Torricelli and his great discovery

It all began with the Grand Duke of Tuscany. There can be no doubt about that. When the new pumps came they would not suck or draw water, and then there was trouble. It is no wonder the Grand Duke was angry. He had set his heart on having those pumps in time to raise water for the fountains that were to play on his grounds during an approaching festival. He had instructed the pump-makers to construct for him the best pumps possible; and now, when they were tried, they would not work. The Grand Duke was angry. He was in the habit of having things go just his way, so when he found the pumps would not work he soundly berated the pump-makers.

It is true, when their royal customer had explained that the pumps were to be used to raise water fifty to sixty feet from the wells, the men told him that the greatest distance they had ever been able to get a pump to suck, or raise water, was about thirty-two feet. But this did not trouble the Grand Duke. He was ready to pay for the extra distance, and bade them go ahead and make better pumps. Willing to please the Grand Duke, and only too glad of the chance of making an extra, charge, they made the pumps, and brought them to the Duke.

They were certainly good pumps; probably the best that had ever been made; but for all that, they were not able to raise the water from the deep wells.

Therefore, the Grand Duke was angry. He told the pump-makers that those pumps must be made to work, and made to work soon. Then there was great excitement. Everyone was suggesting things, both wise and otherwise, but nothing was of any use.

Now, when the Grand Duke found that the pumps could not be made to work, he said: "Tell Galileo to come here."

This man's full name was Galileo Galilei. He was one of the most distinguished philosophers of Italy, and was born in Pisa, Italy, February 18th, 1564. When twenty-four years of age he was elected Professor of Mathematics at Pisa. There is only space to tell a few of the remarkable things Galileo did for science.

It was Galileo who discovered that a pendulum takes the same time to make one complete swing to-and-fro, whether it is swinging through a wide path, or through a very small path. The pendulum Galileo used for this discovery was the great suspended lamp in the Cathedral, and having no watch, he employed the beating of his heart to tell the time. It was Galileo who built for himself an excellent telescope and with it made many wonderful discoveries in Astronomy. Among other things, he proved that the earth moved around the^sun and not the sun around the earth; or, in other words, that the sun, and not the earth, was the centre of the solar system; and in this way he got himself into trouble.

His enemies were powerful and he was obliged to resign his professorship at Pisa. From this place he went to Padua, where he had such wonderful success as a teacher and lecturer that pupils came to him from all parts of Europe.

He remained in Padua for eighteen years, when he was called back to Pisa and became the principal mathematician and philosopher of the Grand Duke.

It was natural, therefore, when no one was able to make the pumps work, that the Grand Duke should say: "Tell Galileo to come here." So Galileo came and the Grand Duke asked him to look at the pumps and get them to working properly. This trouble with the pumps occurred during the year 1641, and, as Galileo was born in 1564, he must at this time have been seventy-seven years of age. Old, blind, and shattered in health from the cruel treatment of his enemies it would not have been surprising if he had been unwilling to give his best thoughts to such work as this of the pumps. It appears, however, that he did the best he could for the Grand Duke, sending word that the trouble would be found in the valves; that if they were made better the pumps would work. The pumps were, therefore, sent back to the makers with instructions to make the valves work more freely. This was done, but still the water would not rise so Galileo was sent for again. Aided by his pupils, the philosopher once more tried to find out the difficult, but, do what they would, they were unable to make the pumps suck or draw water to a greater height than thirty two feet.

It seems difficult in our day, when the causes of natural phenomena have been so clearly traced by the hard labors of the many bright men who have lived before us, to understand fully just why there should have been any astonishment at a pump's failing to draw water for a greater distance than thirty-two feet. We know now that pumps practically operate on the principle of a balance, or pair of scales, the weight of the atmosphere on one side forcing a column of water up the pipe which dips down into the well until the weights of the column of air and the colum of water balance each other. Even if the pump had been so admirably made that it could produce a perfect vacuum in the pump barrel, or, in other words, even if it could suck all the air out of the well-pipe, the most that the pressure of the air, acting downwards on the water in the well, could do, would be to force up a column of water in the well pipe until its height was such that the column of air and the column of water would exactly balance each other.

But neither the Grand Duke nor the pump-makers were to be blamed for their ignorance. At this time, 1641, it was not known that air possesses any weight, and thus it was not even suspected that the atmosphere exerts a downward pressure on the water in the well, or on other things on the surface of the earth.

You may ask how it was possible that intelligent people should have been using pumps for so long a time without finding out what made the water rise in the pump-pipe when the pump sucked all the air out of it. The reason was that, in those early days, men did not go about the study of natural phenomena in the proper way. Instead of actually trying experiments they attempted to reason out the causes of things, and up to that time the world had been satisfied with the following explanation: the water rises in the well-pipe connected with a pump, because, if it did not, there would be nothing left in the pipe, and "nature abhors a vacuum." The world had been satisfied with this explanation up to 1641. And, indeed, this was the explanation that Galileo, great philosopher though, he was, believed; for, when he found by actual trial that the pumps would not suck water through a greater height than thirty-two feet, he sent to the Grand Duke the following remarkable opinion. He said that the pumps would not operate because, although nature abhorred a vacuum, yet she did not abhor a vacuum greater than thirty-two feet of water; an opinion which, had it been delivered by the Delphic Oracle, could have been proved at this late day, when we know so much about atmosphere pressure, to have been true.

But, while Galileo was endeavoring to discover the cause of the pumps not sucking, or drawing water up in the pump pipe higher than thirty-two feet, there was, among the many pupils who were trying to aid him, an especially bright young man named Evangelista Torricelli. As we have seen, no satisfactory solution of the difficulty was reached then; but Torricelli continued to give the problem so much thought that in 1643, less than a year after the death of his old master, he announced to the world his great discovery that water is raised in pump-pipes from wells by reason of the pressure of the air, and not by reason of any abhorrence of a vacuum. By this discovery Torricelli acquired a reputation in the scientific world that immortalized him.

It appears that the apparatus employed by Torricelli in his first experiment, was not, as is generally stated, the glass-mercury tube, but an apparatus made in imitation of the Grand Duke's pump. It consisted of a glass tube sixty feet long, placed in a perpendicular position, with its lower end below the surface of water. He applied a suction pump at the upper end of ihis tube and found that the best he could do was to raise the water thirty-two feet. He then reasoned to himself that, if the true cause of the water rising in the pump-pipe was, as he believed, the pressure of the air against the water in the well, if he employed a denser liquid such as mercury, the height to which the pressure of the air would raise it would be inversely proportional to its density.

Torricelli 's experiment is now generally made as follows. The experimenter obtains a glass tube about four feet long, sealed at one end and open at the other, as shown in Fig. 6. Filling this tube with mercury and placing a finger over the open end, he inverts the tube and inserts the open end below the surface of a quantity of mercury in an open vessel. Then, holding the tube in a vertical position, he takes away his finger from the open end of the tube, when a part only of the mercury runs out, the rest being supported at a height of about thirty inches above the level of the mercury in the open vessel, as seen in Figure. By this simple but beautiful method, it was practically in this way that Torricelli proved, beyond any reasonable doubt, that it is the pressure of the air and not the abhorrence of a vacuum that causes the water to rise from a well to a pump. Figure on top represents Torricelli in his laboratory, making his famous experiment. You can see from the picture that this experiment was made in his laboratory, for a few of the different kinds of apparatus he had used in other researches are also made.

You can also see that here Torricelli is employing the mercury tube and not the water tube. Torricelli 's discovery caused great excitement when it became known in different parts of Europe. Of course, in those early days, no little time was required for news to spread. There was no telegraph or telephone, so that for the news to reach some countries a year or more was necessary.

I regret to say that in many parts of the world, when this great discovery was announced, intelligent men, instead of rejoicing that so great a secret of nature had at last been discovered, refused to accept Torricelli's explanation. They appeared to think that there was something wicked in rejecting the long cherished idea that nature abhors a vacuum.

The discovery of Torricelli was made in 1643. Unfortunately, Torricelli died shortly afterwards, in 1647. His experiments, however, were continued by other able philosophers. Among these was a distinguished French mathematician, Blaise Pascal.

Pascal was born in France in 1623. While a child he exhibited such proofs of ability in mathematics that he was kept in ignorance of geometry lest his fondness for it should interfere with his other studies. But one day his father was surprised at finding the lad (then only twelve years old) demonstrating on the pavement of an old hall in which he played, by means of a rude diagram traced with a piece of coal, a difficult theorem of Euclid, the great geometrician.

Being permitted to continue his studies Pascal composed, when only sixteen years of age, a treatise on the conic sections, a very difficult branch of mathematics concerning circles, ellipses, hyperbolas, and parabolas, which aroused the admiration and astonishment of the greatest living mathematicians. When only nineteen years old he invented an arithmetical computing machine, and by the time he was twenty six he had composed many mathematical works and made many experiments in pneumatics and hydraulics.

Pascal began experimenting with Torricelli's great discovery in 1646, one year before Torricelli's death. In repeating Torricelli's experiment, instead of employing mercury, Pascal used longer glass tubes closed at one end, which he filled with different kinds of liquids, such as wine or water, inverting these tubes in open vessels filled with the same kind of liquids.

Instead of meeting with approbation, Pascal's experiments were bitterly opposed. In order to demonstrate the correctness of Torricelli's discovery beyond any possibility of doubt, he planned the following experiment, which I will let him describe in the following extract from a letter he sent to M. Perrier, his brother-in-law:

"I have thought of an experiment, which, if it can be executed with accuracy, will alone be sufficient to elucidate this subject. It is to repeat the Torricellian experiment several times in the same day, with the same tube, and the same mercury; sometimes at the foot, sometimes at the summit of a mountain 500 or 600 fathoms in height. By this means we shall ascertain whether the mercury in the tube will be at the same, or a different height at each of these stations. You perceive, without doubt, that this experiment is decisive; for, if the column of mercury be lower at the top of the hill than at the base, as I think it will be, it clearly shows that the pressure of the air is the sole cause of the suspension of the mercury in the tube, and not the horror of a vacuum; as it is evident there is a longer column of air at the bottom of the hill than at the top; but it would be absurd to suppose that nature abhors a vacuum more at the base than at the summit of a hill. For, if the suspension of the mercury in the tube is owing to the pressure of the air, it is plain it must be equal to a column of air, whose diameter is the same with that of the mercurial column, and whose height is equal to that of the atmosphere from the surface of the mercury in the basin. Now, the base remaining the same, it is evident the pressure will be in proportion to the height of the column, and that the higher the column of air is, the longer will be the column of mercury that will be sustained."

This great experiment was made on the 19th of September, 1648, on the highest mountain in France, the Puy de Dome, near Clermont. As Perrier climbed to the top of the mountain with the mercury tube, the mercury fell until it was three inches lower in the tube than it was at the base. The experiment was repeated on different sides of the mountain at different times up to the year 1651, and always with the same result.

Pascal made other similar experiments by carrying the mercury tube to the top of high steeples in Paris, and thus established, beyond any peradventure, the fact that the mysterious power which was known under the name of "nature's abhorrence of a vacuum" was really the pressure of the atmosphere.

Torricelli's great discovery produced many important results. It was followed in 1664 by the invention of Otto Guericke, in Magdeburg, of the air-pump.

Another important result was the employment of Torricelli's mercury tube or, as it is now generally known, a barometer, for readily measuring the height of mountains or other elevations, as well as for a weather-glass, or instrument for determining coming changes in the weather. By means of a careful system of observations of the varying heights of the barometer at different points of the earth's surface, the United States Weather Bureau and other similar bureaus are capable of making fairly accurate prognostications of coming changes in the weather.

Various forms are given to the barometer.

One of the simplest forms is shown in Figure. Here a tube nearly four feet high is supported in a vertical position in a large reservoir filled with mercury. The height of the mercury column is measured from the upper surface of the mercury in the reservoir. In filling the barometer tube with mercury it is necessary to employ mercury that has been recently boiled, since, otherwise, the air the mercury contains would escape into the upper part of a tube and so injure the vacuum.

When nearly full the tube is placed on a sloping furnace and heated until the mercury boils. In this way all moisture and air are driven out and a fairly high vacuum is obtained in the upper part of the tube, which is known as a Torricellian vacuum.

Here the barometer tube is placed inside a copper tube provided with two slits on opposite sides. The scale divisions are engraved on this tube, as is more distinctly shown on the right in Figure. In the left the details of the reservoir are represented.

Although we live at the bottom of the atmosphere, with the air pressing with enormous force against our bodies, yet it was not until Torricelli's discovery that the existence of this pressure was discovered. The reason is evident. The atmosphere is a very nearly perfect gas and, therefore, transmits pressure equally well in all directions; that is, upwards, downwards, sidewise, and obliquely. While it is true that the air exerts a considerable pressure against any part of the human body, as, for example, the back of the hand, yet it exerts an equal opposite pressure against the palm of the hand, these two pressures balancing each other. Moreover, an equal and outward pressure acting through the blood exists, so that we are unconscious of any pressure whatever. If, however, the pressure be removed from one side of the hand or any other part of the body, the opposite pressure will at once make itself felt in a very decided manner.

There are a number of simple experiments by means of which the pressure of the atmosphere can be shown. For example, if a glass cylinder, open at the bottom, and covered at the top with a tightly stretched piece of bladder be placed on the plate of an air-pump, as shown in Figure, and a vacuum be created inside the cylinder by drawing out the air by means of an air-pump, as the pressure on the lower surface of the bladder is relieved the pressure on the upper surface manifests itself, so that, in a few moments, the bladder is burst with a loud report.

Otto Guericke contrived a piece of apparatus consisting of two hemispheres of metal as in Figure, provided with well ground flat ends so made as to be accurately fitted together. These ends were smeared with lard and then placed one on top of the other, the lower hemisphere being connected with an air-pump. As the air is removed from inside the hemispheres, the pressure of the air on the outside forces or presses them together with such force that, unless the hemispheres are small, it is impossible to pull them apart until a stopcock is opened so as to permit the entrance of air. These hemispheres are named after the city in which Guericke lived, the Magdeburg Hemispheres. It is said that the first piece of apparatus of this kind Guericke made and exhibited in public, was of such a size that when the air was exhausted from the inside, eight strong horses, four at each end, were unable to draw the hemispheres apart.

Since it is the pressure of the air that causes the mercury to rise in Torricelli's tube, it is not difficult to discover the exact value of the atmospheric pressure. Suppose, for example, we employ a tube the area of the opening of which is exactly one square inch, and that with such a tube, properly filled with mercury, the pressure of the air at a certain time and place is sufficient to cause the mercury to rise thirty inches in the tube. If, now, these thirty inches of mercury be weighed it will be found that they weigh approximately fifteen pounds. Therefore, the atmosphere at that particular part of the earth is pressing on every square inch of surface with a force of approximately fifteen pounds.

The atmospheric pressure is much greater than you might suppose ; for it exerts on the body of a man of ordinary size a total pressure of more than fifteen tons. This pressure is not felt, however, because it is exerted equally in all directions, and the opposite pressures exactly balance each other.

The total weight of the atmosphere is equal to that of a huge globe of copper sixty-two miles in diameter; or, if it were possible to make a pair of balances large enough to hold all the atmosphere on one scale pan, about 138,000 cubes of solid copper, each one mile high, one mile wide, and one mile broad, would be required to balance it.

“The wonder book of the atmosphere” by Edwin J. Houston, Ph.D., author of "The Wonder Book of Volcanoes and Earthquakes"