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35. Proof that air exerts pressure. Since air has weight, it is to be inferred that air, like a liquid, exerts force against any surface immersed in it. The following experiments prove this.

Let a rubber membrane be stretched over a glass vessel, as in Fig. 27. As the air is exhausted from beneath the membrane the latter will be observed to be more and more depressed until it will finally burst under the pressure of the air above.

Again, partly fill a tin can with water and boil the water. The air will be expelled by the escaping steam. While the boiling is

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still going on, let the can be tightly corked, and then be placed in a sink or a tray and cold water poured over it. The steam will be condensed, and the weight of the air outside will crush the can (see Fig. 28).

36. Cause of the rise of liquids in exhausted tubes. If the lower end of a long tube is dipped into water and the air exhausted from the upper end, water will rise in the tube. We prove the truth of this statement every time we draw lemonade through a straw. The old Greeks and Romans explained such phenomena by saying that "nature abhors a vacuum," and this explanation was still in vogue in Galileo's time. But in 1640 the duke of Tuscany had a deep well dug near Florence, and found to his surprise that no water pump that could be obtained would raise the water higher than about 32 feet above the level in the well. When he applied to the aged Galileo (see page 80) for an explanation, the latter replied that evidently "nature's abhorrence of a vacuum

does not extend beyond 32 feet." It is quite probable that Galileo suspected that the pressure of the air was responsible for the phenomenon, for he had himself already proved that air has weight; and, furthermore, he at once devised another experiment to test, as he said, the "power of a vacuum." He died in 1642, before the experiment was performed, but he had suggested to his pupil Torricelli that he continue the investigation.

37. Torricelli's experiment. Torricelli argued that if water would rise 32 feet, then mercury, which is about 13 times as heavy as water, ought to rise but as high. To test this inference he performed in 1643 this famous experiment:

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Let a tube about 4 ft. long, sealed at one end, be completely filled with mercury, as in Fig. 29 (1), closed with the thumb, and inverted; then let the bottom be immersed in a dish of mercury, as in Fig. 29 (2). When the thumb is removed from the bottom of the tube, the mercury will fall away from the upper end of the tube, in spite of the fact that in so doing it will leave a vacuum above it; and its upper surface will, in fact, stand about 13 of 32 ft. (that is, between 29 and 30 in.) above the mercury in the dish.

FIG. 29. Torricelli's
experiment

Torricelli concluded from this experiment that the rise of liquids in exhausted tubes is due to an outside pressure exerted by the atmosphere on the surface of the liquid, and not to any mysterious sucking power created by the vacuum, as is popularly believed even today.

38. Further decisive tests. An unanswerable argument in favor of this conclusion will be furnished if the mercury in the tube falls as soon as the air is removed from above the surface of the mercury in the dish.

To test this point let the dish and the tube be placed on the table of an air pump, as in Fig. 30, the tube passing through a tightly fitting rubber stopper A in the bell jar. As soon as the pump is started, the mercury in the tube will, in fact, be seen to fall. As the pumping is continued, it will fall

nearer and nearer to the level in the dish, although it will not usually reach it, for the reason that an ordinary air pump is not capable of producing so low a pressure as that which exists in the top of the tube. As the air is allowed to return to the bell jar, the mercury will rise in the tube to its former level.

39. Amount of the atmospheric pressure. Torricelli's experiment shows exactly how great the atmospheric pressure is, since this pressure is able to balance a column of mercury of definite length. As the pressures along the same level ac (Fig. 31) are equal, the downward pressure exerted by the atmosphere on the surface of the mercury at

A

FIG. 30. Barometer falls when air pressure on the mercury surface is reduced

c is equal to the downward pressure of the column of mercury at a. But the downward pressure at this point within the tube is equal to hd, where d is the density of the mercury and h is the depth below the surface b. Since the average height of this column at sea level is found to be 76 centimeters (about 30 inches), and since the density of mercury is 13.6 grams per cubic centimeter, the downward pressure inside the tube at a is equal to 76 times 13.6 grams, or 1033.6 grams per square centimeter. Therefore the atmospheric pressure acting on the surface of the mercury at c is 1033.6 grams, or approximately 1 kilogram per square centimeter. In English units

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The pressure of one atmosphere is, then, about 15 pounds per square inch, or about 1 ton per square foot.

40. Pascal's experiment. Pascal thought of another way of testing whether or not it is indeed the weight of the outside air which sustains the column of mercury in an exhausted tube. He reasoned that since the pressure in a liquid diminishes on ascending toward the surface, atmospheric pressure ought also to diminish as one passes from sea level to a mountain top. As there was no mountain near Paris, he carried Torricelli's apparatus to the top of a high tower and found, indeed, a slight fall in the height of the column of mercury. He then wrote to his brother-in-law, Perrier, who lived near Puy-de-Dôme, a mountain in southern France, and requested him to perform the experiment on a larger scale. Perrier wrote back that he was "ravished with admiration and astonishment" when he observed that on ascending 1000 meters (or of a mile) the mercury sank about 8 centimeters (or about 3 inches) in the tube. This was in 1648, five years after Torricelli's discovery.

FIG. 32. A mercury barometer

At the present day geological parties ascertain differences in altitude by observing the change in the barometric pressure as they ascend or descend. A fall of 1 millimeter in the barometric height corresponds to an ascent of about 12 meters.

41. The barometer. The modern barometer (Fig. 32) is essentially nothing more nor less than Torricelli's tube. Taking a barometer reading consists simply in measuring accurately the height of the mercury column. This height varies

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This picture is on the cover of the book which describes the experiments of Otto von Guericke. In the presence of
Emperor Ferdinand III, sixteen horses are trying to separate the Magdeburg hemispheres, after the air between them has
been exhausted. These hemispheres are still preserved in the museum at Berlin. Their interior diameter is 22 inches

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