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18

ENCYCLOPÆDIA PERTHENSIS.

.

P N E U MATIC s.

The gene

(Concluded from Volume Seventeenth.) NOTHER eafy method is this: Let an appa- Various experiments for this purpose have been rafus abcdef, pl

. CCLXXXI. fig.48.) be made, made. Those made by M. de Luc, General Roy, consisting of a horizontal tube a e of even bore, a Mr Trembley, and Sir George Shuckbourgh, are ball dge of a large diameter, and a swan-neck tube by far the most accurate ; but they are all conbf. Let the ball and part of the tube geb be filled fined to very moderate rarefactions. with mercury, so that the tube may be in the same ral result has been, that the elasticity of rarefied horizontal plane with the surface de of the mer. air is very nearly proportional to its density. No cury in the ball. Then leal up the end a, and con- regular deviation from this law has been observed, Dect f with an air-pump. When the air is abstract. there being as many observations on one side as on ed from the surface de, the air in ab will expand the other; but it is certainly worthy the atteninto a larger bulk ac, and the mercury in the tion of philosophers to determine it with precision pimp-gage will rise to some distance below the in the cases of extreme rarefaction, where the irbaronetric height. This distance, without far- regularities are most remarkable. The great source ther calculation, will be the measure of the elas. of error is a certain adhesive Nuggishness of the ticity of the air presling on the surface de, and mercury when the impelling forces are very finall; therefore of the air in a c.

and other Auids can hardly be used; because The most exact method is to suspend in the re- they either smear the inside of the tube and diceiver of an air-pumpa glass vessel, having a very minith its capacity, or they are converted into narrow mouth over a cistern of mercury, and then vapour, which alters the law of elasticity. abftract the air till the gage rises to some deter- Upon the whole, we may assume the Boylean mined height. The difference e between this law, viz. that the elasticity of the air is proporand the barometric height determines the elafti- tional to its density. The law deviates not in any city of the air in the receiver and in the fufpend. fensible degree from the truth in those cases which ed veffel. Now lower down the veffel by the nip- are of the greatest practical importance, that is, wire till its mouth is immersed into the mercury, when the denfity does not so much exceed or fall and admit the air into the receiver; it will press fort of that of ordinary air. the mercury into the little vefsel. Lower it ftill With respect to the action of the particles on farther down, till the mercury within it is level each other, the investigation is extremely easy. with that without; then stop its mouth, take it We have seen that a force 8 times greater than out and weigh the mercury, and let its weight be the pressure of the atmosphere will compress com4. Subtract this weight from the weight v of mon air into the 8th part of its common bulk, the mercury, which would completely fill the and give it 8 times its common denfity: and in this wole vesel; then the natural buik of the air will case the particles are at half their former distance, beg-w, while its bulk, when the elasticity e in and the number which are now acting on the sure ide rarefied receiver, was the bulk or capacity w face of the piston employed to compress them is of the vessel. Its density therefore corresponding quadruple of the number which act on it when it

is of the common density. Therefore, when this to this elasticity e, was

And thus may the eightfold compressing force is distributed over a ration between the dentity and elasticity in all fourtold number of particles, the portion of it Tiss be obtained.