A B shall be equal in length to the height at which it stands in the barometer at the time. The air in the shorter leg will now be compressed by the weight of the atmosphere, and also with an addi. tional equal weight of a column of mercury. The mercury now in the shorter leg will be risen to E, and D E is only the half of DC: that is, the pressure of a double atmosphere compresses the air to half the space which it naturally occupies. If another column of mercury were added to the length A B, the air in D C would be reduced into one fourth the space that it formerly occupied.

As all the parts of the atmosphere press upon each other, the air near the surface of the earth is denser than that which is at some height above it. The height to which the atmosphere extends has never been exactly ascertained; but at a greater height than 45 miles it will not refract the rays of light from the sun. We shall conclude with an account of some experiments on the air pump.

The resistance of air to falling bodies is shewn by exhausting a tall glass receiver of its air, and in that state the lightest body, as a feather, will fall to the bottom as soon as the heaviest metal, fig. 4.

If a cup of porous wood containing mercury be placed on the receiver of an air-pump, and the air from below be exhausted, the external pressure of the atmosphere will force the mercury through the wood in a sort of shower.

Let the air be taken from the pores of a piece of dry wood, which is then to be held under mercury while the external air is admitted into the receiver, the mercury will be forced into all the pores, as may be seen by splitting the wood.

If two brass hemispheres of three or four inches in diameter, made for the purpose, be put together, and the external air exhausted, the pressure from without will require 150 lbs. to separate them: but if the external air be taken away they will separate of themselves, fig. 5.

This is a square phial with a small valve at top. I will put it under the glass receiver K of the airpump, and exhaust the air out of the receiver, and the air will escape from the phial at the same time. The air now being suddenly admitted into the receiver, will dash the phial to pieces, because the valve prevents it from getting into the phial again, fig. 6.

The elasticity or spring of air is shewn by tying up a very small quantity of it in a bladder, and putting it under the receiver K: the receiver is now to be exhausted of air, and the little confined in the bladder will by its elasticity completely fill the bladder.

This square phial is full of air, and the cork accurately cemented in, so that no air can escape. I will put it under the receiver K and exhaust the air from the receiver; the air within not being balanced by any without will burst the bottle in pieces.

Put a shrivelled apple under the receiver, and exhaust the air; then the air within the apple will make it as plump and handsome as when it was first gathered; but by the admission of the air it will return to its shrivelled state.

If a fresh egg with the small end cut off be put in a glass under the receiver, and the air taken away, the small bubble of air contained in the great end VOL. IV.

K

of the egg will expand, and force the contents of the egg from the shell.

Some beer made warm and put under the receiver will appear to boil when the air is nearly exhausted from the glass.

The smoke of a candle will ascend in the air, but in an exhausted receiver it will fall to the bottom, which shews that it generally ascends because it is lighter than air.

The sound of a small bell may be heard while it is under a receiver full of air; but as soon as the air is exhausted, there will be no more sound. Hence air is necessary to the propagation of sound. Ani, mals will not live, nor candles burn, for a single instant, in an exhausted receiver.

A, fig. 7, is a strong copper vessel, having a tube that screws into the neck of it, so as to be air-tight, and long enough as nearly to reach the bottom; r is the handle of a stop-cock. Having poured some water into the vessel, and screwed in the tube, the condensing syringe is to be adapted and the air condensed. The stop-cock is to be shut, while the syringe is unscrewed, then on opening the cock, the air by its great density acting upon the water in the vessel, will force it out into a jet of a considerable height. This is called the artificial fountain.

The sound of a bell is much louder in condensed than in common air.

A square phial that would bear the pressure of the common atmosphere, when the air is exhausted from the inside, will be broken by condensing the air around it.

Fig. 8, is the representation of the section of an