How the kite holds still in the wind and steers with tail and wings
Balancing breast to the wind, and the wing-rudder that makes a wedge of the air
This recto (101r) opens Leonardo's study of the flight of the kite (nibbio), illustrated with a soaring bird at the top right and a lettered wing figure below. He describes how the bird holds motionless in the air above the wind's velocity, its breast to the wind, neither driven back nor sinking, moving only its tail up and down and pressing its weight and obliquity against the gust with a force he measures in pounds. When the wind would overturn or pitch it, the bird raises or lowers its tail so that the entering wind rights it. The lower text explains how the rudder of the wing (a b) and the tail (n m) together temper the bird's descent and recoil, steadying it by 'making a wedge of the wind'.
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The kite holds still in the wind, breast forward
On one side the kite lowers its wing with fury, whence it is about to fall, and at once raises it again so as not to fall faster. The bird stays motionless above the wind's velocity, struck in the breast, moving only its tail from up to down and pressing its weight and obliquity against the gust with the force of a pound, matched by a pound of wind. If the wind would overturn or pitch it, it raises or lowers the tail so the entering wind rights it.
The wing-rudder and tail steer the bird, making a wedge of the wind
If the wind strikes the wing a b from below, the rudder of that wing is struck from above and tempers, while the tail n m makes up where the rudder fails. Should the bird be about to recoil, the wing-rudder a b sets itself, with the tail, against that descent and recoil. So the bird steadies itself in the air, making a wedge of the wind.
Wind force balanced pound against pound
Leonardo treats the encounter of bird and wind as a balance of measured forces: if the bird wishes to fall against the wind with the power of one pound, the wind exerts force and strikes its breast with the power of another pound. He likens the still air struck by wind to wind striking still air, an equivalence he says is proved by the fifth proposition.