Peru, "the clouds, which were gathered below the mountain's brow, appeared like a tempestuous ocean, all dashing and foaming, while the lightnings were breaking through the waves, and the thunders rolling beneath his feet, far below the spot on which he stood. In the mean time he enjoyed a serene and cloudless sky, and left the war of the elements to the unphilosophical mortals on the plains below."

Those who have taken a flight to the higher regions of the atmosphere in balloons, have beheld scenes of a similar kind. Mr. Baldwin, who ascended in a balloon from Chester, in 1785, relates that, at a certain elevation, the earth was entirely hid from his view by an immense mass of vapours, which he compares to a sea of cotton, tufted here and there by the action of the air, and, soon after, the whole became an extended pavement of white cloud. The reason of all this is obvious: the clouds are vapour, or water rarefied by heat. Vapour is lighter than air near the surface of the earth, and, consequently, ascends in it; but, in the higher regions, the air is thinner and lighter than these vapours, and, of course, is unable to support them beyond a limited height; which circumstance undeniably proves that the air is lighter the higher we ascend.

The pressure of the atmosphere may now be illustrated by a few simple experiments. The instrument called the air-pump affords, on the whole, the best means of illustrating the pressure of the atmosphere. This instrument

bears a certain resemblance, in its principle and action, to the common house-pump. It consists of a hollow cylinder or tube, in which a piston is alternately raised and depressed by means of an iron rod attached to the handle of the pump. In the piston there is a valve, which opens by any pressure from below, and is shut by any superincumbent pressure, like the flapper of a pair of bellows. When the piston is forced down, the valve is opened from below by the pressure of air, or whatever fluid the pump may contain. The fluid then gets above the valve, and is lifted up by the raising of the piston, and carried out of the pump, the valve being then shut by the pressure of the air above it. At the top, is a metal plate ground to a perfect plane surface, on which is placed an inverted glass jar or receiver, whence the air is to be extracted. A hole in the plate is connected with a tube which communicates with the pump-barrels. By working the handle of the instrument for some time, the receiver will soon be nearly exhausted of all the air it contains, and the effects produced in a vacuum, or place void of air, may then be exhibited. The following, among other experiments, may be shown by the air-pump. If the receiver be open at both ends, and the upper orifice be stopped by the hand, when the air is exhausted, the pressure of the external air will be such as to prevent the removal of the hand, and will cause a certain degree of pain. If a piece of bladder be tied tightly over

the orifice, as the exhaustion proceeds, the bladder will be pressed inwards, and will finally burst with a loud noise. In the same manner, if a flat piece of window-glass be placed upon the orifice, when the air is exhausted, the glass will be broken into a number of pieces by the external pressure of the atmosphere.

But as comparatively few persons have an opportunity of performing experiments with the air-pump, a few simple experiments, equally convincing, which every person has it in his power to perform, may here be described.

1. Take a wine or an ale-glass, and fill it with water; take a smooth piece of writing paper, and press it firmly against the edges of the glass, so that no air get in between the paper and the water, then turn the glass upside down, and the water will be supported by the pressure of the atmosphere upon the paper. That it is the external pressure of the atmosphere upon the paper which supports the water will appear, when we consider that the paper, instead of being convex, by the pressure of the water downwards, is concave, by the pressure of the air upwards. If a lighted candle be placed under the paper, with its flame touching the paper, we may hold it for an indefinite length of time, without its producing any effect upon the paper, or setting fire to it.

2. Take a wine-glass, and burn a piece of paper in it, and, while the paper is burning, if we place the palm of our hand firmly upon the edges of the glass, the glass will stick fast

B

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, 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

Fig 1

Fig 2

•

H

K

G

above.

from

4. Take a tin

vessel, six or eight

inches long, and

about three in diameter, with its mouth

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 IK. 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, over 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.