For the differences of the motions tending towards the ſame parts, and the ſums of thoſe that tend towards contrary parts, are, at firſt (by ſuppoſition), in both caſes the ſame; and it is from thoſe ſums and differences that the colliſions and impulſes do ariſe with which the bodies mutually impinge one upon another. Wherefore (by Law 2.) the effects of thoſe colliſions will be equal in both caſes; and therefore the mutual motions of the bodies among themſelves in the one caſe will remain equal to the mutual motions of the bodies among themſelves in the other. A clear proof of which we have from the experiment of a ſhip; where all motions happen after the ſame manner, whether the ſhip is at reſt, or is carried uniformly forwards in a right line.
