Whether equal powers of percussors, made with equal lengths of motion in various times, will produce equal lengths of motion with their movables, or not.

The answer is given by the seventh [proposition] of the ninth [book], which says: 'Among movers of equal power and motion, joined with their movables, the one that most quickly separates its movable from itself will be the one that drives that movable farthest from itself.'

Here the adversary says that movers of equal power will not vary their velocities, and consequently, in equal motion, will not drive equal movables farther one than the other. The answer is that there are two kinds of mover, of which one is sentient and the other not. That which is sentient has life, and the other is without life. But that which has life moves its movables by means of the extension and contraction of the muscles that compose its limbs, which extension and contraction is done with more or less speed, with one and the same power and cause, of which the swifter is not the more powerful; nor is any other difference to be found, save that the greater or lesser speed condenses the air more or less, through which the arm of the mover moves. But the insentient mover, such as ballistae or mangonels and other similar machines that, with counterweights or the force of cords or of bent wood, drive from themselves...

The bird that, carried by its impetus, finds itself flying higher than the place where it wishes to alight, spreads and lowers its tail, in which then, striking the air by means of its motion, it bends its straight course and makes it curved and ends it above the place where it alights.

m - a b - c - f n d

The kite, which descends toward the east with great obliquity in a north wind, will have its motion bent by that wind toward the southeast, unless it lowers the right horn of its tail, inclining its motion somewhat toward the northeast. This is proved: let the bird be a b c d, which moves toward the east along the straight line n m, and the north wind strikes it crosswise along the line f n, and would bend it toward the southeast, were it not for the right horn of the tail, which lowers itself to receive the wind. This wind strikes it from the center of gravity backward over a longer span than it does from the center of gravity forward, and thus its straight motion is not bent.