... were I to curve this motion and make it a half-circumference, then such a bird would find itself, at the end of that motion, with its beak turned toward the place whence this deflection was caused. And if this is done against the oncoming of the wind, the end of the deflected motion will be made much higher than was the beginning of the incident motion; and this is the way the bird rises high without beating its wings, and by circling. The remainder of the said circumference is completed in the direction of the wind by incident motion, always with one of the wings low and likewise one side of the tail; and it then makes deflected motion toward the departing course of the wind and remains at last with its beak turned toward the going-away of that wind, and then it makes incident and deflected motion anew against the wind, always circling.

When the bird wishes suddenly to turn to one of its sides, then with speed it thrusts the tip of the wing of that side toward its tail; and because “every motion tends to its own maintenance, or rather every moved body always moves so long as the impression of the power of its mover is preserved within it,” therefore the motion of that wing, driven furiously toward the tail, still retaining at its end part of the aforesaid impression and being unable of itself to continue the motion already begun, comes to move the whole bird along with it until the impetus of the moved air is consumed.

The tail, thrust with its face and striking the wind with it, makes the bird move suddenly in the contrary direction.

c o m a - d n b. When the bird is in the disposition a n c e and wishes to climb high, it will raise the shoulders m o and will find itself in the figure b m n o d e, and it will press the air between the ribs and the tips of the wings, in such a way that it condenses it and gives it upward motion, and generates an impetus in the air, which impetus of air, by its condensation, will thrust the bird upward.