The descent of a bird without wind or beating of wings.

a c b e f h g - K i - b

When the centre of gravity of the bird is farther forward than the centre of resistance of the wings, then that bird will descend along an oblique line, always keeping that obliquity. In the descent of the bird, that descent will be found to be made with swifter motion which is less oblique.
That bird travels by a longer journey whose descent is mixed with many rebounding motions, that is in waves, as is figured above. Let us say that the motion of the bird is ordered to go from a to b, and when it has travelled half a mile, let it make from c d as much rebounding motion as the nature of the impetus of that descent gives; then let it return its wings to their first obliquity and descend by the motion of another half mile, d e; and then let it rise again and re-ascend until it finishes its course at the appointed place.

10 miles - 10 - a - miles 10 - b
If the bird that does not beat its wings does not wish to descend quickly to the low, it will, after some oblique descent, raise itself by a rebounding motion and wheel about, climbing in the manner of cranes when they break up the ordered lines in their flying and gather into a flock, and go with many turns, rising in a spiral; and then, having returned to their first line, they resume their first motion, which descends with a gentle obliquity, and then they gather again into a flock and, wheeling once more, rise up.

The bird that rows more with one wing than with the other will move in a circular motion.

My magnificent Messer Simone, in the past days, my Messer Simone, only this to inform you ...