Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 2.djvu/440

392 courſe of whom from the appearances of things, does certainly belong to Natural Philoſophy.

Hitherto we have explain'd the phænomena of the heavens and of our ſea, by the power of Gravity, but have not yet aſſign'd the cauſe of this power. This is certain, that it muſt proceed from a cauſe that penetrates to the very centers of the Sun and Planets, without ſuffering the leaſt diminution of its force; that operates, not according to the quantity of the ſurfaces of the particles upon which it acts, (as mechanical cauſes uſe to do,) but according to the quantity of the ſolid matter which they contain, and propagates its virtue on all ſides, to immenſe diſtances, decreaſing always in the duplicate proportion of the diſtances. Gravitation towards the Sun, is made up out of the gravitations towards the ſeveral particles of which the body of the Sun is compos'd; and in receding from the Sun, decreaſes accurately in the duplicate proportion of the diſtances, as far as the orb of Saturn, as evidently appears from the quieſcence of the aphelions of the Planets; nay, and even to the remoteſt aphelions of the Comets, if thoſe aphelions are alſo quieſcent. But hitherto I have not been able to diſcover the cauſe of thoſe properties of gravity from phænomena, and I frame no hypotheſes. For whatever is not deduc'd from the phænomena, is to be called an hypotheſis; and hypotheſes, whether metaphyſical or phyſical, whether of occult qualities or mechanical, have no place in experimental philoſophy. In this philoſophy particular propoſitions are inferr'd from the phænomena, and afterwards render'd general by induction. Thus it was that the impenetrability, the mobility, and the impulſive force of bodies, and the laws of motion and of gravitation, were diſcovered. And to us it is enough, that gravity does really exiſt, and act according to the laws which we have explained, and abundantly ſerves to account for all the motions of the celeſtial bodies, and of our ſea.