Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 1.djvu/81

Rh

In attractions, I briefly demontrate the thing after this manner. Suppoe an obtacle is interpoed to hinder the progres of any two bodies A, B, mutually attracting one the other: then if either body as A, is more attracted towards the other body B, than that other body B is towards the firt body A, the obtacle will be more trongly urged by the preure of the body A than by the preure of the body B, and therefore will not remain in æquilibrio: but the tronger preure will prevail, and will make the ytem of the two bodies, together with the obtacle, to move directly towards the parts of which B lies; and in free paces, to go forward in infinitum with a motion perpetually accelerated; which is aburd and contrary to the firt Law. For, by the firt Law, the ytem ought to perevere in its tate of ret, or of moving uniformly forward in a right line; and therefore the bodies mut equally pres the obtacle, and be equally attracted one by the other. I made the experiment on the loadtone and iron. If thee plac'd apart in proper veels, are made to float by one another in tanding water, neither of them will propel the other; but, by being equally attracted, they will utain each other's preure, and ret at lat in an equilibrium.

So the gravitation betwixt the earth and its parts is mutual. Let the earth FI (Fig. 5) be cut by any plane EG into two parts EGF and EGI, and their weights one towards the other will be mutually equal. For if by another plane HK, parallel to the former EG, the greater part EGI is cut into two parts EGKH and HKI, whereof HKI is equal to the part EFG, firt cut off: it is evident that the middle part EGKH, will have no propenion by its proper weight towards either ide, but will hang as it were, and ret in an equilibrium betwixt both. But the one extreme part HKI Rh