Why water over sand makes uneven waves — and what it proves about air
Unequal sand grains roughen the water over them; Leonardo argues that air too is full of unevenly moving parts.
This text page carries no diagram. Leonardo observes that because the grains of level sand differ in shape and size, water flowing over them drives each grain with a different force, so the sand's flatness is spoiled and its ripples come out unequal. He likens water moving over sand to air moving over water, and to bodies of different weight moving through still air. From the fact that a water surface — being a uniform body — cannot ripple as unevenly as granular sand does, he infers that the air itself is full of parts in nonuniform motion, which is why things carried by contact with the air move unevenly.
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Uneven grains make uneven ripples in the water above
Because the grains of level sand are of unequal shape and size, the water flowing over them drives each grain with a different strength of motion, spoiling the sand's flatness and making its waves unequal. Leonardo contrasts this with the surface of water, a uniform body, which cannot ripple in the same uneven way.
The air is full of unevenly moving parts
By analogy with water moving over sand, Leonardo treats air moving over water. Since a uniform water surface cannot show the uneven waves that granular sand does, he concludes that the air is full of parts having nonuniform motion, so that bodies moved by contact with the air move unevenly too.