Section XI.
Of the motions of bodies tending to each other with centripetal forces.
I have hitherto been treatin of the attractions of bodies towards an immoveable centre; tho' very probably there is no ſuch thing existent in nature. For attractions are made towards bodies; and the actions of the bodies attracted and attracting, are always reciprocal and equal by law 3. ſ that if there are two bodies neither the attracted nor the attracting body is truly at reſt, but both (by cor. 4. of the laws of motion) being as it were mutually attracted, revolve about a common centre of gravity. And if there be more bodies, which are either attracted by one ſingle one which is attracted by them again, or which, all of them, attract each other mutually; theſe bodies will be ſo moved among themſelves, as that their common centre of gravity will either be at reſt, or move uniformly forward in a right line. I ſhall therefore at preſent go on to treat of the motion of bodies mutually attracting each other; conſidering the centripetal forces as attractions; though perhaps in a phyſical ſtrictneſs they may more truly be called
