Water eddies (retrosi): two kinds of vortex in a river
Backflow spirals sketched in the margin, plus a note on silt and marine shells carried into mountains
Two swirling eddies are drawn in the right margin, illustrating the theory set out in the text: how water that is reflected back into a current forms rotating vortices (retrosi). Leonardo distinguishes two kinds of eddy, one turning up and down near the river bed and one turning side to side at the surface, and analyses how the reflected water penetrates the current more strongly below than above. A separate opening note argues that rivers could only have carried so much silt, mixed with marine animals, up onto the high land if the mountains had largely stood clear of the water.
On this page
Depth and width of current and of the reflected eddy water
Using the labels h g, e a c and f b d, Leonardo sets a b c d as the depth of the current together with its width, and c a f b as the depth and width of the water reflected back in the eddy. He argues that the revolutions of the reflected water, on returning to the river's current, penetrate it more in its lower parts than at the surface.
Why the surface current resists the returning water
The current is faster above than below, and consequently more powerful above; for this reason it is less penetrated by the percussion of the reflected water at the surface than lower down. Leonardo cites his own seventh proposition as the ground for the current being swifter at the top.
Two species of eddy: bottom and surface
Eddies made by the percussion of reflected water are of two kinds. One is generated toward the bottom and turns up and down along the length of the river, produced by bubbles falling back toward the bed; the other lies at the surface and turns this way and that across the river's width, produced by the circular motion striking the surface current.
Silt and sea creatures carried onto the high land
Had the mountains not for the most part stood uncovered by the waters, the rivers could not have carried so much silt into the sea as the deposit now found at great height, mingled with the marine animals that were enclosed within it.