Fig. 114.
Fig. 115.
167. Pressure of the Air on Liquids.—If you plunge a tumbler into a vessel of water, and turning it over hold it so that its open part is just under the surface, it will remain full. The reason is that the weight of the air pressing upon the surface of the water in the vessel prevents the water in the tumbler from passing downward. Now if you introduce a bent tube under the tumbler, as in Fig. 113, and blow through it, the air that you force up into the tumbler presses the water down, taking its place. That is, the pressure of the air acts in opposition to the pressure of the air outside upon the surface of the water in the vessel. You take a jar, a, Fig. 114, and filling it with water, turn it over with its open end downward, the water will remain in the jar. You have here a representation of the pneumatic trough used by the chemist in collecting gases. To fill the jar a with gas he puts the mouth of the retort from which the gas issues under the jar a, and the gas passing upward expels the water, as the water is expelled by the breath from the tumbler in Fig. 113. In Fig. 115 (p. 125) is represented an experiment which shows not only that the pressure of the air sustains the column of water in the cases cited above, but also that it makes no difference in what direction this pressure is exerted. Take a large tube, a, closed at one end and open at the other, and fill it even full with water. Place, now, a piece of writing-paper over its mouth, and carefully invert the tube, as seen in the figure. The paper will remain, and the water will not run out. It is the pressure of the air that sustains the water, and the paper only serves to maintain the surface of the water unbroken. If the paper were not there the particles of the air would insinuate themselves among those of the water, and pass upward in the tube. You can try this experiment with a wine-glass, and may even succeed with a tumbler. We see in these experiments the reason that a liquid will not run from a barrel when it is tapped, if there be no vent-hole above, unless there be so large an opening made as to let the air work its way in bubbles among portions of the liquid. It is this entrance of the air that causes the gurgling sound in pouring a liquid from a bottle.
168. Amount of Atmospheric Pressure.—If, instead of the jar a, in Fig. 114, you have a tube thirty-four feet high, and closed at the top, situated as the jar a is, it will remain full of water. If the tube be longer the water will stand only at thirty-four feet, leaving a vacuum above it. It makes no difference what the size of the tube is; the result will be the same in all cases.[2] That is, a column of water thirty-four feet high can be sustained by the pressure of the atmosphere. It is easy, therefore, to estimate the weight or pressure of the air. The pressure of the column of water is found to be fifteen pounds to the square inch of its base, and this, of course, is the amount of pressure or weight of the atmosphere which it balances. Mercury is thirteen and a half times as heavy as water, and therefore the air will sustain a column of it only about thirty inches in height.
Fig 116.
169. Barometer.—The weight of the atmosphere varies to some extent at different times, and the barometer is an instrument for measuring these variations. It is constructed on the principles developed in the previous paragraphs. In Fig. 116 is a representation of the instrument. A B is a glass tube about 34 or 35 inches long, closed at one end. It has been filled with mercury, and then inverted in a cup of the same liquid, C. The vacuum above the mercury is called the Torricellian vacuum, from Torricelli, an Italian, who first developed the principles of the instrument. The mercury generally, as stated in § 168, stands at about the height of thirty inches. But it varies from this with the weather. When the weather is bright and clear the air is heavier than this, and, pressing upon the mercury in the vessel, forces it up higher in the tube. But when a storm is coming the air is apt to be lighter, and therefore pressing less strongly on the mercury in the vessel, the mercury in the tube falls. The barometer is of great service, especially at sea, in affording the sailor warning of an approaching storm. An incident is related by Dr. Arnot which strikingly illustrates its value in this respect. He was at sea in a Southern latitude. As the sun set after a beautiful afternoon the captain foresaw danger, although the weather was perfectly calm, for the mercury in the barometer had suddenly fallen to a remarkable degree. He gave hurried orders to the wondering sailors to prepare the ship for a storm. Scarcely had the preparations been made when a tremendous hurricane burst upon the ship, tearing the furled sails to tatters, and disabling the masts and yards. If the barometer had not been observed the ship would have been wholly unprepared, and shipwreck, with the loss of all on board, would have been the result.
A water-barometer could be made, but it would be an unwieldy thing, for the tube must be over 34 feet long. Besides, it would not answer in very cold weather, as the water would freeze. So short a column of the heavy fluid, mercury, balances the weight of the atmosphere that a barometer made with this is of very convenient size; and then there is no danger of the mercury's freezing, except in the extreme cold of the Arctic regions.
170. Barometer a Measurer of Heights.—The atmosphere, as stated in § 152, diminishes regularly in density as we go upward. The rate of this diminution has been accurately ascertained, and therefore we can estimate heights by the amount of pressure on the mercury in the barometer. At a height of 500 feet the barometer will be half an inch lower than in the valley below. At the summit of Mont Blanc it stands but half as high as at its foot, indicating a height of 15,000 feet. Du Luc, in his famous balloon ascension from Paris, saw the barometer at one time standing at about twelve inches, showing an elevation of 21,000 feet.