Oblique Flight of Birds in the Wind
How a gliding bird crossing the wind turns by drawing in one wing (a, n, c, d, p, f)
Under the heading 'Birds. On oblique motion', Leonardo debates with an imagined adversary about how wind lifts a bird. A bird struck from below by the wind always rises, but when bird and wind travel the same path at equal speed the bird gains a degree of descent for every degree the wind moves. The second study, keyed to letters a, n, c, d, p, f, describes a bird gliding eastward across a southerly wind without beating its wings: by drawing in the right wing and extending the left, it bends its straight course toward the contracted wing. Marginal sketches show a bird set in a graduated square and a bird in the streaming wind.
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A bird struck from below by the wind rises
The adversary holds that a bird struck from beneath by the wind always rises. This holds when it flies against the wind, but when bird and wind move the same path at equal speed, the bird must gain a degree of descent for each degree the wind advances.
Gliding bird turns by drawing in the right wing (a, f, c, d, m, n)
A bird gliding east without beating its wings, crossing a southerly wind, draws in its right wing and extends the left. By this inequality of wings it bends its straight motion toward the contracted side: flying the line a f with equal wings n b, it turns from c f to d as it gathers the right wing from m to n.
Bird and wind on one path: degrees of descent
Leonardo measures the exchange in degrees: in the time the wind moves one degree in the region of equilibrium, the bird gains a corresponding degree of descent. The graduated square scores this motion into equal steps.