How Air Moves Before and Behind a Moving Body
Eddies and the wave of air, and why the air behind cannot drive the body forward
Leonardo analyses how air behaves around a body driven through it. Less air moves ahead of the body than behind it; the air rushing in to fill the void it leaves matches the body's speed as a whole, yet its whirling eddies (retrosi) are locally much faster. He rejects the idea that this trailing air drives the body, invoking his 'seventh' proposition — no moving body can exceed the speed of the power that moves it — and notes that the bow-wave of air scarcely runs ahead. A pen study of spiralling vortices at the lower right shows the trailing air turning back on itself.
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Air moves less ahead than behind the body
The motion of the air is less in front of the body that penetrates it than behind it. The air filling the void left behind matches the body's speed as a whole, but its whirling, circular eddies are in themselves much faster than the body's own motion.
The seventh: nothing exceeds the power that moves it
It might seem that the faster air behind the body is what drives it, but by the seventh proposition this cannot hold: no moving body will ever be faster than the velocity of the power that moves it. For the same reason the wave of air scarcely passes ahead of the body.
The trailing vortex of air
The air running behind the body is moved by the impetus imparted to it; through its great dilated wave it strikes the surrounding air, turns back, and forms a wide circulation that diminishes toward its extremities until it stops and no longer follows the body.