Whether the Curved Wingtips Are Necessary
The fast-moving tip condenses the densest air beneath the wing, springing back to reverse its curve.
Leonardo asks whether the curved outer parts of a bird's wing are necessary, and answers that the air beneath them is the densest under the whole bird, because the wing-tip, being farthest from its pivot, moves fastest and so strikes the hardest. Invoking his 'seventh of percussion' - that a blow is greater the longer the motion of the striker - he treats the bent tip like a spring or a drawn bow that keeps the contacted air compressed. He adds that as the wing rises the tip trails, straightens, and reverses its curve, so a concavity turned skyward on the down-stroke turns earthward on the up-stroke. A marginal note states that the part of a rigid rod farthest from its mover is the fastest, in direct proportion to distance.
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The densest air lies beneath the curved wingtips
Leonardo asks whether the curved outer parts of the wing are necessary, and finds the air beneath them the densest under the whole bird, produced in the beating of the wings. The tip, being farthest from its fixed pivot, moves fastest and so condenses that air most.
Percussion grows with the length of the striker's motion
Citing the 'seventh of percussion' - that a blow is greater the longer the motion of the striker - he reasons that in one descent of the whole wing the part farther from the pivot travels fastest and strikes hardest. The air struck by the faster percussor is the more condensed.
The bent tip springs back and reverses its curve
The bent wing-tip acts like a spring or a bow drawn by force, keeping the air in contact with it compressed. As the wing rises the tip trails and straightens, then bends the opposite way, so a concavity turned skyward on the down-stroke turns earthward on the up-stroke.
Speed proportional to distance from the mover
A marginal note generalises the rule: the part of a rigid rod farthest from its mover is the fastest, and velocity stands to velocity as distance to distance, provided the moving rod does not bend.