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Updated: May 31, 2025
A bird's wing is so large, and is moved so rapidly, that the resistance of the air is enough to raise the bird a short distance each time the wings are flapped downward; but after each down-flap there must be an up-flap, and the air resists this just as it does the down-flap; so, unless there were some arrangement to prevent it, the bird would drive itself down each time it raised its wings, just as far as it had raised itself by the down-stroke before, so that it would never get into the air at all.
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