The pole of the shoulder of birds is that which is turned by the muscles of the breast and of the back. And from this is generated the discretion of lowering or raising the elbow according to the will and necessity of the animal that moves.

d - c a - b. I conclude that the rising of birds without beating the wings arises from nothing other than by means of their circular motion within the motion of the wind; which motion, when it departs from the oncoming of that wind, comes declining down to the place where the deflected motion is created; after which, and thus circling, it has described a semicircle and finds itself with its face turned to the wind; and it follows the deflected motion up over the wind, always circling, until, with the help of the wind, it makes its highest height between its lowest lowness and the oncoming of the wind, and remains with the wing weary at the wind; and from that highest height, circling anew, it descends again to the last incident motion, remaining with the right wing to the wind. As to say: the wind goes from a to c, and the bird moves from a and goes declining from a b c, and at c takes the deflected motion up to c d a, and by the favour of the wind it finds itself much higher with the end of the deflected motion than with the beginning of the incident motion; which end of the deflected motion is situated perpendicularly above the said beginning of the incident motion.

The equal resistance of the wings in their bird is always born from their being equally remote, with their extremities, from the centre of the gravity of such bird. But when one of the extremities of the wings comes nearer to the centre of the gravity of the bird than the other extremity, then the bird will descend on that side where the extremity of the wing is nearer to the centre of gravity.