Birds.

second

first

b — simple obliquity — a

d — compound obliquity — c

f — decomposed obliquity — c

The descent of birds is of two natures, of which one is with certainty upon a determined place, the other is a descent that is ambiguous over two or more places. But the first carries its wings open with the tips raised higher than its back, and with equal height of these it descends with a straight and simple obliquity to its destined place.

But the second bird descends with the tips of its wings held lower, with its breast and tail bending now to the right and now to the left, sometimes with simple obliquity and sometimes with compound, and at times with decomposed [obliquity].

a d — c b — f e

The bird that is struck on its flank by the wind moves that wing which is toward the wind so much more, and with a swifter motion, than the other wing, as the wind is of more impetuous motion. This is proved: let the bird be a b c, which moves along the line a f and has the wind d e, which strikes it on the flank on wing a b, and it would carry it off the line of its course were the wing a b not of swifter motion than the wind. For which reason the wind that struck the side of the bird, and from that with a whirling rebound turned toward the wing, which closed again in its beat, so that wing had a second support against the said bending of the bird's straight motion. It is therefore concluded that the great and swift motion which the wing struck by the wind makes more [than the other], together with the wind that struck the bird and rebounded against the said wing, is what does not let the straight course of birds be bent by the wind. And still, if the opposite wing were not slow and of little motion, it would strike into the course of the wind, and the wind against it; and thus the wind would be most powerful to accompany such a bird along its course, etc.