c – f e – d – b

The adversary says he could not deny that the bird cannot fall backward, nor head-downward, along a perpendicular line, but it seems to him that it could descend edgewise, holding its wings entirely open and putting one of the wings, together with the head, beneath the centre of its gravity: that is, it will descend along the line a–b, the wings remaining entirely extended in their natural extension. To this it is answered the same as to the last point, namely that if this bird, in such a disposition and without otherwise helping itself, had to descend perpendicularly, it would be contrary to the fourth [proposition] of the second [book] of the Elements, where it was proved that every body which freely descends through the air will descend in such a way that its heavier part becomes the guide of the motion; and here the heavier part is found in the middle of the extremities of the open wings, that is, in the middle of the 2 lighter parts — which such a descent, as is proved, is impossible. Therefore we have proved that, the bird being with wings open and head somewhat raised, it is impossible for it ever to fall or descend along a perpendicular line, but it will always descend along an oblique line, which line any small movement of wings or tail converts, together with the oblique descent, into reflex motion.
Nature has granted that all large birds stay at such a height that the wind which augments their flight is of straight course and powerful; because their low flying among the mountains — where the wind whirls about and is always full of eddies and vertiginous motions, and where, through the fury of the condensed wind driven through the gorges of the mountains, the bird could not at its pleasure shield and govern itself with its great wings so as to avoid the collisions of the slopes and high rocks and trees — would not sometimes fail to be a cause of their destruction. Whence, at the great heights, when by some accident the wind should turn it, in whatever manner it always has time to redirect itself and safely re-temper its flight, which will everywhere be free. But let it always pass above the clouds, so as to avoid the wetting of its feathers. The beginnings of things being often the cause of great outcomes — as we see a small, almost imperceptible motion of the rudder have the power to turn a ship of marvellous size, laden with the greatest weight and amid such a mass of water that weighs it down on every side, and against the courses of impetuous winds with the embrace of such great sails — therefore we may be certain that a small motion of wing or tail, to enter under or over the wind, is for those birds that sustain themselves above the course of the winds without beating of wings apt and sufficient to prevent the descent of the aforesaid birds.