The motion p s will be made without power of its mover, because natural motion suffices for it; and the motion a s will be made entirely by the power of its mover. Among moving bodies of equal shape and weight that with equal impetus and distance strike objects of equal resistance, that one will rebound to a more distant site which is caused by a percussion of lesser power. And conversely it will rebound to a nearer place which parts from a percussion of greater power. If the greater percussion generates a smaller bounce than the lesser percussion, this happens because, the impetus being of greater power than the percussion, that is as regards the motion of heavy bodies, it is necessary that, the more the motion of the moving body partakes of the greater than of the lesser power, the bounce too be greater or smaller according to the causes that produce it. The bounce that in the figure written above will be of greater length and lesser height is that from which the moving body will depart from b and, once it has struck the object s, will rebound to d. Such a bounce is born of the power of the impetus and partakes of it by as many degrees as those by which b is remote from a, and partakes of the degrees of the percussion by as many degrees as those by which b is remote from p. Therefore we shall say that this b is remote from a by 1 degree and from p is remote by 13 degrees; so will the opposite angle of the rebound do likewise, which is d s c, that enters 13 times into the angle p s c, just as the angle a s b did, which entered 13 times into the angle b s p.