Bird flight: wing feathers, landing, and steering in the wind
With a geometric note on curved parallel lines tangent to a triangle
Under the headings 'Of flight' and 'Theory,' Leonardo studies how a bird's overlapping tiers of feathers strengthen the principal feathers and how those beneath the penultimate feathers stay flexible in flight. He explains that a bird alighting on a high perch must overshoot it, turn back, and glide down without beating its wings, and works out how a bird crossing a sidewind steers along a curved path to reach its target. A separate note treats curved parallel lines whose ends are equidistant from a point and tangent to the two sides of a triangle. The margins carry a feathered wing, a bird descending onto a column, and a bird maneuvering against the wind.
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Curved parallel lines tangent to a triangle's two sides
Parallel lines whose extremes are each equidistant from one and the same point are always curved, and one is shorter than the other, when they are tangent to the two sides of the same triangle. The construction is lettered a c e - d b.
Overlapping tiers strengthen the principal feathers
The tiers of feathers on the wings, laid one over another, are made to strengthen the principal feathers. Leonardo notes the plan to study the wing's anatomy first bare, then feathered.
A program to dissect the bird's wing
First make the anatomy of the birds' wing, then of its feathers stripped of their down, and then with the down. The instruction lays out a staged dissection of the wing's bones and feathers.
Flexible feathers beneath the penultimate feathers
All the wing feathers growing beneath the next-to-last feathers are flexible in flight, and the most flexible are those that do not cover one another, that is, those pierced through so the air passes.
Alighting on a high perch by overshooting into the wind
A bird flying against the wind to land on a high place must fly higher than the place, turn back, and descend upon it without beating its wings. Otherwise, giving up flight to land, it would be thrown backward by the wind.
Steering across a sidewind along a curve, a g m n c
A bird a b flying from a to c while the wind f strikes its flank directs its motion along line a g; the wind continually bends its course along the curve m n c, so that it reaches the intended place c.