If the bird wishes to turn quickly onto one of its sides and continue its circular movement, it will beat its wings twice on that side, rowing the wing backward while the opposite wing stays still; or with a single beat against two beats of the opposite wing.

Because the wings are quicker to press the air than the air is to flee from beneath the wings, the air condenses and resists the motion of the wings. The mover of these wings, overcoming the resistance of the air, rises contrary to the motion of the wings.

That bird will descend with swifter motion whose descent is of lesser obliquity.

The descent of that bird will be of lesser obliquity whose wing-tips and their wing-bones (omeri) are closer together.

These are the lines of the motions made by birds in their rising, made by two lines, of which one is always curved in the manner of a screw, and the other is rectilinear and curvilinear.

That bird will rise high which, with a circular motion in the manner of a screw, makes its reflex motion against the oncoming of the wind and against the flight of that wind, always turning onto its right side or onto its left side.

As if the north wind were blowing and you, to windward, ran by a reflex motion against the said wind, and when, in your straight ascent, that wind were disposed to overturn you, then you are free to bend by the right or left wing, and with the inner lowered wing you will follow a curved motion; with the aid of the tail you curve toward the lower wing, always descending and curving around the low wing, until you again make a reflex motion to windward following the course of the wind. And when you are about to be overturned, that same low wing will curve your motion and you will return against the wind beneath it, until you have gained the impetus. And then rise to windward, toward its oncoming, and by the impetus already gained you will make the reflex motion greater than the incident.

The bird that climbs always keeps its wings to windward and without beating them, and always moves in a circular motion.

And if you wish to go to the west without beating the wings, while the north wind blows, make the incident motion straight and to leeward toward the west, and the reflex to windward toward the north.