How descending birds control their swoop
Wings drawn in to gain speed, spread wide to check the fall
Leonardo describes the oblique motion of a descending bird: it begins the swoop with wings held tight and gradually spreads them to consume the impetus that heavy bodies gain at every degree of descent. When too much wing-spread slows the motion, the bird again draws its wings in and the descent grows swift once more. A wavy line at the top of the page diagrams the resulting path through the air, captioned as the road made by descending fliers through their opening and closing of wings.
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Oblique descent by opening and closing the wings
The oblique motion of a descending bird begins with the wings tight and spreads them little by little to consume the impetus that heavy things gain at each degree of descent. When too much spreading slows the motion, the bird again draws its wings in, and the descent again grows swift.
Diagram of the flight path
A wavy descending line traces the road made through the air by fliers as they descend, alternately spreading and closing their wings. It visualizes the undulating trajectory produced by the behaviour described in the text.