On eddies and their utility.

h - e - d c b a f - g - S - n

The water that passes through the canal h d n strikes at g and divides in such a way that one part turns back and circles along the line g f e; and along the line e b g it still continues the motion it began, but it is full of minute eddies, which penetrate right to the bottom, because there is the whole thickness of the water that moves by such revolution; and these minute eddies are generated because in that spot the water g f e meets the current of d n. It is proved.

Of the bottom.

In the part of the bottom g f e nothing remains, because there is the current of the greater eddy. In the course e b g little remains, and that of middling lightness, because there the slow course of the minute eddies raises little from that bottom. On the mound a all the light things are unloaded, because there is the slowest motion of the water. On the mound c heavier things are left, because that mound is created by the current of the canal, which slows in the encounter with the said minute eddies.

Why do they form in hollows? Because the water is slower in the middle of the eddies than at the sides, and where it is slower, the more it leaves light things.