Theory of birds: banking flight and a weighted wheel
Unequal wing-motion makes curved and oblique flight; a sixteen-sector wheel with three weights at upper right.
Under the heading 'Theory of birds,' Leonardo states that a long, uniform-sided body about its central line flies straight through the air only for as long as the impetus lent by its mover survives in it. A long passage on birds banking in the horizontal plane, keyed c a - b d, works out how wings unequal in length or in height produce curved or combined curved-and-oblique flight, citing his 'fourth' proposition that equal wing-movement gives straight motion. At the upper right a wheel divided into sixteen sectors, its radius likewise divided and carrying three weights (a - 16), accompanies the note 'sophistical, a thing that seems,' and three small marginal figures show a bird in straight, horizontal-curved, and spiral flight.
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Straight flight lasts as long as the impetus
A moving body of long figure and uniform sides about its central line flies straight through the air only for as long a time as the impetus lent by its mover lives on within it.
Banking flight from unequal wing-motion
A bird flying a curved line in equilibrium moves one wing longer and faster than the other without raising or lowering either. If the curve is combined with obliquity, one wing also rises and falls more than the other. Leonardo cites his 'fourth' proposition, that wings of equal movement give straight motion, and its converses for wings unequal in length and in height. The labels c a - b d key the figure.
A sixteen-sector wheel loaded with three weights
At the upper right a wheel is divided into sixteen sectors, with its radius likewise divided and three weights attached, marked a - 16, set beside the note 'sophistical, a thing that seems.'