Why the incident and rebounding motions are caused between equal angles.

The moving body driven by the blow gains, in every degree of time, degrees of height, and draws near to its first mover. The moving body driven by the impetus gains, in every degree of flight, degrees of distance from its mover. Therefore we shall say that what the moving body gains in height above the place of the percussion is caused by that percussion, and what that moving body gains in distance from the said place of percussion is caused by the impetus. Hence we shall say that a greater or lesser blow drives that moving body to a greater or lesser height, and likewise we may say that a greater or lesser impetus leaves that moving body at a greater or lesser distance from itself.

The moving body a, being moved along the line a b, is said to be moved by the simple impetus. The moving body d, being moved along the line d g, will be moved by the simple percussion, which is generated by the mover f descending down along the line f d. Therefore the power of the percussion lies in the line f d, and in the line a b is the power of the impetus; whence it can be said with certainty that if the moving body c runs to the place of the percussion d equally distant between the two said powers, it will be a motion partaking equally of the one power and the other: so that it will not rise except to a middle place, that is, partaking of the height of the blow, yet not having the lowness of the impetus.

Percussion is of two natures, namely when the object flees from the moving body that struck it, and when that moving body rebounds backward from the struck object. In the first, the struck object may be of equal weight and shape to the moving body that strikes it. Then, in such a percussion, the struck object leaves in its place the moving body that struck it, and itself continues the remainder of the motion that was left to the first moving body.