Bird lifted by reflected wind; plan for a treatise on flight
The bird rises without beating its wings; the treatise to be set out in four books
Above a silhouette of a bird carried up on rising lines of wind, Leonardo observes that a bird climbs straight upward without beating its wings when the wind, reflected from below, strikes it. He then lays out a plan to divide his treatise on birds into four books: flight by beating the wings; flight without beating them, on favourable wind; flight in general across birds, bats, fishes, animals and insects; and lastly instrumental (mechanical) motion.
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A bird rises without beating its wings on reflected wind
Leonardo notes that the bird lifts straight upward, without any wingbeat, when a wind reflected from below strikes it, one of his observations on soaring supported by air deflected off an obstacle.
Plan to divide the treatise on birds into four books
He sets out the structure of a treatise on flight in four books: first, flight by beating of wings; second, flight without beating the wings, by favour of the wind; third, flight in general, as of birds, bats, fishes, animals and insects; and last, instrumental motion.