That which is gained in height in the bounce.

The power of the impetus endures throughout the whole rebound motion and ends at the beginning of the natural motion.

The elevation of the moving body in its rebound motion is observed to the same degree as the power of percussion accompanies that motion. The power of percussion does not accompany the motion of the moving body except at the summit of the highest point of its bounce. The power of percussion does not accompany the rebound motion except at the summit of its height. The moving body rises in its rebound motion to the same degree as the power of percussion accompanies it.

The moving body moves away from the place it struck as far as the power of the impetus that accompanies it endures. And it rises as much as the percussion with its power accompanies it. Therefore the whole length of the rebound motion arises from the simple power of the impetus, and all its length from the simple power of the impetus. Whence it is concluded that the power of the impetus will bear to the power of percussion the same proportion as the height of the rebound motion bears to its length. The moving body moves away from the struck place as long as the power of the impetus that moves it endures: the power of the impetus accompanies it, drives it off, conducts it. And it rises from the struck place as much as the percussion with its power pushes it and conducts it. Therefore the whole length of the rebound motion arises from the simple power of the impetus, and all its height from the simple power of the blow. Whence we shall say that the power of the impetus bears to the power of percussion the same proportion as the height of the rebound motion bears to its length.