The wind moves like a river; the three accidents of a leaf
Air carrying clouds, the motion of air, and a leaf's shadow, luster and transparency
In red chalk, Leonardo argues that the air moves like a river, dragging the clouds along as running water carries floating things; were the wind merely to push the clouds, they would condense between the air and the mover and squeeze out sideways like wax pressed between the fingers. Air moves, he adds, when drawn to fill a vacuum or driven off by the rarefaction of the clouds' humour. A final section, illustrated by a small leaf sketch at the lower right, states that a leaf sometimes shows three accidents at once, shadow, luster and transparency: with the light at n, the leaf S and the eye at m, one sees a illuminated, b shadowed and c transparent.
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The wind carries the clouds as a river carries floating things
The air moves like a river and draws the clouds along with it, as running water draws the things floating on it. Were the wind only to push them, the clouds would condense between the air and the mover and take a lateral impetus from their two ends, like wax squeezed between the fingers.
The motion of the air
The air moves when it is drawn to fill a vacuum, or driven off by the rarefaction of the humour of the clouds.
A leaf's three accidents: shadow, luster and transparency
Sometimes a leaf shows three accidents at once, namely shadow, luster and transparency: if the light were at n, the leaf at S and the eye at m, it would see a illuminated, b shadowed and c transparent.