Since the wings are swifter to press the air than the air is to escape from beneath the wings the air becomes condensed and resists the movement of t… - Leonardo da Vinci

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Since the wings are swifter to press the air than the air is to escape from beneath the wings the air becomes condensed and resists the movement of the wings; and the motive power of these wings by subduing the resistance of the air raises itself in a contrary movement to the movement of the wings.