i

to4 Os Pneumatics .

to.thedeast force impressed, its parts are easily moved among one another,it presses according to its perpendicular height, and its pressure is everyway equal.

That the air is a fluid, confisting of such particles as have no cohesionbetwixt them, but easily glide over one another, and yield to the flighteftImpression, appears from that ease and freedom with which animassbreathe in it, and.move through it without any difficulty or sensible xe-sistance.

But it differs from all other fluids in the three following particulars.i. It can be compressed into a less space than what it naturally possesseth,which no other fluid can. 2. It cannot be congealed or fixed, as otherfluids may. 3. It is of a different density in every part, upward fromthe earth’s surface, decreasing in its weight, bulk for bulk, the higher itrifesj and therefore muss also decrease in denssty. 4. It is of an elasticor springy nature, .and the force öf its spring is equal to its weight.

That air is a body, is evident from its excluding all other bodies outof xhe space it possesses: for, if a glass jar be plunged with its mouthdownward into a vessel of water, there will but very little water getin$o the jar, because the air of which it is füll keeps the water out.

As air is a body, it muss needs have gravity or weight: and that itis weighty, is demonstrated by experiment. For, let the air be takenout of a vessel by means of the air-pump, then, having weighed thevessel, let in the air again, and upon weighing it when re-filled withair, it will be found considerably heavier. Thus, abottle that holds awine quart, being emptied of air and weighed, is found to be aboutj 7 grains lighter than when the air is let into it again; which stiewsthat a quart of air weighs 17 grains. But a quart of water weighs14625 grains; this divided by 17, quotes 860 in round numbers;.which fhews, that water is 860 times as heavy as air near the surfaceof the earth.

As the air rises above the earth’s surface, it grows rarer, and confe-quently ilighter, bulk for bulk. For sin ce it is of an elastic or springynature, and its lowermost parts are pressed with the weight of all thatis above them, it is plain that the air must be more dense or compactat the earth’s surface, than at any height above it; and gradually rarerthe higher up. For, the density of the air is always as the force thatcompresseth it: and therefore, the air towards the upper parts of theatmofphere being lefs pressed than that which is near the earth, it willexpand ilfelf, and thereby become thinner than at the surface of theearth.

Dr.