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These experiments show that the atmosphere presses in all directions, upwards, downwards, and laterally. This subject has been dwelt on somewhat particularly, because the atmospheric pressure forms an important element, and a mechanical power in the construction of steamengines, atmospheric railways, and other modern inventions, which are now of such great utility in propelling carriages along railways, and steam-vessels across seas and oceans.
Facts illustrated by the pressure of the atmosphere.
Let Us now attend to a few facts which the pressure of the air tends to explain and illustrate.
1. The atmospheric pressure explains the nature of the process vulgarly termed suction. When we attempt to take a draught of water out of a bason, or a running stream, it is commonly said that we draw in the water by suction; whereas the fact is, that instead of drawing the water into the stomach, we only draw the air into the lungs, and the atmosphere performs the other part of the operation. The process is simply this:—We immerse our lips into the water, so as to prevent the entrance of air into the mouth; we then make a vacuum in the mouth by drawing the air into the lungs, after which the pressure of the atmosphere upon the surface of the water forces it upwards into the mouth. That such is the process of receiving a draught of water when the mouth is held downwards, appears from this circumstance, that if the lips do not touch the water, we might draw in the air by what is called suction for twenty years, and not receive a single drop into the mouth.
The same principle explains the action of a child sucking the breast of its nurse. ' The operation of cupping is performed in the same way. In this case, the operator takes a small glass, close at the top, and holding it for some time over the flame of a candle, or lamp, the air is thereby rarefied, and part of it drawn out. The glass is then suddenly placed on the part of the body to be cupped, and adheres to the flesh by the external pressure of the air.The flesh rises in the glass, and the blood and serosities are forced from the wounded vessels into the glass by the atmospheric pressure on the parts around.
2. It is owing to. the atmospheric pressure that two polished surfaces, which accurately fit each other, adhere with great force. This fact is well known to glass-grinders and polishers of marble. A large lens, when ground very smooth, requires more than the strength of a single individual to pull it directly from the tool. If the surface is only a square inch, it will require fifteen pounds to separate them perpendicularly, though a very moderate force will make them slip along each other. Were the surface six inches square, the force requisite to separate the two pieces would be equal to five hundred and forty pounds. But this cohesion is not observed, unless the surfaces are wetted or smeared with oil or grease, otherwise the air gets between them, and they separate without any trouble. That this cohesion is owing to the atmospheric pressure, is evident from the ease with which the plates may be separated in an exhausted receiver by means of the air-pump. The same cause contributes in a powerful degree to give effect to the cohesion of bodies by means of mortar and cements. When two pieces of wood are to be glued together, their surfaces are first made as smooth as possible; a glutinous substance is then applied to fill up all the pores and inequalities; they are then pressed together, which prevents the air from insinuating itself between them, and the external air then presses upon them with a ■ force of fifteen pounds on every square inch. There can be no question that the stability ot our houses and garden walls depends, at least in a great measure, upon the same principles; for the more completely every crevice between the bricks or stones is shut up, by means of mortars and cements, from the insinuation of the external air, the more firm and stable is the building.
To the same cause is to be attributed the action of a boy's sucker in lifting large stones from the ground. The sucker is made of stiff wetted leather fastened to a string; the moisture upon the leather, when it is pressed down upon the stone, prevents the air from getting in between the leather and the stone, and if the sucker be four inches square, it will require a force of two hundred and forty pounds to separate it from the stone. In certain cases, such contrivances, on a large scale, might be sometimes useful as a mechanical power.
3. Another circumstance which is accounted for on this principle is, the strong adhesion of snails, periwinkles, limpets, and other molluscous animals, to the rocks on which they are found. The animal forms the rim of its shell so as to fit the shape of the rock on which it intends to cling. It then fills its shell either with its own body or with water. In this condition, it is evident, that we must act with a force equal to fifteen pounds on a square-inch before we can detach it from the rock. This may be illustrated by filling a drinking-glass to the brim with water, and, having covered it with a piece of thin wet leather, place it upon a table, and it will be found to require a very considerable force to pull it straight upward. But, if we place a snail adhering to a stone in an exhausted receiver, it will drop off by its own weight. It is owing to the same cause that bivalve mollusca, such as oysters and mussels, keep their shells so firmly shut, and require such a degree of force to open them. But, if we grind off a bit of the convex shell, so as to make a small hole in it, the air gets in, and it opens with the greatest ease. The same thing takes place when it is put under the exhausted receiver of an air-pump. It has been lately discovered that it is owing to the same principle that flies and other animals