If you take as your pattern the wings of feathered birds, these are more powerful in structure of bone and sinew because they are penetrable, that is… - Leonardo da Vinci

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If you take as your pattern the wings of feathered birds, these are more powerful in structure of bone and sinew because they are penetrable, that is to say the feathers are separated from one another and the air passes through them. But the bat is aided by its membrane, which binds the whole together and is not penetrated by the air.