150

Sir Isaac Newton’s BookI.

known, nor yet into how narrow a compafs it is capableof being comprelTed. Mr. Boyle found it by experimentcapable both of expanlion and compreflion to fuch a degree,that he could caufe a quantity of air to expand it felf over afpace fome hundred thoufand times greater, than the fpace towhich he could coniine the fame quantity a . But I fhalltreat more fully of this fpring in the air hereafter b . I amnow only to conlider what refillance to the motion of bodiesarifcs from it.

il. But before our author Ihews in what manner thiscaufe of reflftance operates, he propofes a method, by whichfluids may be rendered elaftic, demonftrating that if theirparticles be provided with a power of repelling each other,which fliall exert it felf with degrees of ftrength recipro-cally proportional to the diftances between the centers ofthe particles; that then filch fluids will obfervo the famerule in being comprefied, as our air does, which is this, thatthe fpace, into which it yields upon compreflion, is recipro-cally proportional to the comprefling weight c . The termreciprocally proportional has been explained above d . And ifthe centrifugal force of the particles acted by other laws, fuchfluids would yield in a different manner to compreflion e .

13. Whether the particles of the air be endued withfuch a power, by which they can ad' upon each other outof contad, our author does not determine; but leaves that

a See his Trait on the admirable, rarifaition ofthe air.

b Book II. Cb.

c Princ.philof. Lib. II. prop. at.

'f Book I. Ch. 2. § 30« Princ. philof. Lib. II. prop. 23. in fchol.

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