b - S - n m. The thick thumb n of the hand m n is that which, when the hand lowers, comes to lower more than the hand, in such a way that it closes and impedes the exit for the flight of the air from the lowering of the pressed hand, so that in that position the air condenses and resists the rowing of the wing. And for this reason nature has made in that thick thumb a bone of such strength, to which are joined the strongest tendons and short feathers of greater strength than the feathers that are in the wings of birds, because upon it the bird leans, upon the already condensed air, with all the power of the wing and of its force, for it is that by which the bird moves forward. And this thumb performs for the wings the office that the claws perform for the cat when it climbs upon the trees. But when the wing procures new force by its returning up and forward, then the thick thumb of the wing sets itself in a straight line with the other fingers, and so with its cutting edge it splits the air and does the office of a rudder, which always slices the air for whatever motion, high or low, the bird wishes to climb. The second rudder is placed on the opposite side, beyond the centre of the gravity of the bird; and this is their tail, which, if it is struck by the wind from below, being beyond the said centre, comes to make the bird lower its forward part. And if that tail is struck from above, the bird raises its forward part. And if that tail twists somewhat and shows obliquely its underside toward the right wing, the forward part of the bird turns toward the right side. And if it turns that obliquity of the underside of the tail toward the left wing, it will turn its forward part toward the left side, and in each of the two ways the bird will bank. But if the obliquely placed tail is struck by the wind from above, the bird will turn, circling and climbing on that side where the upper face of the tail shows its obliquity.