Flying things.

south, east, north - west

The line along which the birds are drawn is the central line of their motions.

When the bird is driven by the wind and on a more oblique course than the course of that wind, the bird will be so much the faster than the said wind.

When the bird, driven by the wind, has its motion parallel to the motion of the wind, it has a speed equal to that of the wind.

Of birds driven by the wind without beating their wings, the bird is faster than the wind by as much as the bird's course is more oblique than the wind's course. It is proved thus: let the wind move in the site of equality one degree of space in one degree of time, and let the bird, driven by the same wind, move the same degree in the same time.

These motions up to here are equal; but by means of the obliquity of the bird's motion, we suppose that it acquires a second degree of motion in the same time in which the wind acquired its first degree. Therefore it follows that the bird can be, in the same time, twice as fast as the wind that drives it.

a b c - e - d

But this oblique motion does not acquire its distance in the site of equality, but between that site of equality and the center of the world. As, if the site of equality were the line a c, along which the wind moves from a to c, and the bird moves with the favor of the wind and with the favor of its own weight from a to d: I say that in the same time in which the bird would have moved (without the favor of its weight) from a to b, it will have moved, with the favor of the wind and of its weight, from a to d; so that the one motion to the other is in sesquialteral proportion, from a to d over the motion a b. But as regards the removal of the aspects, a d is similar to a b, as the perpendicular b d teaches us, etc.