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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 diving-bell has been much used of late in recovering valuable articles from the wrecks of ships that had sunk in deep water, and in blowing up such wrecks as are sunk near the mouths of rivers, and form impediments to navigation—by means of powder ignited by the electric spark. Major General Pasley has been employed for a considerable time past in blowing up the wreck of the Royal George man-of-war, which foundered at Spithead, near Portsmouth, in 1783. Many of the valuable articles belonging to this largest vessel of the British navy have thus been recovered. Five or six divers have been constantly employed, and no serious accidents have occurred. On the 20th June, 1842, 3065 cubic feet, or 61 loads of timber had been brought up from the wreck, beside 219 pigs of iron ballast, weighing 32 tons, 17 cwts. One of the best divers sent up 19 pigs or nearly 3 tons in one day. Most of the iron and brass cannons have thus been got up; and during the first 6 or 7 months of 1842 more than four thousand pounds

* The figure referred to is in “Mental Illumination,” p. 93 fig. 2.

* “Mental Illumination,” p. 93, fig. 3.

Ibid., p. 93, fig. 1.

page 150 of powder have been expended in these operations.—Dr. Payerne has lately invented a method for producing pure air, in such experiments, fit for the respiration of man, and for supporting flame without communication with the external air, which he has frequently successfully exhibited in London, in the diving-bell of the Polytechnic Institution; and on the 2d September, 1842, he put his invention to the test by descending at Spithead, along with Major Pasley, to the depth of 75 feet, when the water stood only 6 inches high at the bottom of the bell. The air they breathed in the bell was perfectly good; and the whole apparatus for purifying it was contained in a case not larger than a common portable writing-desk, which requires only the turning of a small winch occasionally, and no science on the part of the person in charge. When the diving-bell is once filled with compressed air, either by letting it escape from vessels previously filled with it, or by four men pumping for less than half-an-hour, no more pumping is necessary, as the air in the bell never requires to be changed. In the common mode of working the diving-bell, on the contrary, a powerful pump, manned by 6 or 8 men, would be required, as at Spithead, which must be kept constantly at work the whole time that the men are down in the diving-bell; and this incessant pumping is so laborious, that from 12 to 16 men, working in two reliefs, are necessary for the purpose of expelling the water.—Such inventions as that now stated may be applied to many practical and beneficial purposes, and perhaps to purposes more diversified and extensive than we can, in the meantime, anticipate.