Observe the motion of the surface of the water which resembles that of hair, and has two motions, of which one goes on with the flow of the surface, the other forms the lines of the eddies; thus the water forms eddying whirlpools one part of which are due to the impetus of the principal current and the other to the incidental motion and return flow.
[Footnote: See Pl. XXV. Where also the text of this passage is given in facsimile.]
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.