substance, not carelessly, but prudently, deftly, and in the proper
way, and when the thing does not succeed let him not in ignorance
denounce my discoveries, for nothing has been set down in these books
which has not been many times performed and repeated. This nature
knowledge is almost entirely new and unheard of, save what a very few
writers have handed down concerning certain common magnetic powers.
Therefore I but seldom quote ancient Greek authors in my support,
because neither by using Greek argument nor Greek words can the truth
be demonstrated more precisely, for our magnetic doctrine is at
variance with most of their principles.
“Nor have I brought to this work any pretence of eloquence or
ornamentation of words, I have only put difficult and unknown things in
such a form of speech, and in such words, as to be clearly understood.
Sometimes I have to use new and strange words, not in order to throw a
veil of mist over the facts, as alchemists are in the habit of doing,
but that hidden things which have no name, never having been observed
before, may be plainly and correctly described. To the early fathers
of philosophy let due honour be paid, for by them wisdom has been
handed down to posterity. But our age has detected and brought to light
very many facts which they, if they were now alive, would gladly have
accepted. I have therefore not hesitated to expound by demonstration
and theory those things which I have discovered by long experience.”
Dr. Gilbert, as announced by himself, began by studying the various
ores of iron, and soon found that it is only the black iron ore which
is naturally magnetic. He made many experiments with the magnetic
needle, which had been used before his time by the Italians and the
Arabs, and long before them again by the Chinese. He was the first to
show that the natural magnet, or lodestone, is not the only possible
form of magnet, and that a bar of iron may be made into a magnet
by simply hanging it in the direction in which the magnetic needle
points, or by heating it, and hammering it while holding it in the same
direction. He considerably increased the power of the natural magnet by
providing it with an iron cap, or armature, at each end.
[Illustration: Fig. 18.--Magnetising a Bar of Iron by hammering.]
Dr. Gilbert disposed of many mistakes made by previous investigators.
One of these was that, in order to preserve its strength, the
lodestone had to be fed on iron filings. They tried to test it by
taking a lodestone of a certain weight and burying it in iron filings,
which they had also weighed carefully. After leaving it for many
months they took out the stone and weighed it, and thought they found
the stone a little heavier, and the filings a little lighter. The
difference, however, was so small that they doubted whether there was
any. Dr. Gilbert said that some of the filings might easily have stuck
to the stone and made it appear heavier. In any case there was no
evidence that the stone had to absorb any nourishment to keep up its
strength.
Another mistake was committed by Paracelsus, who thought that the power
of the magnet could be increased ten times by heating it nearly to
red heat, and slaking it in oil of saffron. In this way, he said, a
lodestone could draw a nail out of a wall, and accomplish many other
wonderful things which are not possible for any ordinary lodestone.
This, however, was just the reverse of the truth. Dr. Gilbert found
that a lodestone treated in that manner does not gain power, but
actually suffers a certain loss of strength.
It was Dr. Gilbert who first proved that the strongest magnetic force
is found at two opposite points of magnetic bodies. These points he
called the Poles. He proved that two poles which point in the same
direction, when the magnet is free to move, repel each other, and those
which point in the opposite directions when suspended attract each
other.
[Illustration: Fig. 19.--The Earth as a Magnet.]
Magnets may be of very different shapes, and whatever shape a
piece of iron may have it can be magnetised, and when fully magnetised
it will show two poles.
Dr. Gilbert asked himself whether a magnet could have the shape of a
sphere. He cut a sphere out of iron, like a cannon ball, and magnetised
it by touching and rubbing it with a lodestone. Having done that he
took a small magnetic needle and brought it near the magnetic sphere of
iron.
He found that the magnetic needle behaved just as if the iron sphere
were the earth itself, pointing to the poles and showing different
directions at different distances from them. The thought flashed into
his mind that the earth itself might be a great magnet. This thought
was entirely new. The ancients had imagined that the attraction of the
magnetic needle was due to some magnetic mountains in the north, or
some star in the tail of the Great Bear, a group of seven stars which
appear to move round the north pole of the sky.
[Illustration: Fig. 20.--A magnetic Compass.]
This supposition could not be maintained as the facts became better
known. In the first place, since the stars appear to be carried round
the north celestial pole, one would naturally expect that the direction
of the magnetic compass would change in the course of every twenty-four
hours, which it did not.
[Illustration: Fig. 21.--Old form of Magnetic Dip. Circle.]
Then, as to the magnetic mountains, if they were situated at the
north pole, the compass would point in a horizontal direction only
in the region near those mountains. In southern lands and seas the
needle would have to point below the horizon, since the pole itself
is at a considerable angle below the horizon. The angle between the
needle and the horizon would have to become less and less as the ship
sailed towards the pole. This is, however, just the reverse of what
actually takes place. When a needle is balanced carefully before it is
magnetised, then on touching it with the magnet, and so magnetising
it, it is found that it balances no longer. The pole which points
to the north points down, and the pole which points to the south is
raised. This might be taken to indicate the existence of magnetic
mountains at the pole of the earth. But on proceeding to the equator,
this so-called “dip” of the needle becomes less, and on sailing towards
the pole it becomes greater and greater, until a point is reached at
which the needle points vertically downwards.
Dr. Gilbert found that a small needle brought near a magnetic sphere
behaves in a similar manner. At a distance midway between the two poles
the needle points in a direction parallel to the axis of the sphere. At
the two poles the needle points vertically downwards towards the centre
of the sphere. If this variation of the “dip” were quite regular,
sailors might discover their latitude on the sea by simply observing
the “dip,” and Dr. Gilbert thought that it was quite possible. In this
he was mistaken, because not only does the “dip” vary in an irregular
manner, but it also varies from century to century, being greater or
less at the same place from time to time.
Another property of the new compass discovered by Dr. Gilbert was the
magnetic “declination,” or the deviation of the compass from the true
north. This deviation he attributed to the irregular distribution of
land and sea. He thought that the mountains attracted the magnetic
needle towards themselves, and so disturbed the action of the earth as
a whole. Here again he was mistaken.
[Illustration: Fig. 22.--The Compass Needle does not point due North
and South except at a few parts of the earth. The deviation from the
true North is shown in the diagram at a few Places in England.]
The observations made since his time have proved that the deviation,
like the “dip,” is not constant, but varies from century to century. It
is, therefore, absurd to suppose that the mountains of the earth, which
keep their outline unchanged for many centuries, should be the cause of
the observed deviation.
Dr. Gilbert, like those who went before him, made mistakes. But his
mistakes we can easily forgive. So far as he was able, he reasoned only
from actual facts. He could not make personal observations extending
over a century, and he had at his hand no records of observations
extending over long periods. So far as the facts known to him extended,
he reasoned wisely and well. New facts observed by others who came
after him would naturally lead in many cases to new conclusions, and
Dr. Gilbert, had he lived in our own time, would have been the first
to acknowledge and welcome any new facts duly proved and established,
even though they contradicted some of his own theories. In the world of
science there can be no absolute master or dictator. Gilbert himself
was the first to overthrow the opinion that any master mind, however
gifted or distinguished, can govern the thoughts and ideas of all
men who come after him, and in this way he established one of the
principles of modern science.
When all the facts known are collected carefully together, it is
right to arrange them all in a certain order, and look at them from a
certain point of view, guided by certain principles. This principle,
or point of view, is called a scientific theory. The establishment of
such a theory enables us to survey the various facts and bewildering
details at a glance. It is useful as a means of lessening the brain
work required to remember a large number of facts. It also indicates
the directions in which new facts may be looked for, and when such new
facts are found it indicates probable explanations for them. But at
that point the usefulness of a scientific theory ends.
There is no such thing in science as absolute truth. A theory is only
true in so far as it covers all the facts. As soon as new facts are
discovered which do agree with it the theory falls to the ground. As
thousands of people are seeking every day to discover new facts, no
theory can be considered absolutely perfect. Actual facts, or results
of accurate observation, remain the same; but the explanation of the
effects observed changes as knowledge of nature grows from more to
more. This is a matter in which we nowadays differ from the ancients.
We want our science to be living and progressive. We do not want to
lose the fruits of the work of our ancestors. We find a place for it in
our theories, and keep those theories unchanged until we are compelled
to change them by the discovery of a new truth. But we insist that the
new truth shall be a real truth proved as carefully and laboriously as
the older truths were proved. And so we advance, step by step, to the
conquest of realms the limits of which no man can see.
CHAPTER V
GALILEO
THE man who is considered the greatest of all pioneers of
science was born in the year 1564 at the city of Pisa, in the north of
Italy. He first studied medicine, but after a few years he turned his
attention to Mathematics and Science, which suited him much better.
When only twenty-five years of age he was appointed for three years as
Professor of Mathematics in Pisa. He soon showed that he did not intend
to follow slavishly in the footsteps of the old masters. He lived at a
time when a new spirit of independent inquiry was beginning to be felt
in the universities of Europe.
Galileo’s most remarkable early achievement was connected with the
famous leaning tower of Pisa. That tower had been built on a weak
foundation, and after it was finished it began to lean over towards one
side, owing to the ground giving way beneath it. However, the tower did
not fall, but remained in a leaning position. It was used by Galileo in
order to prove that the old Greeks were wrong when they said that a
heavy body would fall more quickly than a light body.
[Illustration: Fig. 23.--Galileo.]
This is true, of course, when we compare a feather with a stone. But
that is due to the resistance of the air. If no air were present the
stone and the feather would fall to the ground together, and the
feather would make a little rattling noise just as if it were a piece
of wire. This, of course, could not be proved by Galileo, as he had no
means of obtaining a space free from air. But at all events he was well
able to compare the rates at which a large stone and a small stone fell.
The old Greeks had stated that large stones and small stones fall at
different rates, and nobody seems to have taken the trouble to find out
whether that is the case. Galileo did try on many occasions. He took
up a large stone in one hand and a small stone in the other hand, and
let them drop together. He found that they reached the ground at the
same instant. When he showed the experiment to his friends, they said
there was a difference in the rate at which the two stones fell, but
that the distance through which they fell was not great enough to allow
this difference to appear. To answer this objection Galileo went to the
top of the tower of Pisa. He took with him a cannon ball weighing a
hundred pounds and a shot weighing one pound. These were contained in
a box, and were dropped by overturning the box. They therefore started
together, and the people who watched the experiment also found that
they arrived at the bottom of the tower at exactly the same moment.
They seemed to hit the ground at the same time.
This famous experiment was the starting-point of a revolution.
[Illustration: Fig. 24.--The Leaning Tower of Pisa.]
The fall of those two balls marked the fall of the old system by which
the advance of science had been so long delayed. Henceforth Galileo
did not stop to consult the old masters before he tried an experiment.
Nature, he said, was always ready to answer questions. The only thing
necessary was to put the question to her in a clear and unmistakable
manner. Nature never hesitated with her answer. He therefore proceeded
during the rest of his life to question Nature constantly and
methodically. In this way he was able to discover a great number of new
laws, and his method has since been accepted as the only sure method by
which our knowledge of the structure and constitution of this world can
be extended.
Wishing to determine the speed of falling bodies more accurately,
Galileo varied the experiment in different ways. In one method, which
he used a great deal, he made spheres or balls of solid brass run down
a groove cut into a wooden board. This groove was lined with smooth
parchment, so that there should be as little resistance as possible to
the motion of the balls. He measured accurately the time at which the
balls were started and the time at which they arrived at the bottom
of the groove. These measurements would nowadays be made by means of
a watch, but at that time there were no watches except water-clocks,
such as had been used in Babylon, and, later on, in Alexandria.
Galileo measured time as best he could with the help of one of these.
He attached a small spout to the bottom of a pail of water, so that the
water ran out in a thin jet. At the instant when he started the ball on
its way down the groove he placed a little cup under the water tap, and
at the instant when the ball reached the bottom of the groove he took
the cup away.
[Illustration: Fig. 25.--A Simple Water-Clock.]
He then weighed the cup with and without the water, and so found the
amount of water that had flowed into the cup. By comparing the amounts
of water obtained in different experiments he was able to compare the
times.
His first object was to find in what way the speed of the body varied
with the distance over which it travelled, or the time during which it
was travelling. He first of all thought that the speed was proportional
to the distance travelled. He soon found, however, that when the
distance is doubled the speed at the end of the fall is not doubled. It
is less than double what it was before. This can be proved by letting
a ball roll along a very smooth surface after it has reached the bottom
of the groove. Galileo found at the end of a long set of experiments
that the speed attained by a body is exactly proportional to the time
during which it is exposed to a force capable of moving it. This
discovery was the origin of the whole science of motion. Galileo called
the speed which a body acquires in the unit of time the “acceleration”
of that body.
Galileo was the first to establish the idea of what is known in science
as Momentum. He found that the amount of motion in a body must be
judged not only by its speed, but also by the weight of the body. It
was evident to him that a heavy body moving at a certain speed is
equivalent to a number of lighter bodies moving with the same speed,
their combined weight being the same as the weight of the heavy body.
In order, therefore, to determine the amount of motion in a moving
body, it was necessary to take into account both the weight of the
body and the speed. Galileo used the word “momentum” to signify the
result obtained by multiplying together the two numbers expressing the
speed and the weight of a body. He also started the idea of centrifugal
force, or the force with which a body stretches a string when it is
swung round by the string.
But the most important work of Galileo was that which he did with the
pendulum. When still quite a young man, it happened that he was at
the cathedral at Pisa during a service. He noticed that a great lamp
suspended from the ceiling far overhead had been left swinging after it
had been lighted. He watched the lamp swinging to and fro for a long
time, and noticed that the swinging gradually diminished. But while the
amount of swinging diminished, the time of swinging appeared to remain
the same. In order to test this he counted the number of times his own
pulse would beat between one swing and the next. He had no clock or
watch in his possession, and was unable, in the circumstances, to use
such a thing as a water-clock, considering that he was supposed to be
engaged at his prayers. By counting his pulse he found that the time of
swinging remained exactly the same until the swing had quite died away.
Any ordinary young man would not have found anything very interesting
in that, but to Galileo the observation was a kind of revelation. He
thought that if he could work out a machine driven by swinging weights,
such a machine could be made to go quite steadily, and could be made
into a clock which would show the time accurately. He did contrive a
kind of counting machine by means of a pendulum which would count its
own swings, but he did not succeed in making a clock that would keep
going any length of time. He designed a clock which could be wound up
and driven by means of a weight, but this was not done until he was
very old and blind, and he could only dictate the description of the
clock to his son, and the latter did not succeed in constructing the
clock until ten years after his father’s death. Nevertheless Galileo
must be regarded as the inventor of the pendulum clock.
In the year 1609 a rumour reached Galileo which started him on an
entirely new line of discovery. He had heard that a Dutch optician had
presented to a German prince an instrument constructed in such a manner
that objects which were distant could be seen as if they were near. A
letter which he received from Paris confirmed the news, but could not
give him any information as to how the instrument was constructed.
Galileo thought about this discovery for a whole night, and in the
morning he had found out the secret of it. He took two lenses, one
of them a magnifying glass, and the other a glass which made things
smaller. The magnifying glass he fixed at one end of a leaden pipe, and
at the other end he fixed the other glass. This end he held to his eye,
and the other end he directed towards a distant object. At first he saw
nothing, but on moving the magnifying glass to and fro along the pipe,
he found the position in which distant objects appeared magnified to
three times the ordinary size. He straightway set to work to improve
the instrument, and in a short time he succeeded in constructing a
telescope which brought objects thirty times nearer and magnified their
surface a thousand times.
[Illustration: Fig. 26.--A modern Astronomical Telescope.]
Galileo went to Venice and showed it to the chiefs of the Republic. He
took them up to the highest church tower in the city, and mounted his
telescope so as to look out to sea.
He said, “Many noblemen and senators, although of great age, mounted
the steps of the highest church towers at Venice to watch the ships,
which were visible through my glass two hours before they were
seen entering the harbour.” This remarkable invention made Galileo
world-famous. The kings, princes, and learned men of the world all
wanted telescopes, and asked Galileo to make them. Such requests even
came from Holland, where the principle of the telescope had first been
discovered.
But although the great men of Europe used the telescope on many
occasions both in peace and in war, Galileo put it to a greater use.
He conceived the idea of turning it to the heavens and examining the
celestial bodies. In doing so he obtained a most surprising series of
revelations. He first of all turned it on the moon, and saw at once
that the markings on the surface of the moon were really mountains and
level plains. He could watch the shadows of the mountains grow and
diminish with the varying direction of the sun’s rays, and he could
even prove that some of the mountains were higher than others. He then
turned his telescope on the great planet Jupiter, and noticed to his
great surprise that Jupiter had near it four smaller companions which
circled round it in a few days. In fact, he found that Jupiter has four
moons where the earth has only one. This discovery was entirely opposed
to the ancient idea, then universally held, that there were only seven
planets or moving stars, namely: the Sun, the Moon, Mercury, Venus,
Mars, Jupiter, and Saturn.
On pointing the telescope to another great planet, Saturn, Galileo
found that another surprise awaited him. It looked as if the planet had
two handles, like a jug. This strange appearance was really owing to
the fact that Saturn is surrounded by a fiat ring, which being slightly
tilted gave the appearance of the two handles. This, of course, was
not known at that time to Galileo, but he certainly was the first to
observe that Saturn had a very unusual appearance.
[Illustration: Fig. 27.--The Planet Saturn and its Rings.]
Galileo next turned his telescope on the sun at a time when it was near
the horizon, and not strong enough to blind him. He observed that the
sun was not evenly bright all over its surface, but that there were
spots on it. These spots came and went, and could be seen to appear
at one edge of the sun, to move round, and to disappear at the other
edge about a fortnight later. This gave Galileo the idea that the sun
revolves on its axis.
After that, Galileo turned his attention to the planet Venus, and
found, contrary to all accepted opinions, that this glorious planet
showed phases like the moon, and was sometimes seen in the shape of a
crescent. Such appearances were not presented by Jupiter or Saturn, and
it struck Galileo that the probable explanation of this must be that
Venus revolves round the sun at a lesser distance than the earth, and
the other planets at a greater distance.
[Illustration: Fig. 28.--Photograph of the Sun on two successive days,
showing Sun-Spots and the Rotation of the Sun.]
Now at the time of Galileo the accepted opinion was that the earth
stood still, and that the sun and all the planets revolved round the
earth. This opinion the professors and philosophers of the time sought
to justify by quoting the Holy Scriptures, pointing out such a passage,
for instance, as that in which Joshua is said to have commanded the sun
to stand still. Galileo replied that the Bible was intended as a guide
in religious matters, but not as a text-book of science, and that the
only method of arriving at truth in science was by carefully observing
the phenomena and seeking for their most reasonable explanation.
However, he was faced with a strong opposition. Some of the enemies
refused to believe their eyes, saying that although the telescope was
useful in observing objects on land or sea, it was misleading when
pointed at the stars. Others refused to look through it at all.
[Illustration: Fig. 29.--The Earth in Space.]
About one of these objectors Galileo wrote to his friend Kepler,
“Oh, my dear Kepler, how I wish that we could have one hearty laugh
together!
[Illustration: Fig. 30.--Galileo’s Pendulum Clock.]
Here at Padua is the principal professor of Philosophy, whom
I have repeatedly and urgently requested to look at the moon and
planets through my glass, which he obstinately refuses to do. Why are
you not here? What shouts of laughter we should have at this glorious
folly! And to hear the professor of Philosophy at Pisa labouring before
the Grand Duke with logical arguments, as if with magical incantations
to charm the new planets out of the sky.”
At length the enemies of Galileo denounced him in Rome, and Galileo
was called before the Inquisition to answer for his strange doctrines.
The court before which he was judged decided that his doctrines were
contrary to divine revelation, and threatened him with torture if he
should continue to teach them. Galileo had no choice but to renounce
his doctrine that the earth moved round the sun, and to do penance for
having taught it. He was now seventy years old, and although he was
allowed to return to his home his useful work was nearly at an end. He
devoted the remainder of his days to Mathematics and to the invention
of the pendulum clock, as already stated.
The centuries which succeeded Galileo have fully justified him and
his teachings. The idea that the earth moved round the sun is now
completely established, and is universally accepted by all educated
people. Not only that, but science is now free to pursue its course
without regard to the errors and prejudices of the old schools of
Philosophy. It is now recognised that no artificial limits can or must
be put in the way of the march of the human intellect, and that the
increase of knowledge makes invariably for the increased happiness of
mankind.
CHAPTER VI
THE BAROMETER
THE ancient philosophers divided all things into four
elements--earth, water, air, and fire. Of these, or a mixture of
them, they thought that all visible things were made up. Nowadays we
recognise about seventy different substances, which we call elements,
and we know that what the ancients called earth is a mixture in
various proportions of about a dozen of our elements. When we use the
word element, we mean a kind of substance which cannot be obtained by
mixing other substances together, and cannot be split up into two or
more substances possessing different properties. We know that water can
be split up into two substances, and that air is a mixture of at least
two other substances. As regards fire, it is now generally thought that
it is composed of hot bodies having the consistency of air. Such bodies
we now call gases. One of the most familiar examples of gas is that
which is used for lighting a room or a street. But the gas is only one
of a great variety of similar substances.
Instead of four elements, we now speak of three states of matter. These
three states are the solid state, the liquid state, and the gaseous
state. When a substance has a certain amount of hardness, when it does
not flow, and does not assume a level surface if left to itself, we
call it a solid. If a substance flows, and fills up a vessel into which
we pour it, and yet remains visible and tangible, we call it a liquid.
Lastly, if the substance fills up completely any space into which we
place it, and expands as far as it can, we call it a gas. Nearly every
liquid and most solids may be converted into gases by heat, and this
circumstance accounts for the great variety of gases which are known
to exist.
A great difference between a solid and a liquid is that a solid left
to itself only presses down upon its support, whereas a liquid also
presses sideways and upwards. In this respect fine sand partakes
somewhat of the nature of both solid and liquid. If the stalk of a
dandelion, or some other hollow stalk, is planted in a box of sand, it
can be crushed by packing the sand very tightly. This shows that the
sand is capable of exerting a pressure sideways. But the coarser the
grains of sand, the less readily does this sideways pressure follow
upon the pressure on the surface. On the other hand, if the sand is
extremely fine it readily exerts pressure in all directions. If we
could produce sand about a thousand times more finely grained than the
finest sand known, we should obtain a substance which would instantly
transmit the slightest pressure on the surface through the whole of the
substance. We should have, in fact, a liquid.
That water is able to exert pressure in all directions may be proved
by means of a small bladder attached to the end of a tube, as in the
toy which is blown up and makes a squeal on letting the air escape.
If, instead of letting the air escape, the tube is stopped up, and the
bladder attached to a long rod and gradually lowered into a tank of
water, the bladder is observed to become smaller and smaller as it gets
lower down in the water. At the same time, it remains as round as it
was originally, thus showing that the pressure of the water is exerted
in every direction, and not on the top, bottom, or sides only.
Another way of showing that water exerts a pressure is by tying a sheet
of thin india-rubber over the end of a long and wide tube of glass,
and lowering the end into the water. The india-rubber is seen to bend
into the tube more and more as it is lowered farther and farther into
the water. This shows that the pressure is in proportion to the depth.
The pressure is not altered by putting any solid object into the water,
so long as it does not change the level of the water. The pressure on
the bottom of the vessel is the same so long as the level of the water
remains constant. If the bottom of the tank is weak in any particular
place, we can protect that place by laying a plate of iron across it,
so long as there is no water between the iron and the weak place. For
if there is any water between the bottom of the tank and the iron
plate, that water hands on the pressure just as well as when the iron
is not there at all.
If instead of an iron plate, we put a solid block of iron suspended
from above, the pressure on the bottom of the tank remains the same.
The pressure simply depends upon the height of the surface of the water
above the bottom of the vessel, and is not changed when the surface is
made either very much larger or very much smaller. This curious fact
was discovered by Stevin. He was Inspector of Dikes in Holland, and had
a great deal to do with the pressure exerted by water upon the dikes
which keep the sea away from the low-lying Netherlands. It was his
business to make sure that the pressure of the water was not sufficient
to break through the dikes.
It was already surmised by the ancient Greeks that air had a certain
amount of weight, but it was not suspected by them that the amount
of air which fills a room would weigh as much as one hundredweight.
If liquids are capable of exerting a pressure in all directions, it
follows that gases, which naturally exert a pressure, will certainly be
able to do the same. But it was very long before the world found that
out. The Greeks thought that an empty space could not exist because
Nature had a horror of it, and hastened to fill up any empty space that
might be produced with any substance that happened to be close by.
Therefore, if a pipe was stopped up with a close-fitting stopper, and
the stopper or piston was drawn in one direction, the air in the pipe
naturally followed the piston. If the end of the pipe was stopped up
the piston could only be drawn with great difficulty.
[Illustration: Fig. 31.--Water retained in the Cylinder of a simple
Pump as the Piston sinks.]
The Greeks thought that this difficulty was due to Nature’s horror of
an empty space, and on closing the tube or pipe with the finger, they
found that the finger or the skin was drawn into the tube. So “anxious”
was Nature to fill up the empty spaces. This supposed horror was made
useful by constructing pumps of much the same kind as those still used
to raise water. The tube of the pump dips into the water, and upon
raising the piston water rises up the tube.
[Illustration: Fig. 32.--Water rising in the Tube of a simple Pump as
the Piston rises while Water above the Piston is being lifted out.]
People succeeded in raising water in this manner as high as thirty
feet, and they did this for centuries without finding any limit to the
height to which water could rise, probably because they could not very
well make pipes much longer than thirty feet.
But in the time of Galileo some people made a pipe forty feet long,
hoping to pump up water through that distance. They found, however,
that no amount of pumping could raise it more than about thirty-three
feet, and that at that point Nature’s “horror of vacuum” or empty space
seemed to cease. This observation showed that there was something wrong
about the supposed horror, and people began to suspect there was no
such thing. Galileo himself, who was consulted on the matter, admitted
that there was a certain difficulty about producing an empty space, or
a vacuum as it is called, but thought that difficulty, which he called
the “resistance of a vacuum,” had certain limits. Galileo died before
he could solve completely the problem of the resistance of a vacuum,
but his friends and disciples pursued it ardently, and finally arrived
at a complete explanation of the difficulty.
The most famous of these successors of Galileo was a man of the name of
Torricelli, who lived in Rome. He had studied the works of Galileo as
a boy of sixteen, and had himself written a book on Mechanics. Galileo
saw the book and invited the young man to stay with him in Florence. It
is said that the two became great friends, and that when Galileo was
very old and blind his declining days were cheered by the conversation
of young Torricelli. When Galileo died, his patron, the Grand Duke of
Tuscany, made Torricelli Professor of Mathematics at the Academy in
the place of Galileo.
It was not long before Torricelli thought of a new and striking
experiment concerning the vacuum. He decided to fill a glass tube with
the heavy liquid quicksilver or mercury instead of water, and expected
that the so-called resistance of the vacuum would be about fourteen
times greater in the case of quicksilver than in the case of water,
so that quicksilver could only be sucked up something between two and
three feet instead of thirty-three feet. He found great difficulty in
getting a suitable glass tube, since the glass-blowers of that time had
not yet learned to make strong glass tubes, although they were very
clever at making all kinds of bottles. Torricelli himself never carried
out this experiment, but it was performed by a friend of his in the
year 1643. It was described by Torricelli in some letters which he sent
in the next year to a friend in Rome. That friend straightway wrote to
some of his friends in Paris, and the news he conveyed to them created
a great sensation.
This was the experiment: A glass tube three feet long, and closed at
one end, was filled with mercury. It was then stopped at the open end
with the finger, and that end was carefully brought under the surface
of a dish of mercury. On removing the finger it was found that some
mercury flowed out of the tube, leaving a space of about six inches
vacant at the top. This space was a vacuum. When Pascal, the great
French man of science, heard of this experiment, he said, “It appears
that the vacuum is not impossible in Nature, and that she does not shun
it with so great a horror as some imagine.”
[Illustration: Fig. 33.--Early form of Barometer.]
The next problem was to give a reasonable account of the extraordinary
observation. Both the Italians and the French were not long in arriving
at a correct explanation, which was based upon the facts observed when
two vertical tubes are joined at the bottom and filled with different
liquids. A heavy liquid in one tube can counterbalance a longer
column of a lighter liquid in the other tube. If two tubes could be
constructed each one hundred miles high, they would reach nearly to the
top of the atmosphere. Now, if one of the tubes was left full of air,
and if instead of air mercury could be poured into the other tube, a
very short column of mercury would balance the whole column of air in
the other tube. The length of the mercury column would be as many times
shorter than the air column as the mercury is heavier than air. The
two tubes would in fact form and any change in the amount of air in
the air tube would be indicated immediately by a rise or fall of the
balancing column of mercury.
[Illustration: Fig. 34.--The pressure of the air upon the mercury in
the short open tube keeps up a column of mercury about 30 inches long
in the closed tube.]
In a barometer it is therefore sufficient to close the tube containing
the mercury at a height of about three feet, so that a tube one hundred
miles high is not required there. Nor is such a long tube required on
the other side either. For, as we have already seen, the pressure of a
liquid simply depends upon the height of its surface above the bottom
upon which it presses, and as the width of the tube makes no difference
the air tube may be made as wide as we please, or may be taken away
altogether, so long as we prevent the air bubbling up into the vacuum.
This can be done by bending round the lower end of the mercury column
until it points upwards. The mercury then rises and falls both in the
long tube and in the short tube, and the pressure of the air upon the
short tube is measured by the difference of level of the mercury in
the two tubes.
It would, of course, be more convenient if the height of the mercury by
itself indicated the pressure of the air, and this can be secured by
dipping the lower end of the mercury tube into a large vessel full of
mercury, so that its level does not perceptibly change when a little
mercury flows in or out of the barometer tube. This is, in fact, the
modern form of the mercury barometer.
Torricelli himself was quite aware of the importance of his experiment.
He wrote, “I do not mean simply to produce a vacuum, but to make an
instrument which shows the changes of the air, now heavy and dense, and
now lighter and thin.”
Pascal said that if it is the pressure of the air which raises the
mercury under the vacuum, then the mercury in a barometer must stand
lower on the top of a mountain than it does when the barometer is on
the ground near sea-level. He asked a brother-in-law who lived in the
south of France to take a barometer up a high mountain. It was found
that the mercury column fell three inches, since less air remained
above to exert any pressure. “This,” said the observers, “ravished us
with admiration and astonishment.” Pascal also took a balloon half-full
of air up a mountain. He found that it gradually filled out until it
was quite tight, and that it collapsed again on descending. This
result showed that the pressure which was not sufficient to fill out a
balloon on the lower ground was quite sufficient to counterbalance the
reduced pressure of the air upon the mountain.
There were some who would not accept the view that the pressure of the
air was sufficient to uphold a column of mercury against gravitation.
They could not believe that the enormous pressure required could be
furnished by such a rare substance as air. They weighed the pressure
exerted by mercury on the supporting surface, and found that a
column of mercury thirty inches long presses with a force of fifteen
pounds upon every square inch of the supporting surface. They would
not acknowledge that the air could exert such an enormous pressure,
and showed that this pressure, exerted by the air on the surface of
the human body, must amount to several tons. The inventors of the
barometer replied that that enormous pressure on the human body was
counterbalanced by the pressure of the air which is contained within
the body itself, so that no inconvenience is felt.
One philosopher claimed to have found that the mercury hangs by
invisible threads from the upper end of the tube, and that he could
feel those threads when he stopped the upper end of the tube with his
finger.
But that air is easily able to exert great pressure was proved
afterwards by an Irishman called Robert Boyle, who showed that when air
is compressed to half its volume, it exerts a pressure of quite thirty
pounds to the square inch.
The vacuum above the column of mercury was intensely interesting to its
discoverers. So far as they knew, it was a perfectly empty space, and
the first of its kind ever discovered. In appearance it was just as if
it were filled with air, and it was just as transparent, so that it was
evident that light had no difficulty in shining through empty space.
This fact suggested the idea of trying whether sound could also travel
through empty space, and the Experimental Academy established in Rome
after Galileo died, tried in various ways to discover whether sound was
propagated through a vacuum. But they did not succeed in arriving at
any decision. A little bell which they brought into a vacuum, suspended
by a thread, kept on ringing quite audibly. But in that case the sound
might easily have been communicated by the thread to the glass, so that
the experiment proved nothing. The real decision of the question only
became possible with the next triumph of human genius, the invention of
the air-pump.
[Illustration: Fig. 35.--An Aneroid Barometer.]
The barometer is now a household instrument, used for indicating the
weather. The mercury falls rapidly when a storm is approaching, and
warns the mariner to make for the harbour or the open sea. Every ship
is now provided with a barometer, but not always with one containing
mercury. The instrument used frequently has a round dial, like a clock,
with two hands.
One of the hands can be set from outside, and marks the position of the
second hand at a certain time. The second hand moves to the right and
left, as the pressure rises or falls. The hand is driven by a mechanism
which connects it with a metallic box from which the air has been
pumped. The pressure of the air presses more or less on the lid of the
box, and so drives the index on the dial.
CHAPTER VII
THE AIR-PUMP
WHILE Galileo’s plan of work was to consider a theory, and
then to test it by experience, other men of science went to work in the
opposite way.
[Illustration: Fig. 36.--Shape of tube used by Boyle to show that air
exerts pressure.]
They made a large number of experiments, and gradually allowed the
results of the experiments to force them into a certain way of looking
at things. One man of science who adopted the second method was Robert
Boyle, an Irishman, who made himself famous by his studies on what he
called “the spring of the air,” meaning the pressure which compressed
air can exert. He took a long glass tube and bent it into a hook at the
lower end, which was closed. He then poured in mercury through the top,
until the bent end contained only a little air, which was compressed
by the weight of the mercury in the long tube. He poured in more and
more mercury, and found that the space filled up by the air became
smaller, the greater the pressure of the mercury was. He found, in
fact, that the air behaves very much like an elastic spring, which can
be stretched and compressed.
After some experience of this kind, Boyle heard of an apparatus which a
German had invented, and with which he performed the most astonishing
experiments. This man’s name was Guericke. Guericke had the idea of
finding out all about the stars, and how they moved through space. He
believed that they did not move through air, because if they did they
would gradually be stopped by the resistance of the air. He decided
that if he wished to find out how they moved, he would have to obtain a
space similar to that in which the heavenly bodies moved. This space,
he said, would have to be an empty space. He therefore tried to make an
empty space.
The first method Guericke adopted was a very curious one. He took a
big cask filled with water, and containing only one opening. Into that
opening he screwed a pump, and tried with all his might to pump the
water out of the cask. This was more difficult than he had expected,
for the air, pressing on the water with the force of fifteen pounds per
square inch, held the water in the cask. If the opening had been free,
the air would have got into the cask in bubbles, and would have pressed
the water out through the opening. But Guericke did not allow any air
bubbles to get into the cask, and so he had to work against the whole
pressure of the air in trying to pump the water out of the cask. If he
could but pump it out, he was convinced that he would leave only empty
space in the cask.
[Illustration: Fig. 37.--Guericke’s experiment of trying to draw Water
out of a full cask in order to leave a Vacuum.]
Well, he did get a large portion of the water out of the cask by
working the pump with all his might, but as soon as the water flowed
out, he heard a gurgling, or bubbling, sound made by the air which was
pressed in through all the joints which were not quite air-tight. Then
he put one cask inside another, but he did not succeed much better,
because again there was a noise resembling hissing and the twittering
of birds. This experiment continued for several days, and in the end
the cask was found full of air. Guericke therefore was convinced that
he could not get an empty space while using a wooden cask.
He then tried a ball made of beaten copper. Three men pumped the air
out of this ball, and no sooner had most of the air been pumped out
than the ball collapsed with a loud report, crushed by the pressure of
the outside air. He then had another copper globe made. It was divided
into halves, and these were joined by means of a ring of leather which
was well soaked in a mixture of wax and turpentine. When that globe was
emptied of air by means of the pump, the halves stuck so well together,
that no human power was able to separate them; but when air was let in
by turning a cock, they immediately fell apart. This experiment was
performed again and again, and on one famous occasion it was performed
before the German Emperor and his Parliament. On this occasion the two
half-globes were fixed so firmly together by the pressure of the air
merely, that it took sixteen horses to pull them apart. The total force
with which they stuck together was about equal to the weight of a ton.
When Boyle heard of these experiments, he made an air-pump of his own.
He arranged it so that he could easily pump the air out of a glass
globe which had a valve.
[Illustration: Fig. 38.--Old Print showing horses trying to pull apart
two Magdeburg Hemispheres from which the air had been pumped.]
He could put in various objects from above by opening the valve, and
then pump out the air and see what happened. The whole globe could
be taken off the pump very easily, since it had a brass nozzle which
fitted into the pump. The brass nozzle was fixed to the glass by means
of a cement consisting of pitch, resin, and potash.
After the air-pump had been discovered, everybody set to work to try it
in various ways. Boyle brought a small closed bladder into the globe
and worked the pump. He found that the bladder at once expanded, and
finally burst; evidently because the pressure of the air inside the
bladder was no longer balanced by the pressure of the air in the globe.
He also put a little cup of water under the air-pump, and found that it
gave off bubbles and finally appeared to boil, although it was not hot.
Boyle thought that water could be converted into air, but in this he
was wrong, because we know now that steam is given off from water under
low pressures of air even when the temperature is very low. Boyle found
also that water falling in a vacuum makes the sound of some metallic
object.
Meanwhile, Guericke was also actively improving his apparatus, and
soon he was able to pump out nearly all the air, and to leave only
one-thirtieth of the original amount in the vessel he employed. He
next succeeded in proving that a vacuum does not allow sound to pass
through it. He placed a clock into the vacuum, and as he pumped the air
out more and more, the striking of the clock was heard more and more
faintly, until it became quite inaudible. He was so convinced of the
powerful effect of the suction of an empty space, that he said that if
a man were to blow his breath into a large vacuum, that breath would
be his last.
[Illustration: Fig. 39.--Guericke’s experiment to show the pressure
upon the piston of a cylinder from which air was being extracted.]
The Marshal of the Emperor refused to believe this, but Guericke
undertook to convince him of the truth in another manner. He
constructed a cylinder with a piston which moved in it air-tight. This
piston was suspended by a rope over a pulley, and to the end of the
rope twenty small ropes were attached, and each of these was held by a
strong man. They pulled up the piston until it was at the top of the
cylinder. Into the bottom of the cylinder a small hole was bored, and
into this hole Guericke introduced the nozzle of his empty globe, which
was stopped by means of a cock. On turning the cock, the air in the
cylinder rushed into the empty globe, or rather it was driven in by the
force of the outer air. So great was this force, that the twenty strong
men holding the ropes were pulled violently forward and were quite
unable to keep the piston up. A somewhat similar arrangement was used
for firing shots without powder.
It is remarkable that all this time Guericke had not heard of the
wonderful experiments made in Italy by Torricelli, the pupil of
Galileo. Had he done so, he might have been led to try to obtain a good
vacuum by means of mercury, as is done nowadays. But in some respects
his apparatus was greatly superior to that used by the Italians. In
order to find whether animals could breathe in rarefied air, some of
the Italians brought birds and other small animals into the vacuum at
the top of the column of mercury. It was found, of course, that the
animals could not live, partly because air was necessary for breathing,
and partly because their whole structure and constitution was
disturbed by the pressure of the air within them, which was no longer
counterbalanced by the pressure outside. Such experiments as these,
together with others on combustion, could be performed much better with
the air-pump as constructed by Guericke; although it must be borne in
mind that such experiments on animals are not now necessary, since we
know that animals cannot survive after they are deprived of air.
The effect of the invention of the air-pump was very great. Men
realised for the first time that they lived at the bottom of an ocean
of air; and that if all air were removed, the world would be quite
different. It would be a world in which neither animals nor human
beings could live. The air-pump in the course of the next century led
to two other great inventions, the steam-engine and the balloon.
CHAPTER VIII
THE INVENTOR OF THE STEAM-ENGINE
IT has long been supposed that the steam-engine was invented
by an Englishman of the name of James Watt. But the real inventor of
the steam-engine was Denis Papin, a French doctor, who invented the
engine while in Germany, and described it in a book which he published
in England, where he lived for many years. He made friends with Huygens
in Paris, and assisted him in carrying out his experiments on the
air-pump.
[Illustration: Fig. 40.--Papin’s improved Air-Pump.]
Huygens was very busy with a great variety of inventions, and was glad
to have this young assistant, who would carry out experiments for which
he himself had no time.
Papin made a number of improvements in the air-pump. One of these was
a cock, constructed on the principle of a gas cock or water tap, but
having two channels through it. Another great improvement which he
made in the air-pump was that he put the objects under a glass shade
instead of in a bottle. The shade was placed upon a flat plate, and its
edge was well greased, so that no air could enter from below.
[Illustration: Fig. 41.--One of Papin’s Experiments. A bladder
containing very little air swells up when a vacuum is produced in the
bell-jar, owing to the expansion of the air contained in it.]
In the middle of the plate was a hole, and the air was pumped out from
below through this hole. Air-pump experiments could thus be made with
a great number of different objects, one after the other, by simply
lifting off the shade and replacing it after putting the object upon
the plate. This method was easier than screwing off the cap of the
bottle used by Huygens.
[Illustration: Fig. 42.--Papin’s Mining Pump. Water flowing over the
water wheel Q turns the shaft P, provided with a double crank. This
drives the double air-pump OO, which sends air through the tube R
alternately out of the cylinders H and L at each turn of the cock S.
The pistons wind and unwind the ropes E and F, and draw up water in the
bucket C attached to the wheel B.]
Papin described these improvements in the year 1674 in a little
book called _Experiments on Empty Space, with a Description of the
Machines by which it is obtained_. The little book was dedicated to
his master Huygens, to whom he wrote as follows:--“These experiments
belong to you. I have made nearly all of them according to your
instructions. But I know that they are only a recreation to you. You
would never have written them down, and certainly you would never have
published them. And so, I am sure, you will not mind my doing it for
you.” Ten years after his book was published, Papin was appointed by
the Royal Society of London a curator of experiments, his duty being to
devise and show experiments to the members of the Society.
Papin was greatly interested in mining, and he was always puzzling
about some way of pumping up the water out of a mine. At that time
the pumping had all to be done by hand, and it was done by a string
of buckets, such as are used in wells. Papin had the idea of making a
water-mill--driven by a brook—pump the water out of the mine. But this
method was difficult unless a brook was close to the mine. However,
Papin designed a system by which the water-power could be transported
easily to the shaft of the mine. The water-mill drove the pistons in
two cylinders connected with a long pipe. The pistons compressed the
air and drove it along the pipe till it got to the mine.
At the mine there was a big wheel, or pulley, bearing a rope with
buckets attached to it. Other ropes were attached to the shaft of the
pulley, and were fixed to two other pistons moving in two cylinders.
These cylinders were connected with the pipe containing the compressed
air. First one cylinder was connected, and the buckets were lowered;
then a tap was turned which connected the other cylinder instead, and
this raised the buckets full of water.
This plan was a great improvement on the hand-pump (p. 70), but it was
not quite satisfactory; and it failed when the brook ran dry in summer
or was frozen in winter. Also the pump was rather a clumsy one. Papin
invented a better instrument, which he called the centrifugal pump,
and it is used to the present day. The pump consisted of a round, flat
box containing a paddle wheel which whirled the water round. The water
entered the box at the centre of the paddle-wheel, and on one side of
the box was a pipe through which the water escaped on being whirled
round. Papin tried to use this pump for pumping water, but he found
that he had no machine for moving it quickly enough, nor did he get
such a machine until he had invented the steam-engine. This, however,
he did not succeed in doing for many years, and meanwhile he invented a
number of other instruments.
One of these was a submarine boat. The first man to invent a
diving-bell which could move under water was named Drebbel. He was said
to have taken a trip under the Thames, disappearing entirely under
water and coming up in a distant place. Some years after this, Boyle
asked a medical doctor who had married a daughter of Drebbel, how it
was that Drebbel and his crew were not suffocated under the water. He
was told that Drebbel possessed a certain extract of air which enabled
him to breathe in a confined space. We are nowadays not able to tell
how much truth there was in the story concerning Drebbel.
[Illustration: Fig. 43.--An early form of Diving-Bell.]
But it is certain that Papin succeeded in constructing a diving-bell,
and in using it under water. His diving-bell was not a bell at all,
but a round box 6½ feet high, in which three men could stand upright.
When sunk under water it did not disappear entirely, because a pipe
stuck out of the top through which fresh air could enter or the used-up
air be pumped out. The pump which did this was one of Papin’s own
centrifugal pumps. On one side of the diving-bell was a very wide tube,
through which a man could crawl. It had a little door at the extreme
end which could be opened under water. This would, of course, let in
the water, but Papin, with one of his pumps, drove air through the tube
from within the diving-bell. This stream of air kept the water back,
and the poor man in the tube (he must have been very uncomfortable)
could do some work in the water outside, such as laying a mine for
exploding an enemy’s ship.
Few people know that Papin was the real inventor of the steam-engine.
But everybody acknowledges his invention of a kettle for cooking food
under high pressure. He called it a “digester.” When water is boiling
it cannot be made any hotter by putting greater heat under it. Greater
heat simply makes it boil away more quickly. But it can be made hotter
by preventing the steam from escaping. When the lid is stuck on tight,
and the spout is stopped up, the steam cannot escape until it reaches a
high pressure, and this means a higher temperature at which the water
boils.
Papin, therefore, constructed a kettle which was closed at the top by
means of a heavy weight. He did not close up the kettle with a screw,
since that would have led to an explosion. He only made sure that the
steam could not escape until it was hot enough and strong enough to
lift the heavy weight. People thought at that time that if they could
cook things at a very high temperature, they could get nourishment not
only out of meat, but out of the bones as well. They thought they could
make the bones eatable. Papin thought so too, and he cooked some bones
in his new kettle. The hardest bones of beef and mutton were made as
soft as cheese, and the extract from them made thick jelly, though the
bones themselves were not good to eat. The digester is used to obtain
gelatine from bones in this way at the present time.
[Illustration: Fig. 44.--Papin’s Cylinder, in which the Piston was
raised by boiling water under it.]
When Papin seriously set about inventing the steam-engine, he was
guided by what he remembered about a machine invented by Huygens. In
the year 1674 he had himself constructed this machine. It was intended
to raise water by an explosion of powder. The water was intended to be
used for the great water-works in the palace of the King of France.
A long cylinder containing a heavy piston was fixed in an upright
position. In the bottom of the cylinder was a plug which could be
screwed in. Before it was screwed in, some gunpowder and a slow match
were placed on the plug. Shortly after the plug was screwed in, the
match lighted the powder, the powder exploded, and drove the piston up
to the top of the cylinder. When the cylinder cooled down, the pressure
of the air drove down the piston. The piston was made to pull a chain
going over a pulley to which buckets were attached.
In the year 1690, Papin took the first step towards constructing the
steam-engine by inventing a cylinder in which the piston, instead of
being driven by the explosion of gunpowder, was forced upwards by
boiling some water under it. This plan made the plug unnecessary. The
piston A was driven up by the steam, and fixed at the top by a rod E
until it could be taken off the fire and made to do some work. There
was one difficulty about this cylinder. If the piston fitted tight,
it was not able to sink down to the level of the water at the bottom
of the cylinder, because in doing so the air in the cylinder would be
compressed, and would resist with a great force.
This difficulty was got rid of by Papin by means of another rod M,
which fitted in a hole in the piston. This rod was pulled out so as to
let the air escape. When the piston was down far enough, the rod was
pushed into the hole again so as to stop it up.
The great fault of this machine was that the cylinder had first to be
heated in order to drive out the piston and afterwards cooled, both of
which processes took time. About the year 1704, a German prince gave
Papin the order to build a steam-engine for pumping up water.
[Illustration: Fig. 45.--Papin’s Steam-Engine.]
Just at about that time Papin had seen a design of a machine by a man
of the name of Savery, which was not, however, a success. His own
machine consisted of a boiler which was to be put in the fire and
kept there. The steam from this boiler was conducted into a cylinder
half-filled with water. On the top of this water there was a wooden
float, so that the steam hardly came into contact with the water at
all. The float contained an iron box, into which a red hot iron
ball was placed. The steam, therefore, was not cooled on entering
the cylinder, but drove the float to the bottom of the cylinder and
forced the water into an upright pipe which was surrounded by another
cylinder. This water compressed the air in the second cylinder, and
valves were so arranged that the air could not turn back. It was
therefore used for forcing water up a pipe. Water was then allowed to
return into the first cylinder, and the operation was repeated. The
first cylinder afterwards always remained sufficiently hot to preserve
the power of the steam. In this respect Papin’s machine was a great
improvement on Newcomen’s machine set up at Wolverhampton in the year
1711.
Papin tried his machine in the presence of the German prince in the
year 1706. He did succeed in pumping water seventy feet high, but
the long pipes began to leak, and the German prince was so disgusted
that he would not give the money necessary to carry out any further
experiments. Papin, therefore, went back to London, and in the next
year he published a book called _New Art of Lifting Water with the
Help of Fire in a most Successful Manner_. He had great plans about
his machine. It was not only to pump water out of mines, but also to
fire guns, and to drive ships and carriages.
But Papin died a few years afterwards without being able to carry out
his great ideas beyond constructing a paddle-boat. He was one of the
world’s greatest inventors, but the engineers of his time were not
able to carry out his great inventions. Though his engine differs very
greatly from the steam-engines now made, it certainly contained a
piston working in a cylinder, and a safety valve, which are essential
parts of a modern steam-engine.
CHAPTER IX
ELECTRIC SPARKS
WHEN the ships of the Carthaginians sailed round the coast
of Spain and France so far as Cornwall, to sell their purple cloth in
exchange for tin, they sometimes brought home with them small pieces of
a stone which had the colour of gold, but was transparent, and could be
cut more easily than glass. This stone came from the shores of Germany,
mostly from the Baltic Sea. It was not really a stone, but the hardened
sap of a sort of pine tree, which had lain long underground; and it
sometimes showed traces of forest life, such as bodies of insects,
embedded in it.
This stone, or “amber” as we call it now, was a source of wonder and
delight to the ancient Greeks and Romans. It was thought to possess
all kinds of magic properties. Besides looking very beautiful in a
necklace it was supposed to have a kind of soul of its own; for it
sometimes attracted little pieces of straw, or dust. The ancients could
only imagine that this was the effect of some kind of will or desire
possessed by the amber. They only knew of one other substance which
showed similar power. This was the lodestone which attracted iron, and
which the ancients therefore sometimes called “quick iron,” meaning
“living iron,” just as the lively liquid mercury was called quicksilver.
It was Dr. Gilbert of Colchester who first proved that these two powers
are entirely different from each other. He called the power shown by
amber “electric” power, and the power shown by lodestone he called
“magnetic” power. Electric power, he said, had to be excited by rubbing
the amber with a piece of cloth; but magnetic power could only be
excited in a piece of iron by rubbing it with lodestone, or hanging it
in a direction pointing to the North Pole. It was Dr. Gilbert also who
invented a little instrument resembling a compass which showed electric
attraction. It was a short piece of straw mounted on the point of a
needle in the same way as a magnetic needle.
Although Gilbert had proved that electric and magnetic forces are quite
different, an event occurred in the year 1681 which showed that there
is some kind of connection between the two. In that year a ship bound
for Boston was struck by lightning.
[Illustration: Fig. 46.--Gilbert’s Electric Needle.]
It was found by comparing the direction shown by the mariners’ compass
with that found by means of the North Star, that the north pole of the
compass pointed south, and the south pole north. In fact the needle had
been reversed, and the ship was steered into Boston harbour with the
compass reversed.
Even at that time people had guessed that the lightning had something
to do with electric force, a guess which Benjamin Franklin afterwards
proved to be true. Robert Boyle observed that hair easily shows
electric attraction. It can be “electrified,” as we say now. When a
cat has been lying by the fire for some time and we stroke her with
the hand, we hear a slight crackling noise as if from small sparks.
Some parts of the fur stick together, others stand apart. This is also
observed by ladies when they comb their hair. When the hair is very
dry, individual hairs fly apart.
False hair, which is usually drier than growing hair, shows electrical
attraction very strikingly. Boyle observed this more than two hundred
years ago. He wrote to a friend, “false locks of hair brought to a
certain degree of dryness are attracted by the flesh of some persons.
Of this I had proof in two beautiful ladies who wore them. For at some
time I observed that they could not keep them from flying to their
cheek and sticking there, though neither of them had any occasion
for paint, nor used it. One of them gave me leave to make a further
experiment, and asking her to hold her warm hand at a convenient
distance from one of those locks taken off and suspended in free air,
as soon as she did this the lower end of the lock which was free
attached itself to her hand.”
It was soon found that other substances besides amber and hair show
electric attraction. Isaac Newton hung a round piece of glass in a
brass ring fixed about an inch above a table. He rubbed the glass with
some “rough and raking stuff,” till some very small fragments of very
thin paper laid on the table beside the glass began to be attracted and
moved nimbly to and fro, leaping upon the glass and remaining there a
while, then leaping down and resting there, then leaping up and perhaps
down and up again. This experiment is repeated easily by rubbing a
stick of sealing wax with a woollen cloth, and holding it over small
bits of paper.
[Illustration: Fig. 47.--Guericke’s Electrical Machine made with a Ball
of Sulphur.]
Instead of rubbing amber or glass, Guericke rubbed a ball of sulphur
with his hands. He made this ball by filling a round bottle with
flowers of sulphur and melting the sulphur until it was quite liquid.
Then he let it cool until it was solid, and then he broke the bottle
and took out the sulphur ball. He attached the ball to a stick, and
when he had electrified it by rubbing, he used it to attract feathers,
which he succeeded in leading all round a room by this electric force
only.
About the year 1730, a pensioner of the name of Stephen Gray found
that electric force may be produced at one end of a wire by holding
an electrified body near the other. He expressed this fact by saying
that the wire “conducts electricity.” He found that all bodies may be
divided into conductors and non-conductors. He proved that the human
body is a conductor, and created a sensation by suspending a boy on a
network of silk and showing that he conducted electricity. Instead of
suspending the boy on a silk net, he found that it was sufficient to
let him stand on a cake of resin, which did not conduct electricity.
This experimental work with the electric force was taken up in France
with great interest. The French experimenters found out that when
a body is electrified strongly, sparks may be drawn from it. One
experimenter had himself suspended on a silken net, and got the others
to electrify him. When that was done sparks could be drawn out of his
body by any one who touched him. These sparks made a crackling noise
and produced a pricking sensation. “I shall never forget the surprise
caused by the first electric spark which was ever drawn from the human
body,” exclaimed one of those who were present.
So great was the interest roused by these new observations that people
began to make electric machines. One of the best of these was made by
Professor Bose in 1744. It consisted of a glass globe mounted in a
frame and kept rotating by means of a pulley.
[Illustration: Fig. 48.--Electricity produced by Friction with Glass
and conducted by a Metal Tube.]
One of the experimenters held his hands on the glass globe, and so
electrified it. The electricity was conducted by a long tin tube
suspended by silk threads. The end of the tube next the glass globe was
provided with linen threads, which collected the electricity into the
tube. At the other end stood an assistant on a cake of pitch or resin.
He held a sword in his hand, and from the end of that sword sparks
could be drawn as soon as the assistant touched the tin tube with his
other hand. These sparks could be made to set fire to spirits of wine
held in an iron dish by a servant who stood on the floor.
About the same time another German professor used a glass cylinder
instead of a glass globe, and did away with the necessity of rubbing
the glass with the hand by letting a leather cushion rub against it.
This leather cushion was a kind of artificial hand. But it was not
sufficiently like the human hand to be very useful, and was afterwards
improved by covering with a mixture of tin and mercury. Later still,
glass discs were used instead of globes or cylinders.
It was in the year 1745 that electric experiments became very popular,
owing to the accidental discovery of a piece of apparatus by which
the electric discharge, or spark, could be made much stronger. This
discovery was made about the same time in Pomerania and in Holland.
In both places people were trying to electrify water in a bottle. The
electricity was conducted into the water by a long nail, which stuck
out of the bottle at the neck. The bottle was held by an assistant,
and when enough electricity had been conducted into the water, the
assistant took the nail out of the water. In doing so he received a
great shock which paralysed his arms and shoulders. In Holland, this
experiment was performed at Leyden, and the jar which can be made to
give such a great shock is therefore called a “Leyden jar.”
The man who received the electric shock said that he would not take
another for the kingdom of France. But this feeling was not shared by
others, and many people tried the electric shock just to see what it
was like. Professor Bose went so far as to say he wished he might die
by the electric shock so that the account, of his death might furnish
an article for the French Academy of Sciences.
[Illustration: Fig. 49.--Discovery of the Leyden Jar.]
The new apparatus became so popular that a great number of people
made a living by going from place to place to exhibit it. In France
especially such entertainments attracted large crowds. On one occasion
an electric discharge was sent through a row of 180 soldiers. At
another time several hundred monks were made to join hands in a row 900
feet long, and a strong electric spark was sent through the whole row.
It made all the monks jump at the same moment.
It was not long before these experiments were repeated in America, and
when they did so they soon attracted the attention of a man who was
able to develop them much further. That man was Benjamin Franklin, who
was then engaged in the printing business. He saw the first experiment
in the year 1746, and at once began to investigate the subject. He was
the first to observe what a very important effect metallic points have
in drawing out, or throwing off, what the French called the “electric
fire.” He constructed a Leyden jar, and made many experiments with it.
When the hot summer weather came he proposed to his friends to wind up
his experiments with an electric dinner, for which a turkey was to be
killed by an electric shock and roasted in front of a fire lighted by
electricity.
In the year 1749, Franklin had an idea of making experiments to prove
that lightning was really a great electric spark. Before he saw the
electric experiments, he had thought that the lightning was due to
some kind of sulphurous breath coming from the earth and collecting
in the atmosphere. Other people thought that the lightning was due to
a gas explosion. The truth of such guesses could be decided only by
experiment. For when a thing has to be explained it is not sufficient
that it shall be more or less like some other thing. Things are
sometimes very similar and still have nothing to do with each other.
The flowers of frost on a pane of glass look very much like the leaves
of ferns or other delicate plants. But the force which makes the fern
grow in the wood is quite different from the frost which makes the
crystals grow on the glass, and it would be a great mistake to suppose
that the same causes are at work in both cases. For in one case we must
have a germ, nourishment, and a certain temperature, whereas in the
other case we require moisture and cold only.
Benjamin Franklin had the true scientific spirit. When he found that
the electric spark and lightning both looked bright and made a noise,
he did not jump at once to the conclusion that they had the same
nature. By careful study he found that lightning and the electric
spark have nearly the same colour. The electric spark looks like a
small flash of lightning. Both are equally swift, both are conducted
by metals, both are capable of tearing or breaking bodies they pass
through, both are capable of killing animals instantly, both can melt
metals and fire inflammable substances, and finally both give rise to
the same kind of smell.
It only remained now for Franklin to draw an electric spark from the
clouds to prove that lightning was an electric discharge. To do this
he proposed that a pointed iron rod should be mounted on the top of
some high steeple as near as possible to the clouds, so as to draw
electricity out of them. Near the top of the tower a man was to be
placed on a stool with glass legs, and connected with a pointed rod. If
the rod collected electricity from the clouds it would be possible to
draw sparks from the man. Franklin proposed this experiment in a letter
to the Royal Society of London, but the Society did not encourage him.
He then tried to start a lottery in order to obtain the money necessary
to build a tower for the purpose, since there was no building in
Philadelphia high enough for the experiment.
While Franklin was still making plans, he heard that the experiment
had been performed successfully near Paris. A man named Dalibard had
mounted a metal rod forty feet high on the roof of a small cabin and
rested the foot of the rod on a table inside the cabin. Dalibard set an
old dragoon to watch for thunder clouds, and had a wire ready near the
rod to draw off any sparks that might be obtained. After several days a
thunder cloud appeared, and sparks flew from the rod to the wire. The
dragoon thought they smelt of sulphur and that some demon was at work.
He sent for the priest, but the priest knew that this was not the work
of a demon; and he proceeded to draw sparks out of the rod with the
wire, after which he told Dalibard what had happened. The latter was
delighted, and exclaimed that Franklin’s idea was no longer a guess but
a reality.
Franklin, however, did not regard the experiment as a sufficient
proof, and he conceived the new idea of sending up a kite with an iron
point in order to draw electricity from the clouds. He sent up a kite,
holding the string by a silk ribbon, so as not to get the sparks into
his hand. A thunder storm came on, but no effect was observed although
Franklin had tied a key to the end of the string. But when the rain
began to fall and the string got wet, a strong spark was obtained on
bringing the knuckle near the key. This conclusive experiment had a
great effect all through the civilised world. It was repeated in every
country, and Franklin’s idea of drawing electricity out of the clouds
by means of pointed rods connected with the earth was adopted in many
places. Such rods were called Lightning Conductors.
Sometimes these lightning conductors were not connected with the earth,
but with some apparatus for measuring the amount of electricity, or
with a Leyden jar for collecting it. Such an arrangement was put up
in St Petersburg by a man of the name of Richmann. But while he and a
friend were observing the measuring instrument, the friend suddenly
saw a ball of blue fire come out of the rod and hit Richmann on the
head, killing him instantly. This death of an experimenter did not
excite horror, but rather envy. One great discoverer is said to have
exclaimed that it was not given to every electrician to die in so
glorious a manner. Men of science were interested greatly in the manner
in which the electric shock had affected the head and body of Richmann,
and those effects were investigated closely. Richmann himself was so
devoted to the cause of science that we may well believe that he would
have been proud of being the first to be killed in an experiment of
this kind. Besides, he was also the last, as his successors were able
through his death to protect themselves from similar dangers.
CHAPTER X
THE ELECTRIC CURRENT
THE electric machine was used for extracting what was called
the “electric fluid” out of a body by rubbing. The fluid in its passage
through the air made a spark and a noise. It pricked the finger. In
larger quantities it gave a severe shock, sufficient to kill small
animals. In the form of lightning it killed men.
Franklin called the fluid the “electric fire,” and supposed that it
was a very subtle and delicate fluid distributed through all nature,
and was the reason and cause of all electric happenings. When he found
that two silk threads when charged with electricity repelled each
other, he came to the conclusion that one portion of the electric fluid
repels any other portion. When one body was rubbed against another he
supposed that some of the fluid was extracted from one and absorbed by
the other, but that after the exchange the two bodies attracted each
other in proportion to the amount of electric fluid which had been
extracted. There was, therefore, both an electric attraction and an
electric repulsion. The attraction existed between the electric fluid
and a body which had lost its ordinary quantity of that fluid. The
repulsion took place between the portion of the fluid itself, and also
between two portions of matter which had both been deprived of some
of their electric fluid. Franklin believed that the whole science of
electricity could be built up by studying the motion and distribution
of the electric fluid.
Franklin also tried to find out whether a body, after losing some
of its natural quantity of electric fluid, weighs less than before,
but he could not find any difference in weight. This result made him
think that the electric fluid had no weight. He knew the difference
between conductors of electricity and bodies which did not conduct
it. He supposed that conductors could take up a great quantity of
the fluid, and store it away in the interior of their substance. He
knew that not more than a certain quantity of electricity could be
extracted from a body however hard it was rubbed. He supposed that
the amount of electricity in a body was limited, and that after some
rubbing the limit was arrived at. The fact that the electric machine
gave a constant supply of electricity he explained by supposing that
the electricity returns to it through the air. There was, therefore, a
constant circulation of electricity, a kind of electric current. But in
Franklin’s time there was no way of producing a really steady current
of electricity.
If we compare the electric current with a current of water, we may
express the case as follows:--Lightning and the electric spark are like
splashes of water falling from a great height, and making a great deal
of noise and disturbance. The electric machine gives a kind of current
which may be compared to a brook running along a stony bed, and making
a great deal of noise with but little water. The next great discovery
in electricity was how to produce a steady current, which we may
compare with the full, steady, and noiseless flow of a good water-tap.
This discovery was made in a very roundabout way.
Towards the end of the eighteenth century, it was found that certain
curious fish called “torpedoes,” or cramp fish, were capable of
giving their enemies a severe electric shock. When this was proved,
men of science inclined to the idea that all movements of muscles are
electric. One of them, who made experiments with frogs, ventured the
guess that a kind of electric vapour moves through all muscles and
nerves, and produces not only the movements of animals, but also their
sensations. Great efforts were made to discover this electric vapour or
“animal electricity.”
It was, however, not till the year 1790 that the first actual
observation of something like animal electricity was made by a
professor in Bologna of the name of Galvani. We are told that his
wife, being in poor health, was ordered to eat frogs’ legs, and that
the professor experimented with a few of those; but in his own account
there is nothing about his wife or about frogs’ legs considered as
food. He simply relates how he placed the hind legs of a frog together
with a piece of its backbone upon a table on which his electric machine
stood, but without the conductor of the machine touching the frog. One
of his assistants happened to touch the nerves of the legs with a
knife, and he observed to his surprise that a kind of shudder seized
the legs of the dead frog. Another assistant thought that this only
took place when a spark was coming from the electric machine. Galvani
says:--“Astonished at this new observation he drew my attention to it.
Although I was engaged in some quite different work, I was inflamed
by an incredible zeal and desire to examine it and bring to light the
cause that was hidden in it. I, therefore, touched one or other of the
nerves with a knife, and at the same moment one of the assistants drew
a spark from the machine. The effect was always the same. In every
case the muscles of the legs were violently contracted at the moment
at which the spark burst, just as if the animal had been seized with a
cramp.”
Professor Galvani was quite certain that he had discovered animal
electricity. He found that whenever the frogs’ legs were touched with
two different metals, they gave a jerk. He tried combinations of iron
with brass, and lead with silver, and finally concluded that silver is
the best conductor of animal electricity. Galvani upheld his theory of
animal electricity until the time of his death, which occurred eight
years after his discovery. But his theory was attacked by another
Italian whose name was Volta.
Volta was distinguished already by his invention of a simple little
machine which gave a continual supply of electricity. It consisted of a
cake made of resin mixed with turpentine and wax, which was cast into
a flat mould, and allowed to cool until it was hard. A flat tin plate
was cut large enough just to cover the cake, and was provided with a
glass handle. The cake was beaten with a piece of catskin or woollen
cloth. The tin plate was placed on top of the cake and touched with
the finger, which drew a small spark. After that the plate was lifted
off the cake by the handle, and was then found to have a strong charge
of electricity. By repeatedly putting the plate on the cake, touching
it lightly, and then taking it off by means of the glass handle, any
amount of electricity could be obtained without renewing the charge
on the cake by another beating. That little instrument was called the
“electrophorus.”
Volta was all his life the greatest opponent of Galvani’s theory of
animal electricity, and at the end of a long series of brilliant
investigations and experiments he succeeded in demolishing the theory
entirely. He was helped by a curious experiment. It consisted in taking
a plate of lead and a plate of silver, and putting one on the top of
his tongue and the other under his tongue. As soon as the edges of
the two plates touched each other he perceived a queer taste. There
was no such taste when the pieces did not touch, and two different
tastes could be got by putting one or the other plate on the top of the
tongue. No effect was obtained if the two pieces consisted of the same
metal. He therefore concluded that the effect observed by Galvani was
really due, not to animal electricity, but to the contact between two
pieces of different metals. The frogs’ legs in Galvani’s experiment, or
the tongue in his own experiment, simply acted as a conductor, for the
electricity produced as soon as the two metals touched each other.
Volta experimented with a great many different metals, and he soon was
able to show that the electricity produced by contact can be proved
to exist by one of the measuring instruments called electroscopes,
which he had himself done much to make more delicate and sensitive. He
found also that metals behaved very differently, some of them giving
much electricity, and others little. Zinc and copper were found to be
a good combination. The zinc was found after contact with copper to
have the same kind of electrical condition as glass when rubbed with
silk. That kind of electricity was at that time usually called glassy
or “vitreous” electricity. The copper was found to be in the same kind
of electric state as sealing wax or resin after they have been rubbed
with wool or catskin. This was called “resinous electricity.” The
two kinds of electricity were known to attract each other, and when
they were allowed to combine, they rushed together in a spark, and
completely disappeared, the one just neutralising the other. Following
the practice of arithmetic and algebra, the two electricities were also
called positive and negative.
Franklin thought that the positive electricity was the real electric
fluid, and that the negative electricity was simply the want of that
fluid. It was a mere guess of his, and he might have guessed just the
other way. If he had done so, he would have been much more correct,
for it has since been found out that the resinous electricity is best
considered as the real fluid, and vitreous electricity as due to a
want or shortage of that fluid. However, at that time there was no
way of telling what was the real electric fluid, and even the number
of electric fluids which existed. Volta was sure of the existence of
vitreous and resinous electricity, but in addition to this Galvani
maintained the existence of a third fluid, which he called animal
electricity.
After many more experiments, Volta was able to arrange the chief metals
in a row or series, in such a manner that any metal, when in contact
with any other metal further on in the series, would acquire vitreous
electricity. This series was the following: Zinc, lead, tin, iron,
copper, silver, gold. He found that zinc could not be made to acquire
resinous electricity by contact, nor could gold be made to acquire
vitreous electricity. The strongest effect was obtained by the contact
of metals as far apart in the series as possible. The strongest effect
of all was obtained by touching zinc with gold.
[Illustration: Fig. 50.--Volta’s Pile.]
It occurred to Volta that perhaps the strongest effect might be
obtained by combining several pairs of the same metals in succession.
He usually used zinc and copper, because silver and gold were too
expensive. He fixed four glass rods in an upright position, and cut a
large number of round plates of zinc, copper, and leather or cardboard.
The leather or cardboard he soared in salt water. He laid a plate of
copper at the bottom between the four glass rods. On this he placed a
plate of zinc, then a piece of cardboard, then a plate of copper, then
a plate of zinc, then another piece of cardboard, and so on until he
had made a big pile of such combinations. This pile is known to this
day as Volta’s Pile.
Volta found that such a pile gave a little spark on attaching a wire
to the top of it, and bringing the other end of the wire in contact
with the bottom of the pile.
[Illustration: Fig. 51.--Volta’s Crown of Caps.]
He therefore called such a pile an “artificial electric organ,” in
order to direct attention to its similarity to the electric organ of a
cramp fish. The pile did not last long. It either dried up, or if there
was too much water, the water ran down the sides and interfered with
the proper contact of the metals. Volta, however, improved it by using
salt water in a row of cups in order to conduct the electricity from
one pair of metals to the next. Instead of using round plates, he used
the metals in the shape of flat bands, soldering together a copper band
and a zinc band, and arranging the cups in the form of a crown.
The spark obtained with the pile, or crown of cups, was not sufficient
to prove that the electricity produced by it was the same as that
furnished by the electric machine. But the proof was not long in
coming. In the year 1789, water had been separated into the two gases
of which it is composed by conducting the discharge of a Leyden jar
through the water a great many times. The same effect was found to
result also from the action of the electric current obtained from one
of Volta’s piles. This coincidence was discovered accidentally.
A man named Nicholson had attached a wire to the bottom plate of one
of the piles, and brought the other end into contact with the top
plate. In order to improve the contact, he put a drop of water on the
plate just where the wire touched it. He then observed that very small
bubbles appeared in the drop of water, and although the quantity of
the gas produced was very small, he thought it smelt of hydrogen. He
at once tried the experiment of attaching two brass wires to the upper
and lower end of the pile, and bringing them side by side into water,
without touching each other. He then found that small bubbles were
given off at one wire, and that the other was eaten away. He collected
the bubbles of gas, and found that when it was mixed with air it
exploded on bringing a lighted match to it. This proved that the water
had indeed been broken up into two gases called oxygen and hydrogen. It
was the first service to which the newly discovered electric current
was put.
CHAPTER XI
THE ELECTRIC TELEGRAPH
WHEN two persons engage in ordinary conversation, they
really use two methods of communication. Both methods are extremely
complicated. By means of mouth, tongue, and throat, the speaker
changes the current of air issuing from his lungs, and throws it into
a great variety of vibrations. These vibrations are communicated to
the outside air and find their way into the ears of the other person.
Such is speech. At the same time, people when speaking employ signals
which appeal to the eye. They move their lips, eyelids, eyebrows, and
sometimes their hands, and each motion gives a slightly different
quality, or stress, to what they are saying. These motions produce an
effect upon the waves of light which are constantly passing to and fro
between the two people. In this way speech is assisted by sight.
When two persons are far apart and still wish to communicate they have
to shout. When the distance becomes greater, their voices become less
and less able to cover it, and, finally, they are driven to communicate
by sight only. So, we find them waving their hands, or handkerchiefs,
or hags at the end of sticks. When the distance is many miles, this
method fails again, and communication becomes very difficult. In
ancient times people used to get out of the difficulty by lighting
big fires which could be seen for ten or fifteen miles in the dark.
The smoke from the fire could be seen for about the same distance in
the day-time. Some savage tribes use the same method even now, and
manage to convey a great deal of information by it. But nowadays we
use the electric telegraph for all such purposes, and the story of its
invention is one of great interest.
The immediate predecessor of the electric telegraph was the optical
telegraph invented by Hooke, Newton’s assistant. The first occasion
on which the optical telegraph was put to practical use was about the
time of the French Revolution. In the year 1792 the whole of France
was covered by a system of signals. The system consisted of a number
of towers on the top of which “semaphores” were mounted which looked
very much like the railway signals of the present day. By means of this
system a message could be sent from Paris to the end of France in a few
hours. When the war broke out in Austria in 1809, the great Napoleon
profited by it to fall upon the Austrians unawares, giving them no time
to make preparations to meet him.
[Illustration: Fig. 52.--Optical Telegraph.]
The Austrians were in alliance with the Bavarians, who suffered greatly
by the war with Napoleon, and the Bavarian Government asked one of
the professors at Munich to devise a system of telegraphy which should
be superior to that used so successfully by Napoleon. The name of the
professor was Soemmering.
[Illustration: Fig. 53.--Soemmering’s First Electric Telegraph.]
He started on the work at once, and four days after receiving the
request he had devised the first electric telegraph which was capable
of transmitting the letters _a_, _b_, _c_, _d_,
_e_. That was all. The apparatus was quite simple and very
ingenious. It consisted of one of Volta’s piles made of fifteen pairs
of silver and iron plates separated by layers of felt soaked in salt
water.
This arrangement was the source of electricity. At the station to which
the signals were to be sent there was a bottle or small tank of water
with a large cork in the bottom of it. Through this cork five wires
were drawn, and they led into water with which the bottle was filled.
The five wires were made very long. They were covered with shellac
varnish, and then they could be twisted together without being in
actual contact. The strand of wire was stretched between the sending
station and the receiving station.
At the sending station the strand of wire was unravelled, and the five
loose ends were marked with the letters _a_, _b_, _c_,
_d_, _e_. The top of the Volta pile placed at the sending
station gave positive electricity, and the bottom of the pile gave
negative electricity. Now, suppose that the wire marked _a_ was
brought into contact with the top of the pile, and the wire marked
_b_ was brought into contact with the bottom of the pile. Then
an electric current travelled all along the strand, through the pile
and the two wires, and through the water between the two wires, thus
making a complete circuit. The water in the bottle was then split up
into hydrogen and oxygen, and bubbles of hydrogen appeared at the wire
_b_ where it entered the water. The observer at the receiving
station then understood that his companion at the sending station
wanted to telegraph to him the letter _b_.
Whenever the man at the sending station wanted to telegraph a letter,
all he had to do was to bring the corresponding wire into contact with
the bottom of the pile, and to bring any other wire into contact with
the top of the pile. The wire at the top was usually the wire which
came next in the message, so that the two letters could be telegraphed
at the same time, the second letter being that which gave off a very
small quantity of oxygen gas.
The instrument so constructed was only capable of sending five
different letters. In order to send a complete message, it was
necessary to have as many wires as there are letters in the alphabet.
Twenty-six wires were therefore twisted together in a strand, after
being covered with varnish and silk thread. The ends of the wires at
the sending station were nailed to a frame by means of nails with
perforated heads. A wire was kept touching the top of the Volta pile,
and another was kept in contact with the lower end of the pile.
These wires ended in small plugs which could be inserted into the
perforations of the nails, so that the sender could pick out any two
letters he wished to telegraph. At the other end the wires were laid
through the bottom of a vessel containing water, and were marked
with appropriate letters. The telegraph was then worked on the same
principle as the smaller one made for five letters.
In addition to these letters, the inventor devised a mechanism by
which a bell could be sounded in order to attract the attention of the
man at the receiving end. This apparatus was extremely ingenious. It
consisted of a lever to which a spoon was attached.
[Illustration: Fig. 54.--Soemmering’s Telegraph.]
The spoon lay in the water, just above two of the wires, with its open
surface down. When the current was sent through these two wires, the
hydrogen and oxygen collected in the spoon, and raised it. This set a
little ball rolling down into a funnel. Through the funnel it dropped
into a small pan, and removed the catch of an alarm clock. This set the
alarm going and attracted the attention of the man at the receiving
station. When the message was finished, the man had to put back the
ball to its original place, and wind up and set the alarm clock as
before.
It was not long before the Emperor Napoleon heard of the Bavarian
invention. One of his officials told him about it and asked leave to
explain it to him. The Emperor listened to the explanation, but then
swept it aside as worthless. “It’s only a German idea,” he said. He
considered it impossible to lay a strand of wire across country and to
preserve it from damage. He preferred to rely on his optical telegraph
which had done him such good service.
The electric telegraph did not come into real use until the magnetic
needle was adopted as an indicator. Ten years after the first telegraph
was invented, a Danish electrician made a very important discovery,
which exercised a far-reaching effect upon electric science. He was
trying to discover some connection between electricity and magnetism,
and made all sorts of experiments with magnetic compasses. He found
at last that a magnetic needle is affected powerfully by a current
passing in a wire close by. When a wire bearing an electric current is
stretched along a table from north to south, and a magnetic compass
is placed on the top of the wire pointing in the same direction, the
needle swings round every time the current is sent through the wire,
and swings back into its ordinary position as soon as the current is
interrupted. So long as the current flows, the needle tends to set
itself at right angles to the wire, and will do so almost exactly when
the current is very strong.
This discovery made a great impression on the scientific world, and
many men of science proposed to use it for the purpose of telegraphy.
But it was not until the year 1833 that the first telegraph was
constructed on this principle. Two professors in Heidelberg, whose
names were Gauss and Weber, established a telegraph of this kind
between the Physical Institute and the Observatory. The wire had a
total length of nearly two miles, and they produced at the receiving
station a deflection of a magnetic needle to the right or left. This
arrangement was afterwards improved by a young student, who attached
pens to two magnetic needles, and made them write dots on a strip
of paper passing along under them. Using two different inks, he got
different sets of coloured dots for each letter. These could be easily
recognised afterwards and read off the paper.
The current used in these experiments was not a steady current from a
Volta pile or electric battery. It was a so-called induction current.
The induction current had been discovered in 1831 by a great Englishman
of the name of Michael Faraday, who found that when a wire is placed
by the side of another wire through which electricity can pass, then as
soon as the current is sent through this wire by connecting it with the
battery, a momentary current flows in the opposite direction through
the other wire.
[Illustration: Fig. 55.--Diagram of Morse System of Telegraphy.]
For this to happen, however, it is necessary that the second wire
should also form a closed circuit, and not have any free ends. Such an
induction current, or induced current, can also be obtained by means
of a large magnet. All that is necessary is to take a coil of wire and
suddenly slip it upon a bar magnet, or suddenly slip it off. The more
rapidly the slipping on or off is done the stronger is the current.
In this way, therefore, it is possible to produce an electric current
without any piles, or batteries, or chemicals.
[Illustration: Fig. 56.--S. F. B. Morse.]
This kind of current was found very effective for telegraphy,
especially as the magnet was always ready, whereas the electric
batteries, or cells, were liable to get exhausted and useless. It was
only when the electric battery was improved greatly and made more
trustworthy that it was adopted again for telegraphy, and it is used
for that purpose at the present day.
A great improvement was introduced into telegraphy by S. E. B. Morse,
who invented the alphabet known as the “Morse Code” and also a
recording machine which writes down the message in dots and dashes. The
alphabet is as follows:--
a • — h • • • • o — — — u • • —
b — • • • i • • p — — • v • • • —
c — • • • • j • — — — q — — • — w • — —
d — • • k — • — r • — • x — • • —
e • l • — • • s • • • y — • — —
f • • — • m — — t — z — — • •
g — — • n — •
[Illustration: Fig. 57.--The Needle Telegraph.]
The word “alphabet” appears on the paper strip like this:--
• — • — • • — — • • • • • • — — • • • • —
a l p h a b e t
For work in which no record is required, the needle telegraph is
employed usually. But the Morse code is used in this case also, the
needle moving to the right when a dot is intended, and to the left when
a dash. Telegraph clerks get after a while to understand a message by
simply listening to the sound of the needle. Nowadays the needle is a
stout magnet, which as a rule rests against a piece of iron belonging
to an electromagnet. When the current passes, it knocks against a
different piece of iron, and the operators learn to read the message
from the sound of the knocks. The instrument is therefore called a
_sounder_.
CHAPTER XII
THE TELEPHONE
IN the electric telegraph, the electric current is made to
turn a magnetic needle, or move an electric pen at a distance. When the
distance becomes very great, as in the telegraph cables which connect
us with America, a more delicate instrument is used for receiving the
messages. It consists of a kind of small ink bottle, which is turned
this way or that by the electric current. It squirts out a fine jet of
ink, and the ink falls on paper which is moved past the instrument. In
this way the signals are recorded on paper without any friction.
The electric telegraph, however, is a very simple and coarse instrument
in comparison with the telephone, for the telephone enables us to send
speech itself over several hundred miles, over distances which the most
powerful light from the highest lighthouse could not cover.
That any one can speak into an instrument in London, and that at the
same time a friend in Paris can hear his voice, and understand what he
is saying, is a thing so wonderful that a person must have tried it for
himself before he can quite believe it. Yet what is wonderful about it
is not so much the fact of speech being communicated, as the distance
across which the communication is made. Why should we not be able to
cover such great distances by shouting?
The reason is, that when we shout we create a wave of sound in the air.
Such a wave of sound is in many respects like a wave in water. When we
throw a stone into a pond, the wave starts from the place where the
stone fell into the water, and travels out in circles which become
wider and wider. Not only do the circles become wider, but they also
grow fainter and fainter the wider they get. If the pond is very large,
the wave may get so faint that when it reaches the bank it is too faint
to be perceived.
If, instead of a pond, we have a long channel or gutter filled with
water and make a splash in it at one end, we find that the wave can
travel a very great distance along the channel without growing much
feebler. That is because it cannot spread out sideways. Similarly, if
we speak into a long pipe or tube, our voice carries much farther than
it does in the open air, again because the wave of sound cannot spread
out sideways. This is the reason why it is so easy to talk through a
speaking-tube.
If we could send the same waves along a wire, we might succeed
in carrying the sound much farther than we do even through a
speaking-tube. This fact can be proved by taking two small tin cans,
and stretching a wire from a hole in the bottom of one to a hole in the
bottom of the other. Then, if one person speaks into one of the cans,
and the other person holds the other can to his ear, and the wire is
stretched between them, the second person will hear distinctly what the
first person is saying. In this case the wave of sound travels along
the wire, stretching it and compressing it in turn, until it reaches
the other can, and throws the air into similar vibrations. Such an
instrument could be called a telephone, but nowadays the word is used
only for instruments which are worked by electricity or magnetism.
To understand the difficulties which the inventors of the telephone
had to overcome, let us consider for a moment the nature of the sound
waves which represent speech. Spoken language consists of vibrations of
the air. These vibrations are very small and very quick puffs of air,
which we produce by means of our throat and mouth. Now if we were to
try to give a very rapid succession of puffs, we could not give more
than about ten per second. But if we had a machine which could puff
much more quickly than that, we should find that after the puffs had
become more than twenty per second, there would be a kind of hum. This
result can be illustrated by means of a toothed wheel such as we find
in clocks. When such a toothed wheel is turned quickly, and we hold the
edge of a piece of paper against it, then, as it is made to turn more
and more quickly, the little ticks of the paper merge more and more
into a hum, and finally become a musical note. The faster the wheel,
the higher the pitch of the note.
Every note has its own pitch, that is to say, it has a certain number
of vibrations per second. The notes in the middle of a piano have from
two hundred to five hundred vibrations per second. The same note may be
either loud or soft, just as a wave of water may be either high or low.
The high wave corresponds to a loud sound. Very small ripples on the
surface of water correspond to a very high, but faint, note of music,
like the singing of an insect.
Each note has its own pitch and its own loudness. In addition to that,
it also has a certain “quality” of its own. We may, for instance,
play the same note on a violin, a trumpet, or an organ. They may have
the same pitch and the same loudness, and yet we can distinguish them
from each other easily. What is this “quality” which enables us to
distinguish the note of one instrument from the same note played on
another instrument? It must be something in the position of the waves
which travel through the air and reach our ear.
This question long remained in doubt, until people found out that no
musical sound was quite simple. Even in water one usually sees little
ripples on the top of big waves. A wave with a perfectly smooth surface
is very rare. In the same way, a perfectly simple and pure sound wave
is hardly ever produced. Each wave has little ripples on it. The wave
from a piano has one kind of ripples, the wave from an organ has
another kind, and the wave from a trumpet--which is particularly shrill
and penetrating--has a great number of very tiny ripples, which mean
very high notes. These facts can be proved by playing a soft note on
an organ, and playing a number of very high notes at the same time.
If the high notes are properly chosen, the sound becomes shrill and
penetrating, like the sound of a trumpet.
Musical sounds are therefore quite fixed as soon as we know their
loudness, pitch, and quality. But not all sounds are musical. There
are such things as noises, and many forms of human speech are anything
but musical. As a matter of fact, speech consists of a very rapid
succession of all kinds of notes, which last only a very short time,
and rapidly change from one to the other.
Of these notes, the highest in pitch are the hissing sounds, such as
_s_ and _sh_, and these are most difficult to send through
the telephone. Shortly after the electric telegraph was invented, the
idea occurred to many electricians that it ought to be possible to send
speech along the wire conducting the electric current. They knew that
sound consisted of waves, and they thought that if an electric current
could be interrupted a great many times per second, a musical note
would be produced at the other end of the wire.
An observation made in America in the year 1837 confirmed this idea. It
was found that when a current is sent suddenly through a long coil of
wire wound round a piece of soft iron, a little sound is heard. Such a
coil is called an electromagnet, because the soft iron is a powerful
magnet so long as a current is passing through the coil. Another
sound is heard when the current is broken suddenly. This observation
suggested that if the electromagnet were at the other end of a long
wire, and the current at one end were made and broken rapidly, the
electromagnet would give a musical note at the other end.
In the year 1854, a French telegraphist proposed to send speech by
electricity on that principle. He proposed to stretch a skin over a
drum, and fix one end of a fine wire to the middle of the skin. The
end of the wire should dip into a drop of quicksilver connected with
another wire, and this contact should establish an electric circuit. On
speaking into the drum the skin would vibrate, the wire would move in
and out of the drop, and the electric circuit would be made and broken
in turn. It would be made and broken very rapidly when a high note was
sung into it, and less rapidly in the case of a low note. It would
follow all the various notes of ordinary speech, and would produce the
necessary vibrations at the receiving end.
The Frenchman never carried out his proposal. In the year 1861 it
was again taken up by a German professor of the name of Reis. His
instrument was able to transmit musical notes, but not spoken language.
It was a very wonderful instrument at the time, but it could not give
the correct quality to notes. The sudden induction of the electric
current was too jerky to give the very fine ripples on the sound waves,
and especially to render the delicate shades of human speech. The
inventor died poor and neglected in 1874, and it was not till 1876 that
the telephone problem was solved really.
The credit of the successful solution of this great problem is due to
an American of the name of Graham Bell. As is very often the case,
the successful instrument was extremely simple, so simple indeed that
people were disappointed on seeing it. It could be put together by a
schoolboy, and yet it was able to talk. From the very first, Bell had
seen the correct principle which must govern the construction of a good
telephone. The fault of Reis’s telephone was that it could not follow
the delicate wavelets of sound which constitute human speech. It was
necessary to make an instrument which should be extremely sensitive and
delicate, as much so as the human ear, which is saying a great deal. In
order to do this Bell used Faraday’s induction currents.
We have seen already that when Faraday pulled off the cap of the
electromagnet a momentary current was induced in the coil of wire.
Small currents could be induced by simply shaking the cap to and fro.
Now, Bell had the idea of mounting a small piece of iron in front
of the bar of an electromagnet. The ends of the coil of wire of the
electromagnet were pulled out to a very long distance and passed round
another electromagnet. The small piece of iron was stuck upon a piece
of parchment, which was stretched on a drum in front of the first
electromagnet. On speaking into this drum the parchment was set into
vibration, and with it the small piece of iron. This produced slight
and very rapid currents in the coil of the electromagnet, and these
were transmitted along the wire to the other electromagnet.
[Illustration: Fig. 58.--Graham Bell, inventor of the Telephone.]
Another parchment with a similar small piece of iron was mounted on
a drum in front of the second electromagnet, and it was found that
the second drum reproduced exactly all that was spoken into the first
drum. For the small currents arriving at the second electromagnet made
its magnetism stronger and weaker in rapid succession, and attracted
the iron sometimes more and sometimes less. Every motion of the first
parchment, therefore, was rendered faithfully by the second parchment,
and the vibrations of the air inside the first drum were reproduced
accurately in the second drum. That was all that was necessary for the
complete transmission of human speech.
But this apparatus was still capable of considerable improvement. In
the first place, we know that an electromagnet loses all its magnetism
when the current in its coil is stopped. But Graham Bell found that it
was not at all necessary to use an electromagnet.
[Illustration: Fig. 59.--Bell’s Magnetic Telephone.]
Instead of a piece of soft iron, he found he could use a similar piece
of steel which had been magnetised strongly. Such a piece of steel is
called a permanent magnet. The little horseshoe magnets with the red
paint on them that are sold in shops are such permanent magnets. This
discovery simplified matters greatly, as it was no longer necessary to
use a battery at all.
Another improvement was to do away with the parchment. Graham Bell
found that a sheet of tin would do very well instead of the parchment.
What is called “tin” is usually in reality a sheet of iron covered on
both sides with a very thin layer of metallic tin; and as the magnetism
acts quite easily through tin, the tinned iron acts like a sheet of
pure iron. When a sound wave reaches it, it is set vibrating; and
its vibrations slightly alter the magnetism of the permanent magnet,
and produce the small and rapid currents mentioned above. The whole
apparatus, therefore, consists simply of two steel magnets, two discs
of tin, sufficient wire to make coils round the magnets and cover the
distance between the two stations, and the necessary boxes to keep the
discs and magnets in the proper position. It is surely the simplest
instrument ever invented to perform such a wonderful task. What is
almost quite as wonderful is that this telephone is still in general
use for receiving telephone messages.
It says a great deal for the excellence of the instrument that
thirty-three years of active search for improvements have not been able
to suggest any real improvement in Graham Bell’s instrument. Bell’s
receiver is still used at the present day.
But his instrument is not as good for sending or transmitting speech as
some other instruments which have since been invented.
[Illustration: Fig. 60.—Carbon Transmitter.]
[Illustration: Fig. 61.--Combined Receiver and Transmitter.]
It was found that Bell’s transmitter was rather feeble as regards
loudness, and an improved instrument has been invented in America since
his time.
It was Edison, the inventor of the phonograph, who first proposed to
use carbon for sending the message. The new instrument was in some ways
a revival of the idea of Reis, but instead of breaking and making the
circuit, it was thought better to make the contact between the ends
of the circuit weaker and stronger in succession. This is now done
in an instrument called the Blake transmitter.
[Illustration: Fig. 62.--Microphone.]
An electric circuit containing a battery is interrupted at one
point, and joined again through a little ball of platinum and a ball
of carbon, such as is used in electric batteries. This contact is
connected with a vibrating plate which transmits the speech. The
circuit is never really broken, but is weakened and strengthened
alternately by a more or less perfect contact between the two balls.
The result is that human speech is conveyed with great clearness.
This principle of loose contact has since been employed for another
instrument which is perhaps even more wonderful than the telephone.
It is the “microphone,” which was first constructed by Edwin Hughes
in 1878. A current was sent through a Bell telephone and through two
pieces of carbon joined by a rod of carbon which was laid loosely
across them. The slightest sound produced on the board which held the
carbons was converted into a loud sound in the telephone. When a fly
walked across the board, the patter of its feet could be heard in the
telephone.
CHAPTER XIII
ELECTRIC LIGHT
MAN is not the only living being which sees by artificial
light. Cats’ eyes shine in the dark. So do glow-worms, and in South
America there are some insects which shed so bright a light that they
are used by the natives as lamps. These lights produced by animals are
much superior to the artificial lights which we can produce. They are
very economical. The animal expends very little of its strength in
producing the light, and the light is much cooler in proportion to its
brightness than any other light we know. Ever since man has learned how
to kindle a fire he has been able to light up the darkest night. He no
longer depends upon the sun and moon. But the fire which savages make
with logs of wood not only gives light, but also heat. It gives far
more heat than light. It is so hot that one cannot touch it, and yet
we can look into its blaze without feeling hurt by the glare. It could
be much brighter than it is without blinding us.
Lamps were invented probably by the ancient Egyptians. They consisted
of a shallow vessel full of oil, with a wick dipping into it. Candles
were used later, when it was found that certain kinds of fat could
be made to melt just where the wick touched them. But even the best
candles give a poor light, unless hundreds of them are lighted at the
same time, and then they give a great deal of smoke. The best light of
all would be a light which was very bright, but gave no heat, smoke,
or smell. It would have to be also very cheap, and so arranged that it
could be lighted at once at any time and in any place.
The last century has seen wonderful improvements made in lamps, and
further improvements are made every day. The most convenient light
invented up to the present time is the electric light. It gives no
smoke, it is clear and bright, it can be turned on with a slight
touch, and it is cheap. It is better than oil lamps, since there is
no trimming of wicks and no oil to fill up. It is better than gas,
since it requires no match for lighting it, and there is no danger of
explosion.
[Illustration: Fig. 63.--An Arc Lamp.]
The invention of the electric light belongs entirely to the nineteenth
century. Sir Humphry Davy was the first to produce it. He had a great
battery of two thousand cells at the Royal Institution, and he found
that when he joined the two ends of the battery by wires and separated
the wires again, a vivid light played between the two ends of the
wires. This light or flame was so hot that it melted all metals,
including platinum, which is extremely difficult to melt. He tried
to find some substance which would not melt, but would yet conduct
electricity. He tried charcoal, but found it conducted the current
very badly. He plunged it in mercury, and so obtained a fairly good
conductor, which did not melt in the electric flame, but burned away
rather quickly. However, he showed this light at the Royal Institution
so long ago as the year 1808, and it excited great wonder and curiosity.
For many years this remained the only experiment, and it was only
towards the middle of the century that a substance was found which
burned away very slowly. That substance was discovered at the bottom
of the crucibles used for making coal gas. When all the coal gas had
been driven out of the coal by heat, and only the coke remained, it was
found that a very hard and compact black mass remained at the bottom
of the crucible. That substance was called “gas carbon.” It conducted
electricity well, and made a very good substance for experiments on
the electric flame. It was cut into small rods, and the flame was made
to play between the ends of two such rods.
The flame was always seen to be in the form of an arch or arc, curving
upwards between the two rods. This was due to the heat of the flame
which produced an upward current of air between the pencils. The flame
was therefore called the “electric arc,” and a lamp containing it was
called an “arc lamp.” Such arc lamps are used to the present day for
lighting streets and squares. One such lamp is as bright as several
thousand candles burning together. The light which comes from the white
hot ends of the carbons is the brightest that it is possible to produce
artificially.
But the arc lamp is really too bright for ordinary use. It is quite
blinding, and it would not do to bring it into an ordinary room. For
many years electricians were busy trying to make another kind of
electric lamp which would not give such a glaring light as the electric
arc. The two men who did most to solve this problem were Thomas Edison
in the United States of America and Joseph Swan in England.
Edison was born in the year 1847. He never went to school. All the
education he ever got was given to him by his mother.
[Illustration: Fig. 64.--Thomas Edison.]
At the age of twelve he was put to work on a railway. Having a great
deal to do with newspapers, he managed to pick up the art of printing,
and by and by he printed a newspaper of his own in a luggage van. One
day he rescued a station-master’s child which was nearly run over by
a train. The station-master, out of gratitude, taught him how to work
the telegraph, which had been introduced just then. Young Edison very
soon acquired great skill at the telegraph, and was soon employed as a
telegraphist.
When only twenty-one Edison invented a new telegraph which could
print its own message. That invention was found extremely useful, and
several telegraph companies asked him to invent other things for them
which were wanted badly. Edison set up a factory of his own for making
printing telegraphs, but within a few years he sold his factory, which
had become too small, and set up a large new establishment at Menlo
Park, in the city of New Jersey. That establishment has since grown
to a great size, and Edison is still working in it at many wonderful
inventions.
Edison’s great idea was to use electricity to make life easy and
pleasant. In order to do this he proceeded in a rather different
manner from the earlier discoverers. In Europe the object of men of
science has usually been first to discover a new truth, and then to
find in what way the discovery could be made useful to humanity. Some
scientific men consider that the first object is much more important
than the second.
It is told of a certain professor who had made a great discovery in
chemistry, that his greatest satisfaction was the thought that it was
never to be of the slightest use to anybody. The story, of course, is
not true, or at least it is greatly exaggerated, for no great discovery
in science can be entirely useless. It enlarges our knowledge of the
nature and constitution of the world, increases our power over matter,
and with every increase of power we are better able to improve our
surroundings and make nature do our will.
Edison, however, proceeded in a different way. He first of all asked
himself what great problem wanted a solution. Having found the problem,
and stated it clearly, he collected all the facts and observations that
had any connection with the problem. Then he looked for the best way in
which to attempt the solution, and if necessary he made a vast number
of new experiments and investigations. In this way he not only made
great inventions, but great scientific discoveries as well.
When he set himself to find a convenient electric lamp, he knew that
when a strong current is sent through a thin wire, that wire becomes
red hot, or even white hot, and therefore gives a light. Such a light
is not a glaring one, and therefore is better in one respect than an
electric arc. Edison tried a great many different wires, and although
many of them gave a good light, they did not last long. A platinum wire
could be made white hot for some time, but in the end it always broke
in some place.
As the electric arc had been constructed successfully with carbon,
Edison naturally tried carbon next. Charcoal was found to be too
porous, and gas carbon could not be drawn into wires. It then struck
Edison that if a silk thread were subjected to great heat, it was
turned black. It would, so to speak, be made into carbon like charcoal,
and would not require thinning down, as it would be quite thin. He
tried the threads of silk, cotton, and flax, and heated them strongly
in a closed vessel. They produced very fine threads of carbon, but
these threads broke when they were in the least disturbed. Of all the
substances which Edison tried, a bamboo fibre was found to be the best,
but even that wore away after being used in a lamp for six hundred
hours.
Edison next invented what he called an “electric candle.” It was a
short rod made of platinum powder mixed with lime. But he shortly
afterwards came to the conclusion that nothing but a fine thread
resembling a wire, but made of carbon, would really answer the purpose.
It is strange that this conclusion was reached just about the same time
by Mr. Joseph Swan in England. The two inventors were working along
the same lines, and in the end they arrived at the same point. Edison
obtained good results by mixing lamp black and tar, and drawing it
into a fine thread. Joseph Swan, on the other hand, soaked a thread
of carbon in sulphuric acid, which made it into a kind of parchment.
He then blackened it by exposing it to great heat. Both inventors
were very successful, and produced lamps which were in every respect
satisfactory.
Both men had patented their inventions, and for some time there was
great rivalry between them, but in the end they decided that it was
better to be friends than enemies. They formed an alliance, and their
lamps were all made in the same way, and distinguished by the word
“Ediswan,” which is a combination of “Edison” and “Swan.” Cotton
wool was soaked in a solution of chloride of zinc. This gave a pasty
substance called cellulose. A lump of this was forced by pressure
through a very small hole, and so drawn out into a fine thread. The
thread was then blackened by heating it in a crucible made of plumbago,
and so the fibre was finished. The fibre was called a filament.
[Illustration: Fig. 65.--Electric Glow lamp with Carbon Filament.]
The next thing was to enclose the filament in a glass globe in which
not a trace of air would get near it. For if even the smallest quantity
of air got near the filament while it was glowing, it would at once
burn up. It was easy enough to pump all the air out of a glass globe,
and leave only some other gas in the globe that would do no harm, but
the difficulty was to mount a filament in the globe and to conduct a
current through it from the outside.
This was done by melting thin platinum wires into the glass. Platinum
wires were the only wires which would not break the glass. For when
platinum is heated it expands in the same way and to the same amount
as glass, and therefore it does not break away from it. The filament
was joined to the platinum wires by a special cement. After all this
had been done, the air was pumped out, and the globe was melted off
the pump at the lower end where the little point of glass is seen. The
glass globe is called a bulb. It is provided with a brass collar from
which two points stick out sideways. These points fit into recesses in
another brass collar into which the wires bearing the electric current
are led. This kind of joint is known as a bayonet joint.
The current for the lamps is furnished from a special factory in which
a steam-engine is made to turn an electric machine called a dynamo.
Such a dynamo is constructed on the principle of the induction currents
discovered by Faraday. Faraday’s whole life was spent in discovering
new truths. He did not think first of all of the use his discoveries
might be to mankind. He knew well that there were plenty of engineers
and inventors ready to take advantage of his discoveries. It is very
well that the discoverer and inventor should work hand in hand. The
discoverer shows the way in which great achievements can be attained.
The inventor makes these great practical achievements, and the good
that results from them comes back to the discoverer in the gratitude
of those who have been benefited by his discoveries and the happiness
which followed from them.
CHAPTER XIV
MICHAEL FARADAY
IT is strange that the man who did most for the progress of
the science of electricity in the nineteenth century was the son of a
London blacksmith. That man was Michael Faraday. He did not get much
schooling, and what he did get was not more than reading, writing, and
arithmetic.
[Illustration: Fig. 66.--Michael Faraday.]
His spare time was passed at home or in the streets. At the age of
thirteen he was engaged as an errand boy to a bookbinder. After a year
of running errands he was apprenticed to the trade of bookbinder, but
the trade did not suit him at all, and all the time he could spare
from it he spent in making chemical and electrical experiments. When he
could get somebody to pay his admission fee, he attended lectures on
these subjects.
When Faraday came of age he heard four lectures delivered at the Royal
Institution in London by Sir Humphry Davy, the great Cornishman, who
invented the safety lamp for miners. He was fired with the ambition to
become a man of science. He had a great dislike for trade, and thought
that all men of science were good and kind, and that it was the science
which made them so. He took careful notes of the lectures, and when
they were finished, he had the happy thought of sending his notes to
Sir Humphry Davy, and explaining to him that his dearest wish was
to follow in his footsteps. Davy was the best man he could possibly
have written to on such a subject. He received Faraday’s letter very
kindly, and promised to find some scientific work for him as soon as
he could. He kept his word, and engaged Faraday the very next year as
his assistant at the Royal Institution. At the age of thirty Faraday
married, and brought his wife to the Royal Institution, where they
lived together for six years. In that institution Faraday, in the year
1825, succeeded Davy as director.
Faraday’s first great achievement in electricity was to prove that
an electric current could be produced by means of a magnet, without
any electric battery. He made this discovery in the year 1831.
Eleven years previously, the first connection between magnetism and
electricity was discovered in Denmark. It was found that when a wire is
stretched under a magnetic needle in the direction in which the needle
points, and a current is sent through the wire, the needle turns round
and tries to set itself in a direction across the wire. In this way an
electric current produced some effect upon a magnet. Shortly after that
fact was discovered, a great Frenchman of the name of Ampère found that
one electric current attracts another that flows in the same direction,
while it repels a current which flows in the opposite direction.
It struck Faraday that it might be possible to produce an electric
current in a wire by simply starting a current in a neighbouring
battery. He tried the experiment by stretching two wires side by side,
and attaching the ends of one of the wires to an electric battery. In
order to discover whether a current was travelling through the second
wire, he brought a little magnetic compass up to it to see if the
compass would set itself at right angles to the wire and so indicate
a current. The needle indicated no current. Faraday then attached the
ends of the second wire to an instrument called a galvanometer. That
instrument consisted of a coil of wire in the centre of which the
magnetic needle was placed. The current would pass through the coil
of wire over the needle in one direction and under the needle in the
opposite direction, and in each case it would tend to set the needle at
right angles to the coil. The more turns the coil of wire had, the more
powerful would be the effect. In this way, therefore, Faraday hoped to
observe even a very small current.
But however long the current was kept passing through the first wire,
the galvanometer showed not the slightest current in the second wire.
This appeared very strange, because it was known that bodies may be
made electric, or magnetic, by bringing them into the neighbourhood
of electrified or magnetised bodies. It was natural, therefore, to
suppose that a coil of wire could be made to carry an electric current
by bringing it into the neighbourhood of another coil of wire which was
already carrying an electric current. But in such things it is useless
to endeavour to answer questions offhand. The only proper way is to try
an experiment. Faraday did try the experiment as already described, and
the result showed him that he was mistaken.
But Faraday was not discouraged easily. Thinking that perhaps the
effect could be strengthened by winding the wires round a piece of soft
iron, he took a ring of iron and wound two coils of wire round it,
taking care that no part of the wire was touching any other part, or
touching the iron. He then observed a curious effect, which was quite
different from what he had expected. He found that the galvanometer
gave a little start as soon as he turned on the current in the first
coil. Then it came to rest again, although the current was still
flowing. On turning off the current, the galvanometer gave a little
start in the opposite direction, and soon returned to rest again.
Faraday was in doubt whether this was really such an effect as he had
been looking for. He wrote to a friend, “I think I have got hold of a
good thing, but cannot say. It may be a weed instead of a fish that
after all my labour I have at last pulled up.”
Faraday was a first-class experimenter, and his method is a good
example of truly scientific work. When he found an effect which he
could not quite account for, he changed the experiment in a great many
different ways, in order to disentangle the causes and effects. The
next time he tried to obtain a current by means of a magnet instead
of another current. He took a natural magnet and wound a long coil of
wire round and round it. The natural magnet had a cap of iron, and on
pulling the cap off, a short momentary current appeared in the coil of
wire. This momentary current could be shown either by attaching the
ends of the coil of wire to a galvanometer or by bringing them close
together. When that was done a small spark passed between the two ends
of the wire.
This was another remarkable discovery, and Faraday proceeded to put it
into another different shape. He wound a coil of wire round a bar of
soft iron, and connected the ends with a galvanometer. Nothing happened
so long as the bar of iron was not magnetised. But on suddenly placing
the bar of soft iron between the two poles of a magnet, the bar became
magnetised, and the galvanometer gave a momentary start. He removed the
magnet, and the galvanometer gave a start in the opposite direction.
Faraday expressed this by saying that a momentary current is induced
in a coil of wire by starting or stopping a current in a neighbouring
coil, or by creating or destroying a magnet near the coil.
Such induced currents are also called “induction currents.” They are
familiar to many through the “induction coils” which are used to give
electric shocks. In these induction coils there is a cylinder of iron
surrounded by a coil of copper wire, and another coil of very fine wire
round that again. A current is started and stopped in rapid succession
in the first coil, and each starting and stopping induces a momentary
current in the outer coil. These momentary currents are made to travel
through the person who holds the handles. It is they that produce the
curious pricking sensation.
[Illustration: Fig. 67.--Induced Currents.]
Instead of sending a current through one coil inside another, we
may take a coil through which a current is already passing, and put
it inside another coil connected with a galvanometer. Each time
the current-bearing coil is put in or taken out, the needle of the
galvanometer moves, showing that a momentary current is passing through
it.
When we consider that great cities are now lighted by means of these
induced electric currents, it will be understood what a vast importance
Faraday’s discovery bore within itself. When we pass the electric light
works, a “central station” as it is called, we hear the throbbing of
the engines which turn the machines designed to produce the electric
current. These machines are called “dynamos.” A dynamo consists of
two things. One of them is a great magnet which produces a powerful
magnetic field. The other is a set of coils of wire which, driven by a
steam engine or gas-engine, move through the magnetic field.
[Illustration: Fig. 68.--A Simple Dynamo.]
Each coil, as it moves in and out of the magnetic field, becomes the
bearer of a momentary induction current, for to move a coil through
a magnetic field produces the same effect as to make or unmake an
electromagnet near the coil, or to start or stop a current in a
neighbouring coil.
Great inventors have been working ever since Faraday’s time to improve
and perfect the dynamo; some worked at collecting all the momentary
induced currents into one smooth current always flowing in the same
direction; others at providing ways and means for increasing the
strength of the magnetic field or the speed of the engines. And
thus to-day we have a vast industry occupying millions of men the
world over, all due to a man who worked quietly and steadily in his
laboratory, observing everything, neglecting nothing, and extracting
the nuggets of golden truth from the mean-looking dust and dross of
commonplace facts.
[Illustration: Fig. 69.--Dynamos in an Electric Light Central Station.]
Faraday made a great number of other discoveries. He investigated the
connection between chemistry and electricity, and founded a new science
called “electrochemistry.” He also studied the action of a magnet
on light, and found that a strong magnet is able to give a certain
twist to a beam of light while it passes through certain substances
(such as sulphide of carbon). Whenever he made such a discovery he
was transported with delight. He was always of a vivacious and merry
disposition. He said of himself: “I was a very lively, imaginative
person, and could believe in the Arabian Nights as easily as in the
Encyclopædia; but facts were important to me, and saved me. I could
trust a fact, and always cross-examined an assertion.”
Towards the end of his life he suffered greatly from loss of memory.
He helped himself by carrying about in his waistcoat pocket packets
of cards, on which he wrote everything he wanted to remember. One of
these has been preserved, and on it are found the following excellent
maxims:--
“Remember to do one thing at once. Also to finish a thing. Also to do a
little if I could not do much.”
CHAPTER XV
TELEGRAPHING WITHOUT WIRES
THE telegraph pole is one of the signs of civilisation. Along
every railway the telegraph poles, with their insulating cups and
strands of wires, stand like a military guard, connecting each station
with all the rest, and preserving the necessary communication with
headquarters. Along the high roads also, and sometimes over hill and
dale, these outposts of civilisation stretch their long arms. They
resemble human beings in this respect that they suffer from heat and
cold and exposure to the weather. In hot weather the wires expand,
in cold weather they contract, and in stormy weather they groan and
howl. Sometimes they are blown down, and then they are as useless for
purposes of telegraphy as a fallen soldier is useless for war. When
protection from storm is specially necessary, the wires are sometimes
buried in the ground. They are insulated properly and twisted together
into a cable, which looks very much like a wire rope; and the cable is
laid in the ground surrounded by a leaden tube. Such cables are used
when we want to connect two stations across the sea, and when that sea
is the Atlantic Ocean, the cables have to be very stout and strong.
[Illustration: Fig. 70.—Section of Transatlantic Cable. C, core; G,
gutta-percha; J, jute fibres; F, steel wires embedded in hemp.]
All this trouble would be saved if there were some way of telegraphing
without wires. We do not use wires in communicating with each other
by speech or sign, and there is no reason why a similar system
should not be invented for great distances. This has really been
accomplished, after long-sustained efforts, and now we have systems
of wireless telegraphy by which ships in mid-ocean can keep in
constant communication with the land, and even print a daily newspaper
containing all the latest news.
We have seen that when an electric current is sent through a coil of
wire wound round a piece of soft iron we get an electromagnet. The
electromagnet is capable of turning a magnetic needle at some distance.
If we could make the needle turn at a distance of a mile, we should
have a simple system of wireless telegraphy. It would be sufficient
to send the current into the electric magnet now in one direction and
now in another. The needle would then turn towards the West or the
East, and we could arrange with our friend who observes the magnet as
to what we mean by the various signals. The action of the magnet is
felt all over the world, but it is felt much less at a great distance
from us than within a few feet or inches. In fact, when the distance
becomes more than a few yards, the effect is so small that it is quite
invisible. That is why this simple system cannot succeed. We want
a system the power of which is not confined to close quarters. Our
success will depend upon the rate at which the power of our instrument
diminishes when the resistance is increased.
If we drop a stone into water, it sends out circular waves which become
gradually feebler and feebler as the distance increases. At a distance
of twenty feet they are twice as feeble as they are at a distance of
ten feet. Their crests only reach half the height. Bad as this is, it
is much worse in the case of light, and very much worse in the case
of magnetism. In the case of light only a quarter of the strength
remains when we double the distance, and in the case of magnetism
only one-eighth remains. If, however, we could have some system of
telegraphy resembling water waves it would be a great deal better than
any other kind. And this is precisely the way in which the inventors
of wireless telegraphy attained their triumph.
The first man to produce waves of electricity, and to prove that they
were waves of electricity, was a German professor of the name of Hertz.
He lived at Bonn on the Rhine, and spent a great deal of time trying
to prove that electric force does not spread out into space instantly,
but takes time to reach a distant point. He found that this time was
very short indeed. He found, in fact, that the speed of electricity is
the same as the speed of light. He proved that if electricity is made
to surge up and down an electric wire, a wave of electric force travels
out in all directions. If such a wave falls upon a sheet of metal, it
is reflected just as light is reflected from a mirror. A similarity
to light also appeared in many other properties of the electric waves
or radiations. These could be concentrated by a curved mirror, and
refracted by a large prism of pitch. These waves of electric force are
indeed a kind of light waves. But whereas light waves are so short
that some fifty thousand of them go to the inch, the waves which Hertz
discovered were each several yards long.
The first practical system of wireless telegraphy was invented and
carried out by a young Italian engineer of the name of Marconi.
Strictly speaking, it may be said that Marconi discovered nothing.
What he did was to apply the scientific discoveries of his predecessors
and make them practically useful. Marconi made the electric discharge
pass between two round knobs, and to these two knobs he attached two
wires. One of the wires went high into the air, and the other went down
into the ground. The consequence was that when the discharge passed
between the two knobs, electricity was in turn thrown up into the air
and thrown down into the ground. The effect was very much like that
produced by throwing a stone into water. A wave of electric force
started from the wire and spread out all around it. As soon as any
electricity was thrown into the earth, it spread out along the surface
in great waves, which travelled over the ground at an incredible
speed. As soon as electricity was thrust up along the wire, a want of
electricity was produced in the earth, and this electric vacuum, as we
might call it, spread out along the ground in the same way as the first
pulse. It was indeed a process resembling the crest and the valley of a
wave of water.
This was, therefore, the state of things likely to produce the best
results. Any other vertical wire at a distance along the surface of the
earth would experience the effect of such waves. The electricity would
surge up into the wire when the crest of the wave readied it, and would
rush down again as the wave subsided.
[Illustration: Fig. 71.--Guglielmo Marconi.]
Now, every such motion of electricity constitutes an electric current.
It remained to discover this current, and to devise a delicate
instrument for discovering it even when it is very weak.
The instrument which Marconi used for this purpose was called a
“coherer.” It was a little glass tube filled with some metallic powder.
When an electric wave passes through such powder, the particles
cohere or stick together, and then the powder conducts the current
easily. That being the case it was only necessary to have a battery in
readiness to take advantage of the new state of affairs. That battery
was connected to both the end wires of the coherer, and as soon as the
wave reached the wire and the coherer began to conduct it, a current
from the battery passed through and worked an ordinary telegraph
apparatus. When the coherer was shaken or tapped with a little
hammer, it lost its conductivity and was ready to receive the next
wave. Marconi had a little hammer tapping the coherer at very short
intervals, so that it was always ready to receive any wave that might
be sent out from the sending station as a signal.
That is, in a few words, the principle of Marconi’s system of wireless
telegraphy. It was first tried at a regatta on the Thames, being fixed
up on board a steam launch which followed the racing boats and sent
news of the exact position of the boats to the receiving station on
the shore. It was next tried on the open sea during a regatta off
Kingstown, in Ireland. Later it was used to send messages a distance
of forty miles over Salisbury Plain. For this purpose the wire had to
be taken very high into the air, and this was done by means of kites.
Wireless telegraphy was also tried on board an Italian war vessel
which sailed from Italy to Gibraltar, and round the coasts of France,
Denmark, and Germany as far as Russia. The war-ship remained all the
time in telegraphic communication with a wireless telegraphic station
in Cornwall. But the greatest triumph of all was when Marconi succeeded
in establishing communication between Cornwall, and Connemara, on this
side of the Atlantic, and Canada on the other side. To cover this great
distance large sending stations had to be erected and several thousand
horse-power had to be used. But this system had the enormous advantage
over the cable system of being free from any dangers threatened by the
Atlantic Ocean.
Wireless telegraphy has been the means of preventing great disasters at
sea by enabling vessels in distress to summon the aid of other vessels,
notably during the sinking of the _Republic_ off Nantucket and
stranding of the _Slavonia_ on Flores Island, Azores.
CHAPTER XVI
THE NEW RAYS
IT is not given to many men to make such a wonderful discovery
as fell to the lot of Professor Röntgen in the year 1895. If anyone
wishes to mention something very new and wonderful, some great
revelation of science during the nineteenth century, he usually puts
the Röntgen Rays or X-rays in the first place. This is very natural,
for these rays give men powers of vision which they could hardly expect
ever to possess. They enable us to see through many dark bodies, and
even through the human body itself. The discovery of these rays forms
one of the most romantic chapters in the history of science.
The discovery was made in the course of some work with a vacuum tube. A
vacuum tube is a bulb of glass from which all the air has been pumped.
An electric lamp is a kind of vacuum tube, but it would not be suitable
for the experiments of Professor Röntgen unless the filament were
removed and nothing but the two platinum wires remained. If the current
were sent through the bulb as usual, it would have some difficulty in
passing from one platinum wire to the other, and indeed it could not
pass at all unless some slight trace of air remained in the bulb.
[Illustration: Fig. 72.--Professor W. C. Röntgen.]
Even then a higher pressure of electricity is required than is used
for lighting the filament in the ordinary way. Instead of having the
two platinum wires side by side, they are usually placed at opposite
ends of the tube when the ordinary experiments with vacuum tubes are
being performed. Also, instead of ending in sharp points the platinum
wires usually end in small discs or caps.
When a current is sent through a tube of this kind the trace of gas
in the tube shines with a strange light. The light usually shows
some beautiful colours, and is arranged in layers. These layers are
generally in the neighbourhood of the positive wire, which is called
the anode. At the negative wire we find a layer of white, or violet,
light, and next to this a dark space, which separates it from the
glowing space beyond. When special care is taken to get rid of as much
gas as possible, the dark space expands until it fills the whole of the
tube. Then a strange thing happens. A green glow plays on the whole of
the tube opposite the cathode, and it is in this green glow that the
X-rays are born. But before we describe how the X-rays originate, we
may say a few words of what happens in the vacuum tube, and gives rise
to the green glow.
If we could see the cathode through a microscope of a magnifying power
greatly beyond that of any existing microscope, we should find it to
be the scene of a very intense activity. We should see great jets of
small particles start from the cathode at a tremendous speed, and shoot
away in straight lines until they reached the glass wall opposite,
there producing the green glow. These small particles are thought to be
the real particles of electricity which constitute the electric current.
An electric current consists of a stream of these extremely small
particles, which are called “electrons.” They are much smaller than
the atoms of a wire; and thread their way slowly between the atoms,
knocking against them every now and then, and producing a great deal
of heat. The same number of electrons passes through every section of
the whole circuit in the same time, and naturally, when a wire in the
electric circuit is very thin the electrons must pass at a greater
speed in order to get through and make room for those that come after
them. In a thin wire, therefore, the heat is very great, and when the
wire becomes a filament, the heat is great enough to make it shine with
a bright light. This gives us an electric glow-lamp.
Now, when these electrons arrive at the end of the cathode wire, they
are urged strongly to cross into an open space beyond the wire, partly
by the crowds of electrons following them, and partly by the strong
attraction which the anode opposite exerts upon them. But the atoms
of the wire themselves attract and hold the electrons, and there is
therefore always some difficulty to overcome before the electrons can
leave the wire. Once they are free from the cathode, they are impelled
forward by the repulsion from behind and the attraction in front. Like
stones falling from a great height they rush on more and more quickly.
The force which plays upon the electrons is enormously greater than the
force of gravitation. It is more than a million times greater. On the
other hand, the particles or electrons are very small, and, therefore,
do not produce any very destructive effects, however high their speed
may become. But their speed is something enormous. It is something like
fifty thousand miles per second. No wonder that when these particles
are stopped by a glass wall they produce some very curious effects.
These effects become much more intense when they are concentrated upon
a sheet of platinum placed opposite the cathode. That sheet becomes
white hot and sends out the famous Röntgen Rays, which Röntgen himself
called “X-rays.”
Now as to the history of the discovery of these rays. Professor Röntgen
was one day experimenting with such a vacuum tube, and trying to get
the electrons to go out of the tube into the open air. This experiment
had been done many times before him by using a little window made of
very thin aluminium, which can be penetrated by electrons.
[Illustration: Fig. 73.--Vacuum Tube for producing Röntgen Rays.]
He was studying the effect of these electrons upon a sheet of some
phosphorescent substance. He wished to find out whether the electrons
would still come out if the vacuum tube were enclosed completely in
black cardboard. He also wished to observe the effect in complete
darkness. He was surprised to find that the phosphorescent screen was
lighted up, and that some influence from the vacuum tube reached the
screen through the black cardboard.
Greatly struck by this observation he tried various other arrangements.
He put a deal board between the tube and the screen, but found that
the board made no difference whatever, and that whatever rays from
the tube were lighting up the screen, these rays could pass through
the deal board. He then took away the board and put in its place a
thick book, but again this made no impression whatever upon the rays.
Thinking that perhaps the rays were coming round to the screen in some
unexplained way, he held his hand between the tube and the screen.
Imagine his surprise on seeing, not the shadow of his hand, but only
the shadow of the bones in his hand on the phosphorescent screen. It
must have given him something of a shock, and we may suppose that he
quickly turned on the light to see if his hand had really shrivelled
up into a skeleton. But no, the hand looked just as usual, and yet on
holding it in the path of these wonderful rays, the shadow of the bones
was again distinctly visible, the flesh showing only in faint outline.
It was a most wonderful discovery.
The great news was not long in travelling all over the world. The
experiment was repeated thousands of times, and it was soon found out
that a great new fact had been added to the range of human knowledge.
It was found that metallic bodies such as coins, bullets, and needles
could be located in the flesh by means of the rays playing through it
on to the screen. Instead of the screen, a photographic plate could be
used, and pictures of these bodies could be obtained in a few minutes.
[Illustration: Fig. 74.--A Foot inside a Boot as seen by means of the
Röntgen Rays.]
Surgeons were not slow in using the great discovery in the hospitals,
and every large hospital has had a special staff of men trained in the
use of the Röntgen Rays.
But it often happens that the blessings of a new discovery are
endangered by evil effects which they bring in their train, and this
was very much the case with the Röntgen Rays. The men who spent all
their days taking photographs by means of the new rays found to their
dismay that wherever the rays played upon them for a long time the
skin became dry and hot. The hair fell off, and even the nails dropped
out. Worse than that, very painful sores began to appear after a time,
which it was found almost impossible to heal. In some cases death was
the result of prolonged exposure to the Röntgen Rays; but, as usual,
science eventually succeeded in separating the good from the bad
effects, and protected the skin by means of screens of lead which
cannot be pierced by X-rays. Now this new great aid to surgery and
medicine has been established properly and regulated satisfactorily.
The X-rays were not the only kind of new rays discovered towards the
close of the nineteenth century. In fact they ushered in the discovery
of quite a number of new kinds of radiation, each more wonderful than
the last. We have already mentioned the green glow on the glass, and
the white light on the platinum screen which catches the electrons.
Somebody put forward the supposition that it is the phosphorescent
light itself which sends out the X-rays. He supposed that whenever
anything shines with phosphorescent light, X-rays are sent out, and
these make an impression upon a photographic plate. This supposition
was tested by a great Frenchman of the name of Becquerel. His father
had made a great name by investigating phosphorescence, and what more
natural than that the son should study the behaviour of phosphorescent
bodies as regards X-rays!
But by a very extraordinary and lucky chance he chose among the many
phosphorescent bodies known to him the only body that could have
led him to the great new discovery which he made. That body was a
substance called uranium. He exposed some crystals containing uranium
to sunlight until they glowed with a thin blue light.
[Illustration: Fig. 75.--Henri Becquerel.]
Then he shut up those crystals in a dark box with some photographic
paper, and left them there for a few hours. At the end of that time he
found that the crystals had blackened the photographic paper. This, he
thought, was either the effect of the bluish light, or the effect of
X-rays from the crystals. He decided to try the experiment again, but
he waited in vain day after day for the sun to come out.
This, as the sequel showed, was another fortunate chance. For all this
time the crystals lay ready in a drawer in the neighbourhood of a
photographic plate, waiting to be exposed to the sun. Tired of waiting
any longer, Becquerel thought he had better close the experiment
for the present by developing the photographic plate. To his great
surprise, he found that the crystals had made a strong impression upon
the plate, although it had been protected by black paper, and although
the crystals had not been exposed to sunlight. It then struck him that
perhaps the crystals had been exposed to the sun some considerable time
before, and were now only very gradually giving up the light which they
had absorbed. He therefore kept them for months in complete darkness,
and then exposed another plate to their action. But contrary to all
expectation, he got the same result as before, so that it was clear
that the action of the uranium was quite independent of light, and
continued without interruption.
Even greater surprises were yet in store, for this power of uranium
to give out a new kind of rays which impressed a photographic plate
was found to be quite indestructible. No matter if the uranium was
melted up in a crucible, pounded to a fine powder, mixed with other
substances, melted or boiled, or treated with all kinds of acids, the
effect was always the same so long as the quantity of uranium remained
the same. The new and strange power seemed to lie in the very atoms of
the uranium themselves.
This power was quite new, and therefore had to be called by a new
name. The wife of another French professor of the name of Curie, who
greatly distinguished herself in studying this new power, called it
“radio-activity,” and this name is now used generally. Professor Curie
and his wife went to a great deal of trouble to concentrate the uranium
and make it as highly radio-active as they could. They found that some
preparations of uranium were more active than others, and, therefore,
they found that a certain sediment of the uranium ore was quite
extraordinarily radio-active. They extracted this sediment from several
tons of the ore.
It was an extremely expensive operation, but it was richly rewarded by
another marvellous discovery, for at the end of a great deal of work
they found a new substance which was a million times more radio-active
than uranium. This substance they called “radium.” So intensely active
was this new substance, that if shown in the dark, a small piece of
it made a phosphorescent screen brightly luminous. But the radium
proved as dangerous as the X-rays, for Madame Curie, who wore a small
quantity of it in her sleeve, found that it gave her a sore which took
several months to heal.
After the tragic death of her husband in a motor accident Madame Curie
was appointed Professor of Chemistry in the Paris University, being
the first lady ever appointed to such a post in that ancient city of
learning.
CHAPTER XVII
AIR-SHIPS AND FLYING MACHINES
WHEN we read of the great inventions of the past, we are apt
to think that they would in any case have been invented some time or
other, even though the great inventors themselves had never lived.
But to tell the truth, each inventor had before him a great number of
difficulties which he could only overcome by patient and persistent
effort. It often happened that some attempt to solve the problem had
been made before him, and had failed. Such a failure would lead men to
believe that success was impossible. Somebody would try to prove that
the problem could not possibly be solved, and that one might as well
give it up as hopeless. It is just this kind of attitude which is most
disturbing to inventors and discoverers. They are surrounded by friends
who never cease to tell them that they are simply wasting their time in
trying to solve a problem which cannot be solved.
Such a problem is that of flying through the air. The birds have solved
it long ago, but even a few yearn ago it was taken for granted that no
machine or engine could be built which would lift its own weight into
the upper air. But within the last year the problem of human flight has
been solved completely, and probably in a few years’ time the flying
machine will be seen by nearly everybody every day.
This achievement means an entirely new development of civilisation.
We are fortunate to live in the time of an invention which is more
important than the steam-engine or the steam-ship, and will do more
to bring people together than the telephone or the telegraph. We are
doubly fortunate in being able to watch from day to day the manner
in which the various difficulties of flying are overcome one by one.
Instead of reading of great discoverers of far-off times, we can read
about the triumphs of the great men of our own time. We can follow and
admire the work of the heroes of science of to-day.
The first instrument which enabled man to rise into the air like a bird
was the balloon. Nobody knows when the first balloon was sent up. But
it is reported that at Pekin, the capital of China, a balloon was sent
up at the coronation of the Emperor of China so long ago as the year
1306. In the year 1709, a priest of Lisbon is reported to have ascended
into the air in a balloon filled with hot air, but the first authenic
account we have of such an event dates from the year 1783.
Several years before that, Cavendish in England had found that hydrogen
is fourteen times lighter than air, and that it therefore ascends
through air. This was proved by filling a soap bubble with hydrogen
instead of with air. It floated quickly up to the ceiling and then
burst. This experiment set many people thinking about the possibility
of getting things lifted up into the air by gas. Two of these people
were brothers of the name of Montgolfier, who owned a paper factory in
France. One day they made a great bag of paper and held it over the
fire, thinking that the smoke from the fire would raise the bag into
the air. But the paper was too heavy, and the balloon did not rise.
Most people would have been discouraged by such a failure, but the
brothers Montgolfier guessed the cause of it, and proceeded to manage
things better. They built another balloon in the shape of a box, with
its opening downwards, and this time they employed silk instead of
paper. They held it over a fire of straw and wool, thinking that such
a fire developed a kind of gas which was very light, or which was
repelled by the earth. They found, however, after a while, that the
nature of the fire did not matter so long as it was hot enough. They
found, in fact, that it was the hot air which really caused the balloon
to rise, and that a flame burns upwards simply because it is lighter
than air. How this should be so is quite clear from the principle of
Archimedes, according to which a body loses as much in weight as the
weight of the fluid it displaces.
We have seen already that the weight of the air inside an ordinary
sized room is about a hundredweight. If therefore we construct a
balloon of the same bulk as a room, and fill it with a gas much lighter
than air, the balloon will weigh less than its ordinary weight by about
a hundredweight. If the balloon together with a light gas which fills
it weighs 60 lbs., it will be able to lift 52 lbs., or rather it will
require 52 lbs. to keep the balloon down on the ground. By making the
air inside the balloon hot, about 45 lbs. of air can be made to balance
the pressure of the outside air, so that 60 or 70 lbs. may be the
weight of the balloon itself. But by using hydrogen the weight of the
gas can be reduced much further. For eight pounds of hydrogen exert the
same pressure as a hundredweight of air, so that these eight pounds of
hydrogen can lift 104 lbs.
The silk balloon constructed by the brothers Montgolfier rose up into
the air, but fell down again as soon as it cooled. They therefore made
a new balloon 40 feet high, constructing the envelope of packing-cloth
covered with paper. The lower end was open, and under it was hung
an iron cradle filled with moist straw and wool, which gave a slow
fire, and helped to keep up the heat of the balloon. The new balloon
rose into the air in June 1783, and soon sailed out of sight, bearing
a message up to the clouds that mankind was about to enter their
territory.
The news of this great success quickly reached Paris, and led to many
attempts to imitate and develop the striking invention. In Paris,
the work was taken up by the brothers Robert, makers of scientific
instruments, and Professor Charles, a young and promising physicist.
They made a balloon 12 feet in diameter and filled it with hydrogen
gas. It rose into the air and sailed away before a huge crowd of
people. A few days afterwards, the brothers Montgolfier brought to
Paris a giant balloon 72 feet high, which also rose into the air, but
was held back by ropes. A week after that they attached to the balloon
a car of wickerwork, in which they put a sheep, a cock, and a duck.
The balloon came to the ground somewhere out in the country, and all
the three animals alighted safe and sound.
The success of these first three navigators of the air encouraged the
human inventors of the balloon to venture themselves into cloudland,
and in November of the same year two Frenchmen ascended in a
fire-balloon and sailed over Paris across the river Seine. To show that
the same could be done in a balloon filled with hydrogen, Charles and
Robert made an ascent in a gas balloon a few days later. The balloon
was made in lengths of red and yellow silk. It was provided with a net,
to which the car was attached, with a valve for letting out the gas,
with a barometer for measuring the height, and with bags of sand. The
sand was taken as ballast, and was used to lessen the weight of the
balloon when it was sinking too quickly.
The great hopes built upon the balloon were not fulfilled for many
years. Benjamin Franklin, when he first saw a balloon, said he had seen
an infant which he hoped to see grow into a giant. But for ninety years
no decided improvement was made in the construction of the balloon.
Bold investigators of the air were found to cross the Channel from
Dover to Calais, or from London to Germany, as Green did in 1836 in a
giant balloon filled with coal-gas. Some great heights were reached.
In 1862 two Englishmen named Glaisher and Coxwell rose to a height of
seven miles, the greatest height ever attained by people in a balloon.
Their pulses rose till they gave 110 beats per minute instead of about
70 or 80 beats per minute. Their faces became purple, and finally
Glaisher became unconscious. Coxwell lost the use of his hands in the
extreme cold of the upper air, but he managed to pull the valve rope
with his teeth, and so they both returned safely to Mother Earth.
During the Franco-German war a number of balloons were sent up from
Paris when it was besieged by the Germans. These balloons carried no
fewer than two and a half million letters into the provinces of France.
But the people in the provinces could not answer these letters, as no
balloon could be relied upon to reach Paris. It is not surprising,
therefore, that after the conclusion of the war, the French people
should have made many efforts to construct balloons which could be
guided to any particular destination. One such balloon was built in
1872, and another eleven years afterwards, but those balloons were not
suitably constructed, and the engines and screw propellers were not
powerful enough to take them against the wind.
It was only when motor cars were built in great numbers that people
acquired sufficient experience to build very light and powerful
engines suitable for propelling balloons. This shows how one class of
inventors may help another class. A balloon which can be driven in any
chosen direction is called an “air-ship.” Many such air-ships have
been built within the last few years. They are shaped very much like a
cigar. Some of them have cars made of light rods of steel or aluminium,
and such air-ships have been navigated from one city to another over
distances of sixty miles.
One of the most remarkable of these air-ships is that constructed by
Count Zeppelin, a German officer (Fig. 76). It is 420 feet long, and
38 feet high. It is cylindrical in shape, and is covered with silk or
gold-beater’s skin stretched over a stiff frame of aluminium rods.
It is divided into sixteen air-tight compartments, so that if a hole
should be cut in it at any point, the whole balloon would come to the
ground very slowly. One such balloon travelled down the Rhine in August
1908 for over eleven hours. On its homeward journey it was caught in
a storm and burnt up, but it was thought that it was built on a good
plan, and the Germans at once set to work to build more ships of the
same kind. In 1909 one of these made a trip from the Lake of Constance,
in Switzerland, across Germany, to a place almost within sight of
Berlin.
But while inventors were busy constructing balloons, the solution of
the problem of flight was undertaken along an entirely different line.
[Illustration: Fig. 76.--Zeppelin II. on its Flight over Cologne.]
Many men did not see why we should not to some extent imitate the
flight of our successful rivals, the birds. It is true that from the
earliest times many men have tried to make wings that could fly, but
these attempts always ended in failure or disaster. The human body is
too heavy in proportion to the power of the muscles. Birds are much
stronger in proportion to their weight than we are. Their bones are
built on a lighter plan, and there is no probability that we should
be able to alter the construction of our own bones for the purposes
of flight. Nor is that necessary. There are many ways of keeping a
body afloat in air. Some birds can keep afloat for a long time without
flapping their wings at all, and most people are acquainted with the
trick of making a card fly through the air by giving it a rapid turning
motion.
It was an American, Professor Langley, who first constructed a small
machine which could fly through the air by means of its own mechanism.
He found that the faster a flat surface is moved through the air in
a horizontal direction, the less power it requires to keep it up. He
concluded that if a machine is provided with horizontal wings, and
driven very fast through the air, it will not require a very powerful
engine to keep it up once a fairly high speed is attained. He launched
his apparatus over a lake, and had the pleasure of seeing it fly for
miles before it fell into the water. When this principle had been
established other inventors straightway proceeded to apply it.
Two American motor manufacturers of the name of Orville and Wilbur
Wright constructed a machine capable of carrying a man. Two years ago
the first public trial of a flying machine was successfully made in
Paris, when Mr. Santos Dumont, a young Brazilian balloonist, flew
several hundred yards in a machine heavier than air. This machine
consisted of a number of boxes open at both ends and covered with
tightly-stretched canvas.
[Illustration: Fig. 77.--Blériot’s Aeroplane leaving France.]
At that time it was found impossible to steer such a machine. But on
December 30, 1907, Mr. Henry Farman succeeded at last in Paris in
describing a complete circle in the air, covering a distance of more
than half a mile, without once touching the ground. In July 1908 Mr.
Farman remained in the air twenty minutes, covering a distance of
eleven miles. This speed was afterwards increased to sixty miles an
hour. Meanwhile Mr. Wilbur Wright had gone to France, and he soon
eclipsed all the French records. On September 16, 1908, he flew a
distance of a mile and a half with a passenger on board, and on
September 21 he remained in the air one hour and a half, covering a
distance of fifty-six miles. He remained in the air after darkness
had set in, and it was strange to see his machine fly about like some
gigantic night-bird. On the last day of 1908, Mr. Wright accomplished a
flight lasting two hours twenty-three minutes, and covered a distance
of nearly seventy-eight miles. This has since been exceeded both as
regards distance and duration, and there seems every prospect that
flights lasting several hours will soon be of common occurrence. On
July 25, 1909, M. Louis Blériot, a French engineer, performed the
great feat of crossing the English Channel from Calais to Dover in
thirty-three minutes, in an aeroplane of his own construction. For ten
minutes of the flight he was entirely out of sight of land.
Civilisation has now arrived at a new stage of immense importance.
For the first time in its long history mankind has entered into full
possession of the realm of air. We have now a new road which needs no
repairing, and extends all round the globe. Our race enters on a new
era, and nobody can say what great changes and improvements in our
daily life are yet in store for us.
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