Pagina-afbeeldingen
PDF
ePub

to the hand, producing a certain degree of pain, and it will require a considerable degree of force before the hand can be detached from the glass. In this experiment, the burning of the paper rarefies the air, and nearly expels it from the glass, and then the atmosphere presses with its whole weight upon the hand.

3. Take a glass tube, two or three feet long, of a narrow bore; plunge one end of it in a bason of water ; apply the mouth to the other end, and draw out the air by suction; the water will instantly rise into the tube by the pressure of the atmosphere on the water in the bason; and, if we immediately place our thumb firmly on the upper part of the tube, and withdraw it from the water in the bason, the water will be suspended in the tube by the pressure of the atmosphere, although the tube

is open below; but, Fig 1

when the thumb is removed from the upper part of the tube, the water in it will run out, in consequence of the pressure

of the atmosphere from above.

4. Take a tin vessel, six or eight

inches long, and about three in diameter, with its mouth

Fig2

[ocr errors]

G

K к

II

about a quarter of an inch wide, as E F (fig. 1.) Pierce a number of small holes in its bottom, about the diameter of a common sewing-needle. Plunge this vessel in water, and, when full, cork it up, so that no air can enter at the top. While it remains corked, no water will run out, being prevented by the atmospheric pressure upon the bottom of the vessel; but the moment it is uncorked, the water will issue from the small holes by the pressure of the air from above. The same experiment may be made with a tin-plate tube, about an inch in diameter, open at the top, and having its bottom pierced with a small hole. When filled with water and tightly corked at the top, it may be carried for miles without losing a drop of water, notwithstanding the hole in the bottom.

5. In order to show the lateral pressure of the atmosphere, take a tube, as G H, (fig. 2,) six or seven inches long, having a small hole on each side, as I k. When filled with water, and tightly corked, no water will run out from the sides of the tube, but the moment the cork is taken out, the water will run out at i and K, as represented in the figure.

6. Take a wine-glass and burn in it a piece of paper ; then invert the glass, while the paper is burning, ver a saucer full of water, the water will rush up into the wine-glass, in consequence of the air being rarefied or driven out by the burning paper, and in consequence of the pressure of the atmosphere upon the surface of the water in the saucer.

a

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

in the construction of steamengines, atmospheric railways, and other modern inventions, which are now of such great utility in propelling carriages along railways, and stcam-vessels across seas and oceans.

power

CHAPTER III. 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

« VorigeDoorgaan »