Swimming shows the method of flying
The goose's foot, and why a wider body meets more resistance in water and air.
Under the heading 'On motion,' Leonardo argues that swimming demonstrates the way of flying, and that a broader weight meets greater resistance from its medium. Watching a goose, he notes that the bird closes its webbed foot as it pushes forward, meeting little water and moving quickly, then spreads the foot on the backstroke so that it grips more water and drives the body on. From this he draws the rule that, for a given weight, the wider it is, the more slowly it moves.
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Swimming demonstrates the method of flight
Leonardo takes swimming as a model for flying: both work by pressing against a yielding medium. He notes that a wider surface offers more resistance to the air, just as a spread foot does to water.
The goose opens and closes its foot to swim
The goose closes its foot as it carries it forward, so that little water resists and it advances quickly; drawing the foot back, it spreads it wide to grip more water and drive itself on. The curved outer side of the foot passes through the water more easily going forward than the foot does coming back.
The wider the weight, the slower its motion
From the goose's foot Leonardo generalizes a rule of motion: for a given weight, the broader its surface, the slower it travels through the medium. A foot held always open, or always closed, could make no progress at all.