Raising and Lowering the Wing, and the Wing-Bone
Feathers spread on the up-stroke and close on the down-stroke; the spoon-shaped humerus eases lifting.
Leonardo contrasts the up-stroke and down-stroke of a bird's wing: because the wing is convex above and concave below, it rises through the air more easily than it descends. On the up-stroke the feathers spread apart so the perforated wing slips through the air, while on the down-stroke their intervals close so the wing presses and condenses the air beneath it. A closing note describes how each feather's long weak part rests beneath the short strong part of the next, and how the wing's humerus is hollowed like a spoon so that lifting is easy and lowering meets resistance, driving the bird forward like a rasp. Feather diagrams and a small bird accompany the text at the right.
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Raising the wing is easier than lowering it
Because a bird's wing is convex above and concave below, and convex surfaces penetrate the air more readily than concave ones, the bird lifts its open wing more easily than it presses it down. Leonardo cites the 'third' proposition of the book to this effect.
Feathers spread on the up-stroke, close on the down-stroke
The feathers separate as the wing rises so that the perforated wing slips more easily through the thick air, and their intervals close as the wing descends so that, united, they bar the air and beat it into a denser mass. Each vane also curves into a cover over the next against the wind's entry.
The wing-bone (humerus) hollowed like a spoon
Under the heading 'Omero' of the wing, Leonardo describes the humerus, where the wing's rudder is set, as hollowed beneath like a spoon, convex above and concave below. This shape makes lifting easy and lowering hard, and above all drives the bird forward as it draws back, in the manner of a rasp.
Each feather's weak part tucks beneath the next
Leonardo notes that the feathers under the wing, which must rest and rub upon the sustaining air, lay their long weak part beneath the short strong part of the following feather. He adds that the sinews are more powerful below the wing than above, and that all this serves the wing's motion.