How a Bird Rights Itself After Falling Head-Down
Bending the tail and shifting weight; the centre of gravity below the lighter part
Folio 44r asks how a bird that has pitched into a head-down fall can right itself: by bending its tail toward its back, so that its central line of gravity and the lighter tail come into one line and the straight plunge is turned into a slower, oblique glide. Leonardo grounds this in his propositions that a falling body's centre of gravity always hangs below its lighter part, and that a descent slows as it grows more oblique. A second note treats a bird flipped onto its back, which must pull in its tail and raise its wings behind its head to shift its weight, since the centre of gravity is not at the middle of its length and always hangs below the centre of quantity. The right margin carries small figures of birds in these tumbling and recovering postures.
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Righting a head-down fall by bending the tail
The bird that falls head-downward rights itself by bending its tail toward its back. Its central line of gravity c d, being remote from the tail's central line of levity a b, within a small amount of descent becomes one and the same line with it, so the straight plunge is forced to become oblique and thereby slower and longer.
A falling body's centre of gravity lies below its lighter part
Leonardo invokes his tenth proposition: the centre of the heavy body descending through the air always lies below the centre of its lighter part. From this it follows that a straight descent turns oblique, and grows the longer and slower the more oblique it becomes.
Recovering from an inverted, back-first tumble
If the bird overturns in the air by means of the wind, it must draw its tail in as far as it can and raise its wings behind its head, so that the part from the middle backward becomes heavy and from the middle forward light. In this way it shifts the balance to recover from the inverted fall.
Centre of gravity versus centre of quantity
By the ninth proposition, a centre of quantity not concentric with the centre of gravity keeps the body from resting in the site of equality at its greatest width. By the tenth, the centre of gravity of bodies suspended in air always lies lower than their centre of quantity.
Marginal figures of tumbling and righting birds
Down the right margin Leonardo sketches a series of small bird figures in falling and recovering attitudes: birds pitched head-down, birds curving the tail toward the back, and birds drawing in their tails and raising their wings to right themselves.