There are three principal motions of heavy bodies, that is: violent, transverse, and natural. There are two mixed motions, that is violent-transverse and transverse-natural. Transverse is called that which falls away from the mover, and it divides into two, that is natural and violent. There are two actions of these motions, that is incident motion and reflected motion; incident is the motion that parts the moving body from the mover and leads it to the site of its percussion; and reflected is the motion that divides the moving body from the percussion and raises it to its highest height. Percussion is the impediment of motion, that is the end of incident motion and the beginning of reflected motion. There are two qualities of motions, that is augmentative and diminutive: augmentative is when the heavy body moves toward the center of the world, for in every degree of descent it gains heaviness and speed of motion; diminutive motion is when the heavy body is removed from the center of the world, for in every part of motion it gains degrees of lightness and slowness. There are three differences among the lines that can be traversed by the impetus or motion of heavy bodies, that is: the direct line, the line of equality, and the oblique line. Direct is that which extends from a given site and directs itself to the center of the world; equidistant, or equal, is that which keeps its ends equally remote from the center of the world; oblique is that which is enclosed between the equidistant and the direct line. And these can be infinite, because such space is divisible to infinity; and they vary only in greater or lesser obliquity. More oblique is that which more nearly approaches the equidistant line, less oblique is that which more nearly approaches the direct line. The motions of heavy bodies are of three natures, that is direct, oblique, and curved.

There are two ways in which the moving body strikes its object: one is when the object is immovable, the other when the object moves against the moving body. There are two varieties of the injuries received in the contact of two bodies that strike each other, that is: either the injury of the object is greater, or that of the moving body. It will be of the moving body when this is of lighter quality than the object; and so conversely. There are two ways of the percussion, that is when the percussion consumes itself, and the other when it rebounds.

Motion is born from excess or deficiency. The course of the moving body against its object can be varied by the body in two ways, that is simple motion and compound motion. Simple is that motion in which the object never draws its axis out of its first state; compound motion is when, besides the straight motion the body makes toward its object, there is added to it the rotating motion of itself. The straight rotating motion can be varied in two ways, that is direct-rotating and lateral-rotating. Direct-rotating is when the upper part of the body turns toward the object and its lower part turns toward its mover. Lateral-rotating is when the object turns about the central line of its motion. There are ways in which the mover drives the body from itself: the first is when it is moved by a simple impetus; the second by an impetus compounded with other impetuses; the third when the mover follows its body with equal motion; the fourth when the body is driven off by the percussion. There are three ways of the sites that receive the percussion, of which one is apt to resist and the other to yield. There are two kinds of movers, that is straight or turning. The turning kind varies in three principal ways, that is by the direct, the equidistant, or the oblique. The mover of the turning is of two sorts, that is counterweight, spring, or blow. The counterweight varies in two ways in turning bodies, that is augmentative or diminutive: augmentative is when the moving counterweight recedes from the center of the turning, and diminutive is that of the counterweight of the mangonels. There are two natures of percussions, that is simple and compound. Simple is that which the body makes in incident motion upon its object; compound is when that first percussion passes into the support of the first struck object, as would be the blow given on the sculptor's chisel and then passed to the marble being carved. Again such blow divides into two others, that is simple blow and doubled blow. Of the simple enough has been said; the doubled is when the hammer descends with impetus by natural motion, flees bouncing from the upper blow, and falls to the making of the lower blow, so that this percussion is made at two sites by the two opposite parts of the hammer; and such a blow goes on diminishing in proportion to the number of objects interposed between the blow and the last resistance, as if we should beat a book on its first leaves, for although all touch, the last feels little injury.

The rebound born from above downward is more powerful than that which is born from below upward.