The hand of the wing is that which causes the impetus. And then its elbow is set edge-on so as not to impede the motion that creates the impetus, and when this impetus is then created, the elbow lowers and becomes oblique, and oblique becomes the air where it rests, almost in the form of a wedge, upon which the wing comes to rise; and if it were not done so, the motion of the bird, in the time that the wing returns forward, the bird would come to sink toward the consumption of the impetus. But it cannot sink because, as much as the impetus fails, so much the percussion of that elbow resists this descent and raises the bird up again.

a b; c d; e f. Let us say that the impetus can as 6 and the bird weighs 6, and in the middle of the motion the impetus returns to 3 and the weight remains still 6. Here the bird would come to sink by half a motion, that is by the diameter of the square; and the oblique wing in contrary aspect, also by the diameter of that square, does not let this weight descend, nor does the weight let the bird climb; whence it comes to move by straight motion. As to say: the descent of the bird in the said half-motion would have to descend along the line a b, and by cause of the obliquity of the wings in contrary aspect would have to climb along the line d c; whence for the said causes it moves through the site of equality e f. The elbows of the animal do not all lower at the beginning, because in the chief rush of the impetus the bird would leap up on high, but they lower only as much as is needed to impede the descent according to the will and discretion of the bird. When the bird wishes to run suddenly upward, it immediately lowers the elbows after it has generated the impetus. But if it wishes to descend, it holds the elbows still up after the creation of the impetus.