Weights, Their Movers, and the Rebound of a Heavy Body
Condensation of struck air, the pitch of birds' wings, and reflected versus incident motion.
Leonardo argues that the power of a mover must be proportioned to the weight of its movable and to the resistance of the medium, and that no science of this is possible without measuring the condensation of the air struck by the moving body. He points to birds, whose wing-beats sound lower or higher according to their slower or swifter motion. A marginal diagram of a heavy sphere's descent and rebound (labelled a c b and d f e) accompanies a discussion of why the reflected motion of a stone makes more noise than its incident motion, and how the effect varies with the obtuseness of the angle of incidence.
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Power proportioned to weight and to condensed air
The power of a mover must be proportioned to the weight it moves and to the resistance of the medium. No science of this can be given without first knowing the condensation of the air struck by the movable, which grows denser with the movable's greater speed.
Pitch of birds' wing-beats as a measure of the air struck
Birds show how struck air condenses: by the sound of their wings beating the air they make a lower or higher note according to the slower or swifter motion of their wings.
Descent and rebound of a heavy sphere (a c b, d f e)
In the bounce of a spherical weight the upper part moves in the direction of the whole's motion while the lower part always turns back. The path is drawn as a descent and rebound at an acute or obtuse angle, lettered a c b and d f e.
Noise of reflected versus incident motion
The reflected motion of a stone makes more noise than its incident motion, though it is less powerful, because it arises from compound rather than simple motion; and the noise grows as the angle of incidence becomes more obtuse.