Waves of the Wind and How Birds Ride Sea-Cliff Updrafts
Soaring almost motionless on deflected air, and steering with the tail
On folio 42v Leonardo defines the 'waves' and impetus of the wind and studies how a soaring bird hangs almost motionless near mountains and high sea-cliffs, riding the updrafts thrown up when the wind strikes those masses and bends upward in various revolutions. He argues that the incident (oncoming) and reflected (rebounding) motions of the air alternate, each generating the other so they are never alive at once, and that the bird balances its own weight against the wind by tilting its body. A marginal note explains how the bird steers by lowering or raising the two 'horns' of its tail. The right margin carries small sketches of coiling and spiralling wind-eddies.
On this page
Soaring on updrafts deflected off mountains and sea-cliffs
The bird sustains itself in the air with imperceptible balancing near mountains or high sea-cliffs. The winds striking those roundnesses are forced to keep the impetus they have begun and bend their straight course upward in various revolutions, on whose front the bird settles with open wings. By the obliquity of its body it acquires as much weight against the wind as the wind exerts force against that weight.
Incident motion and reflected motion generate each other
The incident motion is always joined with its reflected motion, the beginning of the reflected joined with the end of the incident. If the motions persist, each is in turn the cause of the other, and the death of the one immediately generates the other, so they are never alive at the same time. The incident motion has a weak beginning and always grows; the reflected motion is the contrary.
Steering by lowering or raising the horns of the tail
When the bird lowers its tail equally it descends by oblique and straight motion, but if the right horn is lowered more, the straight descent curves toward the right, the more so as that horn is lower; the left horn does the same on its side. If the tail is raised equally above the line of the back the bird rises in straight obliquity; raising the right horn curves the ascent to the right, the left horn to the left.
Marginal sketches of coiling wind-eddies
Down the right margin runs a column of small pen sketches of the wind's revolutions: tight coils, a flat spiral, a tornado-like vortex, a helical spring form, and looser crescent and S-curves. They illustrate the various revolutions and eddies made by air struck back against the roundnesses of mountains and cliffs.