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.