That bird rises more swiftly of which the circles of its elevation are of lesser size.
What is here proposed happens because the bird which, without beating its wings, rises with the aid of the wind, receives the wind beneath it in the manner of a wedge; and the wedge that has the thicker angle has its side less oblique, whence it raises higher the thing placed upon it.
The bird never moves upward unless the wind enters beneath it and makes itself a wedge for it, driving it somewhat along the line of the wind's course. Here a doubt arises, that is, whether the wedge does not raise the thing placed above it by a perpendicular line, if that thing is not supported in such a way that it cannot flee before the blow together with the wedge — how then can the bird raise itself upon the wind that makes itself a wedge for it, without that wind carrying it away with itself? Therefore so much the worse will that bird come to climb upward and against the wind, unless indeed it climbed by falling, like water in the empty screw.

b c - n f r - d a

See tomorrow morning whether the bird that wheels, coming against the wind n, stays in the line a b, keeping its head at b, or whether it stays in the line c d.