Why river beds run deepest at the centre; a soaring bird
Rebounding currents and shifting river bends, with a bird riding the wind
Two folios join hydraulics and flight. Leonardo explains why straight rivers are deeper and higher at their centre: the rebound of water off both banks meets midstream in contrary motions that spring up and fall back with acquired weight, and he notes that river bends migrate as depths shift from place to place. On the facing leaf a bird rises in circles on the wind, bending its wings and shoulders toward sky or earth as the wind strikes its forehead or tail, always taking the wind at its centre of gravity. A short geometric note sets a n as the semidiameter of a circle. Figures include a serpentine current, concentric circles and a lettered construction.
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Deepest and highest in mid-channel
The current of a straight river is higher in the middle than at its sides and turns to greater depth there, because the rebound motion of water off each bank meets midstream in contrary motions; unable to penetrate one another, the waters spring up and then fall back with the weight gained in the air.
Why river bends shift place
The windings of rivers move from one place to another because their depths change site. Leonardo adds that water descending into water no longer weighs, and no longer desires to go to the centre of the world.
A bird rising in circles on the wind
The bird rises in circles by means of the wind, always struck from below along an oblique line. When the wind meets its forehead it lifts wings and shoulders toward the sky; when the wind takes its tail it turns the shoulders earthward; so it always takes the wind at its centre of gravity, before, behind or to the side.
Semidiameter of the circle a n
A brief construction note lets a n be the semidiameter of the circle, keyed to a lettered geometric figure at the upper right of folio 134r.