Why a bird flies faster than the wind that drives it
Reckoning flight speed in degrees against still, following, and opposing air
A full page of inverted mirror-script, without diagrams, arguing that a bird's motion is much swifter than the wind's. Leonardo reasons that otherwise no bird could make headway against a wind, then gives a worked example in 'degrees' of speed: a bird moving at six degrees in still air loses two to a head wind and is left with four, but gains the wind's two degrees when flying with it, reaching eight. He closes by noting that the beat of the wing must be judged according to whether the air is still, fleeing, or oncoming.
On this page
Birds outrun the wind that carries them
The motion of flying creatures is much swifter than that of the wind, for otherwise no bird could move against a wind. A bird moves against the wind less than its natural course through still air only in proportion as the wind's degrees of motion are fewer than the bird's.
Flight speed reckoned in degrees: six less two is four; six and two is eight
Leonardo sets the bird at six degrees of speed in still air and the wind at two. A head wind takes two degrees, leaving four; but flying with the wind adds the wind's two, so the bird reaches eight degrees of speed.