DE VI PERCUSSIONIS. CAP. III.

ON THE FORCE OF PERCUSSION. CHAPTER III.

Projectiles, once they are separated from that which impelled them, are not propelled by the fluid medium in which they exist.

CHAPTER III.

Since it is very evident, as has been said, that certain bodies are moved by an external and separate mover, it is usually asked what the aforesaid moving cause accomplishes primarily, or what work it produces in nature. And because we see bodies that are carried, impelled, or suffer a blow from an external mover not only agitated by motion while they are joined to the very body that is moving them—from which they are led as if by hand—but also after the external mover has departed, the moved bodies themselves, left without a guide, execute the motion previously begun, it therefore seems that they have acquired not only instruction and discipline, but also the force and faculty to be able to carry out the motion thereafter. And because the subsequent motion is also an action, and one depending upon another cause rather than coming from nothing, hence it is commonly inferred that there remains in the projectile, after it has been abandoned by the impeller, a cause and motive force from which the subsequent motion can be performed and continued. And since a motive force and faculty is posited as a certain quality, if this had been impressed by the thrower into the moved body, then surely the motive quality—that is, the accident—would migrate from its own subject into a foreign one. Because this is commonly deemed absurd, they have invented a new impelling cause, which is nothing other than the fluid medium itself, such as water or air, which is a motor substituted by the thrower, insofar as, having seized the movable object and carrying it, it continues the former motion until, with the motive faculty of the air extinguished, the motion of the projectile finally dies out and ceases. And although this sort of opinion has been rejected by others with very strong arguments, and especially by Galileo Galileo Galilei, I have nevertheless thought it worth the effort to refute the falsity of the same opinion with new arguments devised by me.

The Peripatetic from the school of Aristotle pronouncement is usually defended in a twofold manner. For some believe that while a stone, for example, is seized and impelled by the hand, before the hand abandons it, it propels the surrounding air no less than the stone. But when the company and propulsion of the hand cease, the abandoned stone, having no motive force of its own, would fall perpendicularly to the surface of the earth as a heavy object. Yet, because it is surrounded on all sides by air previously agitated and set in motion, it happens that the stone is carried by the surrounding air as if by a rapid river, until the agitation of the air languishes and finally is extinguished along with the motion of the same carried stone. Others of the same Peripatetic school afterwards say that the air following the projectile with the greatest rapidity and force returns to fill the space abandoned by the projectile, lest a vacuum—which nature abhors—should follow, and thus, by continuously pressing the rear part of the stone most vehemently, it promotes it, just as boys expel fruit pits to a great distance by squeezing them from behind with their fingers.