Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 1.djvu/73

Rh whoe centres they are; and therefore while thoe two centres retain their tate of motion or ret, the common centre of all does alo retain its tate: it is manifet that the common centre of all never uffers any change in the tate of its motion or ret from the actions of any two bodies between themelves. But in uch a ytem all the actions of the bodies among themelves either happen between two bodies, or are compoed of actions interchanged between ome two bodies; and therefore they do never produce any alteration in the common centre of all as to its tate of motion or ret. Wherefore ince that centre, when the bodies do not act mutually one upon another, either is at ret or moves uniformly forward in ome right line, it will, notwithtanding the mutual actions of the bodies among themelves, always perevere in its tate, either of ret, or of proceeding uniformly in a right line, unles it is forced out of this tate by the action of ome power impreed from without upon the whole ytem. And therefore the ame law takes place in a ytem coniting of many bodies as in one ingle body, with regard to their perevering in their tate of motion or of ret. For the progreive motion, whether of one ingle body, or of a whole ytem of bodies, is always to be etimated from the motion of the centre of gravity. Rh