... struck from above, the power of the wind that strikes it from above is not of its full strength, since the wedge of wind that divides from the middle of the wing-bone (omero) downward lifts the wing upward with almost the same power as that with which the upper wind sends the wing downward.

Wing-profile figure, with:

e f - a b - c d

It is proved. Let the wing-bone (omero) of the wing be f b d, and e f c d is the whole sum of the wind that strikes that wing-bone, of which wind the middle is a b c d, which strikes from the crest of the wing-bone b as far as d. And because the line of that wing-bone b d is oblique, this wind a b c d forms a wedge at the contact b d and raises it upward; and the upper wind a b e f, which strikes the obliquity b f, forms a wedge and pushes the wing downward. Whence these two aforesaid contrary forces do not allow the wing-bone to enter at once below or above the oncoming wind of the bird as its need requires. Whence that need has been provided for by placing a rudder (timone) over that rounded wing-bone, which acts as a shield for it and immediately cuts the wind in the manner that the bird's need requires, as is shown at m n.

Figure of a wing, with:

a n

But if the wind strikes the bird on the right or left wing, then it is necessary that it enter below or above that wind with the tip of the wing struck by that wind, which change takes up only as much space as the thickness of the tips of such wings. This change, being to leeward, the bird turns its beak to the wind; and if it is to windward, the bird will turn its tail to that wind. And here arises the danger of the bird turning its body upside down, if nature had not provided by placing the weight of such a bird's body lower than the position of the wings' extension, as will be demonstrated here.

Figure of the extremity of a wing, with:

Rudder (Timone) - n m