The concavity and usefulness of water eddies
How an eddy's hollow depends on the speed and mass of the revolving water
Leonardo sets out rules for the concavity and usefulness of water eddies (retrosi). He argues that the swifter the water, the deeper the eddy's hollow, while thicker but slower water yields shallower ones; among currents of equal speed, the greater mass of revolving water best holds its concavity. Two marginal spiral and vortex figures contrast an eddy from a narrow current in a great slow pool with one from a broad current, and he refers the reader forward to the 'accidental eddy'.
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Depth of an eddy's hollow set by the water's speed and mass
The eddy generated in faster water has the deeper concavity, while one formed in thicker but slower water has a shallower one. Among waters of equal speed, the greater mass of revolving water best sustains its hollow, since the heavy lateral slow water presses on the eddy's weak sides and can close it up.
Eddy from a narrow current in a great slow pool
A marginal figure shows an eddy generated by a narrow current entering a wide expanse of slowly moving water.
Eddy from a broad current in a great slow pool
A companion figure shows an eddy caused instead by a large, broad current within the same kind of great slow pool.