Eddies formed by a sudden widening of the channel
How an abruptly widened bank breeds boring eddies that undermine it
A bank drawn inward to widen the channel produces a sudden eddy that bores into and deepens the foot of the bank until it collapses. Leonardo reasons from cited propositions: a river that gains sudden width also gains sudden width of water, which lowers its depth, so a sudden current forms and, cast against the widened bank, splits into two eddies, of which the first (c b a) is the more powerful and drives straight downward. If both sides widen at once, eddies form on both and, meeting midstream, they scour a sudden great depth. He insists all these figures must come out of experience.
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A widened bank breeds a boring eddy that undermines it
A bank drawn inward to widen the channel causes a sudden eddy to form that bores into and deepens the foot of the bank and ruins it. Because sudden width lowers the water's depth, a sudden current forms and, striking the widened bank, divides into two eddies, of which the first is stronger.
The straighter eddy mouth c b a penetrates most
Enclosed, the eddy c b a throws itself strongly straight toward the bottom; by the cited ninth proposition an eddy penetrates more the less oblique the lips of its mouth, and this one has them fairly straight.
Two-sided widening scours a sudden great depth midstream
If the channel gains sudden width on every side, it generates eddies on every side; where these join in the middle of the channel's width they make a sudden and great depth. All these figures must come out of experience.