Stories from Science
Book 2
by A. Sutcliffe & A. P. D. Sutcliffe with illustrations by Robert Hunt
Cambridge: At the University Press, 1962
The Romance of the Barometer
A pump consists essentially of a long pipe, one end of which dips into the water which has to be pumped, whilst the other end is connected to a barrel or cylinder. When the handle of the pump is moved up and down a partial vacuum is formed in the cylinder. Then, said the early scientists, because Nature had such a dislike of a vacuum she got rid of it immediately by 'sending' water up the pipe to fill the empty space.
A traditional story relates that in the year 1640 the Grand Duke of Tuscany decided to sink a well in the grounds of his ducal palace. The workmen had to dig much further down than was customary and did not reach water until they had got to a depth of about forty feet. A pump was erected with its pipe dipping into the water; and then the men tried it But to their amazement no water came out of the spout no matter how hard "worked the handle up and down. The men thought that there must be something wrong with the pump; but a careful inspection showed no fault.
This strange occurrence was reported to the Duke, and he, like his men, could not understand why the pump failed. In those days many rich men such as the Duke became the 'patrons' of well-known scientists; that is, they paid a salary to the scientist to enable him to continue his scientific work without having to
do much other work to earn a living. Many years before the failure of the pump Galileo had been appointed the Grand Duke's 'philosopher and mathematician extraordinary . The Duke therefore consulted him about the problem.
It was seen that the water rose to a height of 'eighteen palms (about thirty-three feet) up the pipe of the pump and no further. Galileo 'explained' this by stating that whilst Nature did not like a vacuum its horror ended when water had risen to a height of eighteen palms inside one. But he himself must not have been thoroughly satisfied with this explanation for, being then an old man, he asked one of his young and promising pupils called Torricelli to investigate the problem.
Torricelli, believing that a pump could not lift a heavy liquid as high as it did a light one, decided to use mercury in his investigations, for mercury is thirteen and a half times as heavy as the same volume of water. He therefore expected that the maximum height which mercury could be lifted by a pump would be thirty-three feet divided by thirteen and a half, that is, thirty inches. The big advantage of using mercury instead of water would be that he could then use a tube about a yard long, which would be a convenient size, instead of one at least thirty-three feet long.
He obtained a glass tube of this length which was sealed at one end. First he filled it completely with mercury; then he closed the open end by putting his thumb over it; and finally he turned it upside down and submerged it in a bowl of mercury so that the open end was below the surface. When he removed his thumb from the open end a column of mercury about thirty inches high was left standing in it; and at the top of the tube, where some of the mercury had been, was an empty space (which later became known as the Torricellian vacuum).
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| Torricelli with his tube |
Long before the time of this experiment Galileo had shown that air has weight, as have all substances. Hence Torricelli concluded that the weight of air acting on the surface of the mercury in the bowl resisted the escape of the mercury from the tube. When the weight of the mercury which was left in the tube was ' balanced' by the thrust of the air on the surface of the mercury in the bowl no more mercury could escape into the bowl.
Torricelli then gave a correct explanation of the failure of the pump. The air, he said, pressed on the surface of the water in the well with sufficient force to send it 13 1/2 times 30 inches, say 33 feet up the pipe of the pump, but it could not force it to go any higher.
The experiment had done more than solve the problem of the pump's failure; it had shown a method of measuring the pressure of the air. Soon Torricelli's tube inverted over mercury became known as a barometer; and even today we still state the pressure of air by saying that it will hold up a column of mercury which is so many inches in height.
About the year 1644 the fact that air exerts a pressure came to the knowledge of a young French scientist named Blaise Pascal when he was living in Rouen. He pondered over the statement which had been made that 'we live at the bottom of a sea of air which undoubtedly has weight'. If this were true, he reasoned,
then the less the height of the air above us, the less should be the weight of air pressing on us. Hence if a barometric tube (that is, Torricelli's apparatus) was carried to a great height, say up a tall tower, the length of the column of mercury in the tube should decrease.
then the less the height of the air above us, the less should be the weight of air pressing on us. Hence if a barometric tube (that is, Torricelli's apparatus) was carried to a great height, say up a tall tower, the length of the column of mercury in the tube should decrease.
He decided to see if this was indeed the case by carrying such a tube up a church tower. But although he noticed a slight difference in the height of the mercury, he realised that the tower was not tall enough to yield any conclusive results. He then thought of his native mountains. He came from a village called Clermont, about two hundred miles south of Paris. This village lies at the foot of the Puy de Dome, a mountain rising to a height of three thousand feet.
Pascal was then a sick man and had been ordered by his doctor not to take any vigorous exercise, so he persuaded his brother-in-law, M. Perrier, who lived at Clermont, to do the experiment for him
On 19 September 1648 the summit of the Puy de Dome could be seen appearing trough the clouds at about five o'clock in the morning. So Monsieur Parrier decided to make the experiment that day. He called together his friends and by eight in the morning five men, all distinguished in their respective professions, and all interested in science, were ready to make the ascent.
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| M. Perrier on the Puy |
M. Perrier had provided himself with two glass tubes about four feet long and sealed at one end, two bowls, and about sixteen pounds of mercury. At the foot of the mountain he did Torricelli's experiment with one of the glass tubes and some mercury; he found that the column of mercury in the tube measured 26.4 inches in height.
He repeated this experiment with the other tube and satisfied himself and his companions that the column of mercury was the same height in each tube.
The five men then set out for the summit of the Puy de Dome, leaving behind one of the inverted tubes in the care of a friend who had offered to take readings of the height of the column at intervals throughout the day.
The summit, about three thousand feet above the starting point, was at last reached, and, on doing Torricelli's experiment, the men found that the mercury stood at a height of 23.2 inches. The column was therefore 3.2 inches shorter than it was at the starting point, it is said that, although they had expected some difference, the reading differed so much from the one taken at the foot of the mountain that they could hardly believe their own eyes and decided to repeat the experiment in various ways and in
different places on the mountain top. Thus they repeated it inside a small chapel built on the summit, and again and again in the most exposed places they could find. They even waited until a fog descended on the mountain and again repeated it. But each time they found that at the summit the height of the column was 23.2 inches.
They then began the descent and when they had reached a spot almost half way down the mountain side they decided to repeat the experiment. They found that the height of the column was 25 inches. Upon reaching the starting point they checked the reading on the tube which had been left there and found that it still stood at 26.4 inches.
Next morning the priest of the oratory at the base of the mountain suggested that the experiment should be repeated at the bottom of the high tower of Notre Dame in Clermont and again at the top of the tower. This was done and the difference in the readings was 0.2 inches. The tower was about 120 feet high.
The results of the experiments were communicated to Pascal who at once repeated them, using a high tower in Paris. He obtained approximately the same results as his brother-in-law.
These experiments clearly demonstrated to Pascal that Galileo's theory that air has weight was correct and that we do live at the bottom of an ocean of air which is pressing on us. They also showed that Torricelli's tube could be used to measure the height or altitude of mountains as well as to record atmospheric pressure.
