Impacts and moving bodies, with equal distances and objects, can create percussions varying to infinity. That rebound is shorter which is born of a more powerful blow. The lesser blow produces a longer rebound. That percussion which is of greater power generates a rebound of lesser length. The greater the percussion, the smaller the rebound; and the smaller the percussion, the longer its rebound.

The height of the rebound is understood to be that which the moving body makes from the line of equality of the struck site upwards. The length of the reflected motion is that which is born at the site of the percussion and ends at the beginning of the natural motion. The central line of the weight is that which passes from the center of its gravity to the center of the world. The central line of the motion of heavy bodies is that which passes through the center of the thickness of such motion and ends at the center of the magnitude of such body. The central line of the blow is that which is born from the point of percussion of the moving body, passing through the middle of the weights of which the upper is equal to the lower. The center of gravity of bodies will always be concentric with the central line of the thickness of its motion and of its magnitude, even though such body be irregular.

Motions are of three natures, that is: the augmentative, the natural and observant one (that of animals), and the diminutive, that is the violent. The natural transverse motion ends at the beginning of the natural motion.

The weight m weighs more than the weight n which strikes at r, and therefore this m keeps n so burdened that it cannot rise up in its reflected motion.

That weight loads the pole of the pulley least whose sustaining cord bends over such pulley with an obtuse angle of greater size. Let then a b be the cord which, being straight, is the limit of the ultimate obtuse angle, and by what is said above this cord can give no weight to the aforesaid pulley; a g c gives less than a g d. Now in the right angle all obtuse angles are ended, and in acute angles no more weight is gained at the pole than double the weight suspended from that cord.

Always the cord, straight or bent, is loaded with twice the weight sustained by it. The weight m is wholly in n, which at the site n weighs half of its heaviness, as is shown in the upper figure of the quadrant. Therefore the weight sustained from m to n is one pound, and the weight of the resistance b, which is another pound, comes back as a half at n, whence the pole c feels one pound and a half exactly.

The weight acts through the whole length of the cord that sustains it, and wholly in every part of it, not passing the limit of more than a single bend. Therefore the heavy body c is thus wholly by power in b and in a, as at its own site c. But even though it be wholly in b, the pole of the pulley n feels only half on account of the attachment a, and on account of the real weight c it feels one whole. Therefore the pole n feels such weight one time and a half, and a half it feels ...

Here it is a manifest thing that the pole feels twice the weight attached to it, and feels it along the central line of the world. This one does not feel it twice along the central line of the world, but along the line of the half obliquity h g.