On motion and weight. In equal motions made in equal time, the mover is always of greater power than its moved body. And the mover will be more powerful than the moved body by as much as the motion of that moved body exceeds the length of the motion of its mover. But the difference by which the power of the mover exceeds the power of the moved body will be smaller the less the length of the motion made by the moved body exceeds the motion of that mover. But understand, reader, that in this case one must reckon with the air, which condenses before the moving body the more, the greater the speed of that body; because that air is condensable toward infinity. This cannot happen in the motions made by bodies within water, which is incompressible, as is proved by putting it in a vessel with a narrow mouth, into which — by no device of any mover — will more be put than the natural capacity of that vessel requires. And the contrary is understood of air, which, forced into vessels of very narrow mouth in which there is a little water, and tilting such a vessel so that the enclosed water is set between the perforation of the vessel and the condensed air, then the power of that condensed air drives the water from the vessel with such fury that it penetrates far into the air, until the air remaining in the vessel returns to its first natural rarity. But to return to our subject we say that, among bodies of equal weight, that one is of slower motion which occupies more air with the front that divides the air; and conversely, in occupying less air it does the opposite — not, however, extending into such thinness that weight fails it, because where there is no weight there is no local motion in the air.

No local motion can occur in the air except from a greater or lesser density than the density of that air. And if the adversary should say that the same density which the condensed air acquires before the moving body does likewise before the mover — and all the more before the mover, the more it is in contact with a greater quantity of air struck and condensed before it than its moved body is, as we see in the hand when it throws a stone through the air — here it is answered that it is impossible for the motion of the mover joined to the motion of the moved body to be more or less swift than the motion made by the moved body, nor is there ever in any part of accidental motion a speed equal to the speed of the motion of that mover. And this is proved in accidental motion, where the moving body acquires degrees of slowness in every degree of motion, although the percussion of the moving body is somewhat greater when farther from the mover than when near.

And this we see in the arrow of the bow rested with its point against the wood, which — although the string drives it with all the power of the bow — penetrates very little into that wood, whereas it does the contrary when it has some motion. Some say that in moving, the arrow prepares before itself a wave of air, which wave by means of its motion does not impede the course of that arrow. But this is false, because every moving body wearies and impedes its mover. Therefore the air billowed before the arrow is billowed by means of the motion of that arrow and gives little or no help of motion to its mover, which has to be moved by the same mover; rather it impedes and shortens the motion of the moving body. The impetus generated in still water produces a different effect from that generated in still air. This is proved, because water never condenses within itself by any motion made beneath its surface, as air does when struck within itself by the moving body. But the small particles well teach us that within the water, from the surface to the bottom, it is stirred; these whirl about when the water fills the void that the fish, penetrating it, leaves behind itself. The movements of which water strike and push that fish, because only that water within the water has weight which has motion. And this is the primary cause of increasing the motion to its mover.