Flying without beating the wings: descending and circling on the wind
The first book of soaring flight, and how a bird turns with the north wind
This verso (101v), headed as the opening of 'the first book, which treats of flying without motion of wings', studies soaring and gliding, illustrated with several sketches of birds turning and settling. Leonardo explains that to descend a bird lowers a wing from the middle forward as a rudder along a line a b, then turns its face to that side and spirals down until it faces its landing place. A long passage analyses how a bird uses the north wind (tramontana) to climb in circles, keeping its tail low to be struck obliquely from above, and how it shifts a wing from over the wind to under it (a b to d b) to avoid being overturned, and slants toward the scirocco. Wind directions (east, west, south) govern each maneuver.
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The first book: descending without beating the wings
The first book treats of flying without motion of wings. To descend, the bird lowers a wing from the middle forward, on the side toward which it wishes to go, which makes a rudder to its motion along the line a b. It then turns its face to that site a b and descends by the same obliquity, spiralling ('drilling') until it faces the place where it wishes to alight.
Climbing in circles on the north wind
When the north wind (tramontana) blows and the bird faces east with its tail under the wind and its northern side low, this signals that it will climb in circles by favour of the wind, which keeps the tail low and strikes it obliquely from above. Turning to the west it is struck from below and continues the circle; if it would go toward the scirocco it lowers half the southern wing to catch the wind within.
Shifting a wing from over the wind to under it
As the kite rises in its circular motion and returns its upper wing to meet the wind, it would often be overturned unless it immediately changed that upper wing, lowering and setting it, from the middle onward, under the wind. A lower figure fixes this with the wing a b, which first was over the wind, making itself under the wind in d b.