Water and air trading places in an eddy, and a plan of eddies
Why an eddy 'has no fixed place', with a programme classifying every kind of retroso
A drawing at the upper right shows water issuing from a conduit after forming an eddy, labelled a, b, c, d and noted as 'this eddy has no fixed place.' Leonardo explains that because a b is lighter than c d it cannot supply as much water as c d consumes, so air is drawn in through c d until the two columns balance in weight. He then digresses that thin wavy wooden boards can be cut to imitate camlets or watered silk, before setting out a long programme for classifying eddies: surface and deep ones, mobile and stable, long and round, those that whirl floating things without sinking them, those that submerge and later expel them, and the oblique, straight and flat kinds.
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Why water and air trade places in an eddying conduit
Because the column a b is lighter than c d, it cannot deliver as much water as c d consumes; so air of necessity enters through c d to make up the difference, until the water at c d balances the weight of the water at a b. The eddy, he notes, 'has no fixed place.'
A programme for classifying eddies (retrosi)
He lists the kinds of eddy to be treated: surface ones and those formed at various depths, mobile and stable, long and round, those that whirl light things on the surface without sinking them, those that submerge and whirl things near the bottom then leave or expel them, and the oblique, straight and flat sorts.
Wavy boards imitating camlet and watered silk
As an aside he notes that thin wavy wooden boards can be made to look like camlets (giambellotti) or watered silk (tabì), with various stable markings. It is a note on producing decorative, patterned surfaces in wood.