monly ariſes either from the attrition of contiguous bodies as they ſlide by one another, or from the coheſion of continuous bodies that are to be ſeparated, or from the weights of bodies to be raiſed, the exceſs of the force remaining, after all thoſe reſiſtances are overcome, will produce acceleration of motion proportional thereto, as well in the parts of the machine as in the reſiſting body. But to treat of Mechanics is not my preſent buſineſs. I was only willing to ſhow by thoſe examples the great extent and certainty of the third Law of motion. For if we eſtimate the action of the agent from its force and velocity conjunctly; and likewiſe the reaction of the impediment conjunctly from the velocities of its ſeveral parts, and from the forces of reſiſtance ariſing from the attrition, coheſion, weight, and acceleration of thoſe parts; the action and re-action in the uſe of all ſorts of Machines will be found always equal to one another. And ſo far as the action is propagated by the intervening inſtruments, and at laſt impreſſed upon the reſiſting body, the ultimate determination of the action will be always contrary to the determination of the re-action.
