Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 1.djvu/64
20 Book I.
at once, or gradually and ſucceſſively. And this motion (being always directed the ſame way with the generating force), if the body moved before, is added to or ſubtracted from the former motion, according as they directly conſpire with or are directly contrary to each other; or obliquely joined, when they are oblique, ſo as to produce a new motion compounded from the determination of both.
To every action there is always oppoſed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.
Whatever draws or preſſes another is as much drawn or preſſed by that other. If you preſs a ſtone with your finger, the finger is alſo preſſed by the ſtone. If a horſe draws a ſtone tied to a rope, the horſe (if I may ſo ſay) will be equally drawn back towards the ſtone: for the ſtretched rope, endeavoring to relax or unbend itſelf, will draw the horſe as much towards the ſtone as it does the ſtone towards the horſe. It will obſtruct the progreſs of the one as much as it advances that of the other. If a body impinge upon another and by its force change the motion of the other, that body alſo (becauſe of the equality of. the mutual preſſure) will undergo an equal change in its own motion towards the contrary part. The changes made by theſe actions are equal, not in the velocities but in the motions of bodies; that is to ſay, if the bodies are not hindered by any other impediments. For, becauſe the motions