c a - d b

When the wind strikes the bird below its course, from the centre of its gravity toward that wind, then such a bird will turn its back to the wind. And when the wind is more powerful from below than from above, then the bird would turn upside down, if it were not immediately quick to draw in to itself the lower wing and extend the upper wing. And in this way it rights itself and returns to the position of equilibrium.

b - a - c

It is proved. Let the wing drawn in beneath the bird be a c, and let the extended wing be a b. I say that the powers of the wind that strike the two wings will hold the same proportion as that of their extensions, that is a b against a c. It is true that c is wider than b, but it is so near the centre of the bird's gravity that it resists little in comparison with b.

But when the bird is struck to leeward beneath one of its wings, then it would be possible for the wind to overturn it, if, as soon as it has turned its breast to the wind, it did not extend toward the earth the opposite wing and draw in the wing that was first struck by the wind, which remains uppermost; and so it will come to return to the position of equilibrium.

n m f

It is proved by the fourth [proposition] of the third [book], that is, that "that object is more overcome which is fought by a greater power." Again by the fifth of the third, that is: "that support resists less which is situated farther from its fixed point." Again by the fourth of the third: "among winds of equal power, that will be of greater power which is of greater bodily quantity," and that one strikes with a greater sum of body, which finds a greater object; whence, m f being longer than m n, m f yields to the wind.