CHAPTER VI. The Barometer

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.

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.

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.”

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.

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.

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.