Of flight.

The rudders created in the shoulders that the wings of birds have were devised by ingenious nature for a convenient bending of the straight impetus, which often occurs in the furious flight of birds; for she found it much more convenient, amid the straight fury [of flight], to bend a very small part of the wings than the whole of them. For this reason there were ordained tiny and very hard feathers which, forming a cover one over another, arm and fortify themselves among one another with marvelous power. And these such feathers are fixed in a small and very dense bone, moved by sinews that bend it upon its joint, which are very large in such wings. The motion and the position of this bone in the shoulder of the wings is ordered and established in the same manner as is the thumb in the hand of man, which, being in the midst of four sinews that surround its base with four equal spaces among them, generates by means of those sinews infinite motions, both curved and straight.

We shall say the same of the rudder placed behind, at the rear of the motion of the ship, imitated from the tails of birds, of which experience teaches us with how much greater ease that little rudder is turned in the swift movements of the great ship than is the turning of the whole of that ship.

Why did the inventors of ships not place their rudders in front, as has been said of the rudders placed in front of the shoulders of the wings?

This was not done in ships because the great rising that the wave of water makes into the air, being struck with impetuous percussion by the motion of such a ship, would be the cause, through the weight it has acquired, of rendering the motion of such a rudder most difficult; and besides this it would often break. But because air within air has no weight, though it does have its condensation, which is most useful — for the rudders of the wings work better in the dense thing than in the rare, since the dense resists more than the rare — but the ship does the contrary in the acquired...

The reflected air always turns about, making a whirling motion after its percussion.

When the bird wishes, with wings outspread, to make a circular motion that raises it on high with the favor of the wind, then it lowers one of its wings and one of the horns of its tail toward the center of its circling. And when the bird's motion is circular so as to rise on high without beating its wings, it receives, in the fourth [quarter] of its circling, the wind beneath one of its wings; and thus the wind acts as a wedge to it and lifts it on high, and would turn it upside down, if the other wing did not lean or support itself upon the air that, with whirling motion, strikes beneath that wing — which is the same [air] that struck beneath the other wing and condensed itself.