The principal motions of heavy bodies are three, namely violent, transversal and natural.

The mixed motions are two, namely violent-transversal and transversal-natural.

Transversal is called that which falls at a distance from the mover, and it divides into two, namely natural and violent.

The actions of these motions are two, namely incident motion and rebounding motion; incident is that motion which separates the moving body from the mover and leads it to the place of its percussion; and rebounding is that motion which divides the moving body from the percussion and raises it to its greatest height.

Percussion is the impediment of motion, that is, the end of incident motion and the beginning of rebounding motion.

The qualities of the motions are two, namely augmentative quality and diminutive quality: augmentative is when the heavy body moves toward the centre of the world, so that in every degree of descent it gains heaviness and speed of motion; diminutive motion is when such a heavy body is removed from the centre of the world, so that in every part of the motion it gains degrees of lightness and slowness.

The differences among the lines that can be traversed by the impetus or motion of heavy bodies are three, namely the direct line, the line of equality, and the oblique line. Direct is that which extends from a given place and is directed to the centre of the world; equidistant, or equal, is that which stands with its extremities equally remote from the centre of the world; oblique is that which is enclosed between the equidistant and the direct line. And these can be infinite, because such a space is divisible to infinity, and they vary only in greater or lesser obliquity. The more oblique is that which comes nearer to the equidistant line, the less oblique that which draws nearer to the direct line.

The motions of heavy bodies are of three natures, namely direct, oblique, curved.

There are two ways in which the moving body strikes upon its object: the one is when the object is motionless, and the other when that object moves against the moving body.

There are two varieties of the injuries received in the contact of two bodies that strike each other, namely either the injury of the object is greater, or that of the moving body. It will be the moving body's when it is of a lighter quality than the object is, and conversely.

There are two modes of percussion, namely when the percussion consumes itself, and the other when it rebounds.

Motion is born of excess or deficiency.

The course of the moving body against its object can be varied by that moving body in two ways, namely 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 moving body makes toward its object, there is added the rotating motion of itself. The straight rotating motion can be varied in two ways, namely direct rotating and lateral rotating. Direct rotating is when the upper part of the moving body turns toward the object and its lower part turns back toward its mover. Lateral rotating is when the object turns about the central line of its motion. The ways in which the mover drives its moving body from itself are these: the first is when it is moved by a simple impetus; the second by an impetus compounded with other impulses; the third when the mover follows its moving body with equal motion; the fourth when that moving body is driven off by the percussion.

There are three modes 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, namely straight or revolving. The revolving is varied in three principal ways, namely by the direct, or the equidistant, or the oblique. The mover of the revolving is of two sorts, namely counterweight, spring or blow. The counterweight is varied in two ways in revolving movers, namely augmentative or diminutive: augmentative is when the counterweight that moves draws away from the centre of the revolving, and the diminutive is that of the counterweight of the bricole (swing-beam engine).

There are two natures of percussions, namely simple and compound. Simple is that which the moving body makes in incident motion upon its object; compound is said when that first percussion passes into the support of the first struck object, as would be the blow given upon the sculptor's chisel and then passed to the marble that is being carved. Again such a blow divides into two others, namely simple blow and doubled blow. Of the simple enough has been said; but the doubled is when the hammer descends with impetus by natural motion, and flees rebounding from the upper blow, and falls to the making of the lower blow, and this percussion is made in two places by the two opposite parts of the hammer; and such a blow goes on diminishing by as many as are the number of the objects interposed between the blow and the last resistance, as if we should beat a book on its first leaves, so that although all touch one another, the last feels little injury.

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