ordinary pressure of the atmosphere: and air so rarefied would be a counterbalance to the denser air of the atmosphere,

6. Air more condensed is rendered more elastic, by an equal degree of heat, than air which is more rarefied; and this in proportion to its density. If air witli one degree of density is rendered twice as elastic by 300 degrees of heat; air with two degrees of density will become twice as elastic with 150 degrees of heat; so that the heat required to produce a given elasticity, will be reciprocally as the density. Here I wish for a series of experiments to prove the effects of different degrees of heat on air of different densities; but I cannot recollect where to find them; and I have neither leisure, nor apparatus, to make them for myself.

From this article, it may be conceived, that if air is extremely condensed at great depths in the lower parts of the earth, as it must be by its situation, the application of heat may have such prodigious effects as to produce convulsions in the body of the earth, and even a disruption of the parts, extending to the surface. How far quiescent air, when raised by fire from the bodies wherein it is lodged, may be subject to the same laws

with condensed air, is worthy of inquiry; and the subject comprehends the rationale of the expansive force of gunpowder, of which we have spoken in another place.

1

By considering air as a mixture of air and fire, we may see the reason cf some very obscure phænomena, which have little or no relation to its elasticity. One of these I shall mention here, and reserve others for a fitter place. A blast of cold air, continued for some time, will communicate heat to the ball of a thermometer, and raise it several degrees; as we have already seen in the Discourse on Fire. The air impelled by the blast against the surface of the glass, is stopped there; but the fire that goes with the blast enters the surface, (as the finer grains of any pulverised matter will pass through a sieve,) and produces an agitation within the pores of the included fluid. After the same manner, the air which enters the lungs in respiration may deposit some of its fire at every successive impression, and the fund of animal heat may be kept up in part on this principle*; though I apprehend several

*This idea has lately been pursued by an ingenious writer, in a pamphlet on Animal Heat, &c. which the reader

veral other circumstances must be taken in, before we can give a satisfactory account of it: but that matter is not now before us.

The elasticity of the air, as a force or spring of motion, is applied mechanically in a great variety of philosophical instruments, a particular account of which would carry me out too far, and is not within the limits of my present design. The chief of these is the air-pump; from which the air is exhausted on the principle of its own elasticity. Condensed air, by its pressure on the surface of the water contained in the same vessel with it, gives motion to the stream of an artificial fountain. With great condensation, it will give motion to leaden bullets in an air-gun, much after the manner of gunpow= der, and nearly with the same force. Many other applications of the air's elasticity are described in these books of philosophy, which profess to deliver such things. The treatises which a beginner will find most useful, are Clare's Motion of Fluids, and Regnault's Philosophical Conversations, translated by Dale, vol. i,

On

reader may consult, See Crawford's Experiments and Ob servations on Animal Heat, and the Inflammation of Combustible Bodies.

411

On the Weight and Moisture of the
Atmosphere.

By the word atmosphere, we mean that heterogeneous mixture of air, vapour, and terrestrial exhalations, which surrounds the sphere of the earth, and is extended to the regions of the sky. This fluid mass is found to gravitate, or to have weight upon the earth on which it rests; and its parts are also found to gravitate upon one another, so that the inferior are more compressed than the superior. If a glass vessel, open at the top, is covered closely with the hand, and the air is exhausted from the vessel, so that the counterbalance is taken away from below, the incumbent air is immediately perceived to press on the back of the hand with a vast weight, so as to force a considerable part of the palm into the orifice of the glass. This occasions a sensation as if it were a suction from beneath; but the whole force is actually that of a weight on the upper side. The whole surface of the body is at all times subject to this pressure, which amounts to about 14 pounds averdupois upon every square inch; but force being opposed

to

to force on all sides, we are not sensible of the effect, till the pressure is removed from some part, and the balance destroyed. Air is compressed by the weight of incumbent water, as by the weight of incumbent air; and hence the air contained in the bell used by divers is compressed into half its space when the bell is about 30 feet below the surface of the sea; at the depth of 60 feet, or 10 fathoms, it is compressed by thrice the weight of the atmosphere; at 100 feet it is reduced to less than one quarter of its natural space: and the effect of air so compressed is very hurtful to the blood-vessels and the auditory membrane, so as to endanger life on some occasions. This shews us that the natural weight of the air at the earth's surface is duly tempered, with consideration of the welfare of man's body; which would suffer in some respects, if the weight of the air were much greater or less than it now is. The wisdom of making this provision is ascribed to the Creator, in the book of Job, chap. xxviii. 24, 25: He looketh to the ends of the earth, and seeth under the whole heaven; to make the weight for the winds, and he weigheth the waters by measure.

It was known very anciently, that leathern bottles,