Of Pneumatics.

clastic or springy nature, and the force of itsspring is equal to its weight.

That air is a body, is evident from its ex-cluding all other bodies out of the space it pos-sesses : for, if a glass jar be plunged with itsnaouth downward into a vessel of water, there'will but very little water get into the jar, becausethe air of which it is full keeps the water out.

As air is a body, it must needs have gravity0r weight: and that it is weighty, is demon-setated by experiment. For, let the air beta ken out of a vessel by means of the air-pump,then, having weighed the vessel, let in the aira gain, and upon weighing it when re-filled witha ir, it will be found considerably heavier. Thus,a bottle that holds a wine quart, being emptiedof air and weighed, is found to be about 1 6grains lighter than when the air is let into itagain , which shews that a quart of air weighs16 grains. But a quart of water weighs 14621grains; this divided by 16, quotes 914 in roundnumbers; which thews, that water is 914 timesas heavy as air near the surface of the earth.

As the air rises above the earth’s surface, it.gtows rarer, and consequently lighter, bulk forbulk. For, because it is of an elastic or springynature, and its lowermost parts are pressed withme weight of all that is above them, it is plainthat the air must be more dense or compact atthe earth’s surface, than at any height above it;,and gradually rarer the higher up. For, thedensity 0 f the air is always as the force thatcompresses st. an d therefore, the air towards,the u PP er parts of the atmosphere being lesspre lied than that which is near the earth, it willexpand itself, and thereby become thinner than,at the earth’s surface.

M 4 Vr.

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