The Christian Philosopher; or, Science and Religion
Note II, pp. 31–94.—Experimental Illustrations of the Pressure and Compressibility of the Atmosphere—The Diving-Bell, etc
Note II, pp. 31–94.—Experimental Illustrations of the Pressure and Compressibility of the Atmosphere—The Diving-Bell, etc
The pressure of the atmosphere is most strikingly illustrated by means of the air-pump. But as few persons, comparatively, possess this instrument, the following experiments, which any person may perform at pleasure, are sufficiently convincing on this point. Take a common wineglass, fill it with water; apply a piece of paper over the mouth of the glass; press the paper to the rim of the glass with the palm of the hand; turn the glass upside down; withdraw the hand from the paper; and the water will be supported by the pressure of the atmosphere. That it is the atmospherical pressure, and not the paper, which supports the water, is evident; for the paper, instead of being pressed down by the weight of the water, is pressed upward by the pressure of the atmosphere, and appears concave, or hollow in the middle. If the flame of a candle be applied to the paper, it may be held for an indefinite length of time, close to the paper, without setting fire to it. The same fact is proved by the following experiment:—Take a glass tube, of any length, and of a narrow bore; put one end of it in a basin of water; apply the mouth to the other end, and draw out the air by suction; the water will immediately rise toward the top of the tube; and if the finger or thumb be applied to the top of the tube, to prevent the admission of air, and the tube removed from the basin of water, the water in the tube will be supported by the pressure of the atmosphere on the lower end. Again:—Take a wineglass, and burn a small bit of paper in it; and while the paper is burning, press the palm of the hand upon the mouth of the glass, and it will adhere to the hand with considerable force. In this case the pressure of the atmosphere will be sensibly felt; for it will sometimes require a considerable force to detach the glass from the hand.
The following experiment will also illustrate the pressure of the atmosphere. Take a tin vessel about 6 or 7 inches long and 3 in diameter, with its mouth about a quarter of an inch wide, as E F in Fig. 1.* Pierce a number of small holes in its bottom, about the diameter of a common sewing needle. Plunge the vessel in water; and when full, cork it up, so that no air can enter at top. While it remains corked ne water will run out, being prevented by the atmospheric pressure, but the moment it is uncorked, the water will issue from the small holes in the bottom by the pressure of air from above.—The same experiment may be made by taking a tube, G H, Fig. 3,* 7 or 8 inches long, and about three-fourths of an inch diameter, having a small hole on each side, I K. When filled with water and corked no water will run out, but when the cork is removed the water will run out at I and K, illustrating the lateral pressure of the atmosphere.
The pressure of the atmosphere explains a variety of common phenomena. When we take a draught of water out of a basin or a running stream, we immerse our mouths in the water, and make a vacuum by drawing in the air; the pressure of the atmosphere upon the external surface of the water then forces it into the mouth. The same cause explains the process of a child sucking its mother's breasts—the action of a boy's sucker in lifting large stones—the rise of water in pumps—the effects produced by cements—the firm adhesion of snails and periwinkles to rocks and stones—the scarcity of water in the time of hard frosts—and the fact, that a cask will not run by the cock, unless a hole be opened in some other part of the cask.
The following experiment illustrates the compressibility of air, and at the same time the principle on which the Diving-Bell is constructed. Let A B, Fig. 1,† represent a large tumbler, nearly filled with water. Place a place of cork on the surface of the water, and over the cork an ale-glass, C D, with its mouth downward: then push the glass perpendicularly down toward the bottom of the tumbler, and the cork will appear swimming a little above the bottom—indicating that there is no water above it in the ale-glass, but only air, which prevents the entrance of the water. If the water in the tumbler be supposed to represent the water of a river or of the sea, the ale-glass will represent the diving-bell in which a person may sit with safety in the depths of the sea without being immersed in the water, provided fresh air be supplied. A small quantity of water will be found to have entered the ale-glass, and the deeper it is plunged in any vessel the higher will the water rise within it; which proves the compressibility of the air within the glass.
* The figure referred to is in “Mental Illumination,” p. 93 fig. 2.
* “Mental Illumination,” p. 93, fig. 3.
† Ibid., p. 93, fig. 1.