that floating bodies in water prove the weight of the water As the water must be heavier than the

which supports them. wood that floats upon it, so the air must be heavier than the smoke and vapours, and other substances that rise in it.

Besides this sort of reasoning, we have the most positive proofs on the subject, deduced from facts very familiar to the observation and apprehension of every person in common life. If we close up the valve-part and nozzle of a pair of bellows, after having brought the boards together and forced all the air out, it will be found that a great force, equal to some hundred pounds, will be required to separate them, because they are kept together by the pressure of the external air which surrounds them.

If the air, by means of an air-pump, be taken from a glass flask, made with a valve for the purpose, and if the flask be accurately weighed, it will be found that it weighs, if it be a quart vessel, 15 or 16 grains less when it is exhausted, than when it is, as all vessels usually are, full of air, though in common language we say such vessels are empty. Hence it is found that a cubical foot of air weighs about an ounce and a quarter. By this means we obtain the specific gravity of air, compared with water, the usual standard; for if a cubical foot of water weigh 1000 ounces avoirdupoise, and the same quantity of air weigh 14 ounce; the former divided by the latter gives the superior weight of water above that of air, thus 1000 ounces divided by 14, or in decimals by 1.25, and we gain as the quotient 800, shewing that water is about 800 times heavier than air at the temperature of 60°.

If a glass tube, 32 inches or more in length, be closed at one end, then filled with mercury, and inverted and plunged into a vessel of the same fluid substance, the mercury will remain suspended in the tube, descending to some point between 28 and 31 inches, according as the atmosphere be less or more dense; now it is manifest, that the suspension of the mercury is occasioned by the pressure of the external air upon the surface of the mercury; since if this pressure be taken away, by

placing the tube and the vessel under the receiver, and exhausting the air, by means of an air-pump, the mercury will sink in the tube, and upon re-admitting the air, it will instantly rise as high as it was before, which is an experimental demonstration of the weight, gravity, and pressure of the atmospheric air. This is called the Torricellian experiment, from the person who invented it; and upon this depend the structure and use of the common barometer, because upon the weight of the air, which is perpetually varying, the mercury rises and falls, indicating the probability of certain changes in the atmosphere.

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The same thing may be shewn in another way: instead of using a tube closed at one end, that is, hermetically sealed, as it is called, let it be open at both ends, but the upper part be accurately closed with a piece of bladder or parchment, the mercury will be suspended as before, and remain at the usual height; but if the covering be pierced with a needle, so as to admit the air, the mercury will immediately fall; for in this case the weight of the air presses upon the mercury in the tube, and there being now as much pressure on the mercury in the tube, as on that in the vessel outside the tube, there can be no equilibrium, until the fluid find its level.

Having established the gravity of the air, by which it is similar to all other terrestrial substances, we shall now point out the circumstances in which it differs from them, and from other fluids in particular; these are as follow.

1. The pressure of the atmosphere altitudes above the surface of the earth.

varies at different As all the parts of

the air gravitate, or press upon each other, it is easy to conceive, that those parts next the surface of the earth are more compressed, and of course denser than what those are at some height above it; in the same manner as if fleeces of wool were thrown upon one another to a great height, the fleeces towards the bottom, having all the weight of what is above them, would be squeezed into a less compass than those at, or near the top. Such is the case with the atmosphere, that

surrounds the earth. On the top of high hills, or lofty mountains, the air is found to be of considerable less density than that at or near the level of the sea. The precise altitude of the atmosphere has never been ascertained: it may extend to an immense distance, becoming rarer, in proportion to its distance from the earth. But as it is known that it does not refract the rays of the sun at a greater height than about forty-five miles, this therefore is usually considered as the limit of the atmosphere. It has been demonstrated by Mr. Cotes, that if the altitudes in the air be taken in arithmetical proportion, its rarity will be in geometrical proportion: thus at the altitude of

7 miles, the air is 4 times rarer than at the surface of the earth: 14 16 64

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So that at forty-nine miles above the surface of the earth, the air is more than sixteen thousand times rarer than at the surface. But at the surface, a quart of air weighs but about fifteen or sixteen grains, of course at the height of forty-nine miles, it can weigh but the sixteen thousandth part of a grain.

2. Air is elastic, or capable of compression and expansion. This is proved by the following experiments, which will be better understood, when the structure and use of the air-pump have been explained. 1. By the great expansion of a small quantity of air in a bladder nearly empty, when the air is removed from the external parts under the receiver of an air-pump. 2. By the extrusion of a fluid from a glass bubble, by the expansion of a quantity of air contained in it. 3. By the expulsion of the white and yolk of an egg, through a small hole in the little

end, by the expansion of the air contained in the greater end. 4. By putting an almost emptied bladder into a small box, and laying a proper weight on the lid, which, on exhausting the air, will be raised up by the expansion of the air in the bladder. 5. Also a bladder filled with air, and just made to sink with a weight, will, upon exhaustion, soon rise by the expansion of the contained air. 6. Glass bubbles, and images filled with water, so as to make them just sink in that fluid, will, on exhausting the air from the surface, rise to the top of the vessel. 7. Beer, cyder, water, and porous bodies, emit great quantities of air under the exhausted receiver. 8. A shrivelled apple, when put under an exhausted receiver, will have its coat distended by the internal air, so as to look smooth. 9. If the open end of a tube, whose other end is closed, be immersed perpendicularly in water, the space occupied by the air will be diminished, as the depth of the tube, or the upward pressure of the water is increased: or, if the shorter leg of a bent tube be closed, and mercury poured into the longer, the air will be compressed in the shorter leg into a space continually decreasing as the quantity of pressing mercury is increased; and if some of the mercury be taken from the longer leg, the air in the shorter will expand and occupy a proportionably larger space. 10. The mercury may be raised by the expansion of a small quantity of confined air to the same height in an exhausted tube above the air-pump, as that to which it is raised in the mercurial gauge, by the pressure of the atmosphere below it.

The limits of the condensation and rarefaction of the air have not been ascertained. Dr. Hales contrived to condense a portion of air into the fifteen hundredth part of its usual bulk, which is, perhaps, the greatest degree of condensation that has been ascertained with experimental precision; but it has often been carried to the thousandth degree, and in this case, air is as heavy as, or heavier than, water, though it still maintained its æriform shape, so that it cannot be, as some

have supposed only water in a different form, because being as dense or denser than water, it still retains its expansive powers.

3. The elastic force of the air is equal to the force of compression. For if the air be exhausted from an open tube, whose lower part is immersed in a vessel of mercury, which is subject to the pressure of air that cannot escape; then will this air, pressing upon the surface of the mercury, force it nearly to the same height, as it would have been raised by the pressure of the atmosphere. Besides, if the force with which the air endeavours to expand itself, when it is compressed, were less than the compressing force, it would yield still farther to that force; if it were greater, it would not have yielded so far: of course when any force has so compressed the air, that it remains at rest, the force of the air arising from its elasticity, can neither be greater nor less than the compressing force, but must be equal to it.

4. Heat increases the elasticity of the air, and cold diminishes it; or, which is the same thing, heat expands, cold condenses the air. This property is usually demonstrated by the following experiments: (1) Tie a bladder very close, having in it a small quantity of air, but so that the bladder is flaccid: lay it in this state before the fire, and as it becomes warm, the bladder is gradually distended, till at length it will burst. But if the bladder be removed into a cold place it will contract, and become as flaccid as ever.

(2). If a glass, with air in it, be inverted in water, and then heated, the air in the upper part will expand, till it fill the glass, and expel the water out of it; and part of the air itself will follow, if the heat be continued and increased.

Galileo, as we have seen, was the first who discovered that it was impossible to raise water higher than about thirty-four feet, by what was at that time thought to be suction only; and thence he concluded, that it was not suction, but the pressure of the atmosphere, which was the cause of the ascent of water in pumps. By the same pressure mercury is suspended in a