Oblique Descent of Birds; the Childhood Kite Memory
Simple and composite obliquity of a bird's body and wings, with the resultant direction of descent
This verso opens with Leonardo's famous autobiographical memory of a kite that came to his cradle and struck him inside the lips with its tail, which he takes as a sign that writing of the kite is his destiny. The rest of the page develops the mechanics of oblique flight: obliquity of flat surfaces is 'simple' when turned to one aspect and 'composite' when turned to two, and a bird's obliquity is likewise simple or composite according to whether the central lines of its body and wings are inclined. Using a small balance diagram he resolves the direction of descent as a composition of inclinations, six degrees to the west plus two to the south giving a true descent between west and southwest. Several small crossed-line diagrams lettered a, b, c, d and e, f, g, h illustrate the cases.
On this page
The kite that came to the cradle
Writing so distinctly of the kite seems to be my destiny, Leonardo notes, for in the first memory of my infancy it seemed that, as I lay in my cradle, a kite came to me, opened my mouth with its tail, and struck me many times with that tail inside the lips.
Simple and composite obliquity of surfaces
The obliquities of flat surfaces are of two kinds: simple, turned uniformly to one aspect (south, west, or another), and composite, turned to two aspects such as east and sirocco. On a simple oblique square surface, a body moving upon it travels parallel to the side of the square when the upper edge is level.
Resolving the direction of a bird's descent
If a bird tends to descend westward with six degrees of obliquity along its body, and its open wings tend southward with two degrees, its true descent falls midway between west and southwest. The eight degrees span two winds, and the true motion lies nearer b (west, power 6) than d (south, power 2) in proportion to their powers, found by a mean proportional of inverse proportion.
Simple obliquity in the bird
Simple obliquity in the bird is when it holds only the central line ab of its trunk oblique while the central line c d of the wings stays level. The lettered diagram shows the trunk line inclined and the wing line in the position of equality.
Composite obliquity in the bird
Composite obliquity is when both the central line of the trunk and that of the wings are oblique. Here ef is the central line of the trunk and g h the central line of the wings, both inclined.