Why rivers shift their beds, what foam is, and how water scours a pit
Erosion, deposition and the undermining of banks; air trapped as bubbles; the circular pit under a fall
Three passages examine how running water reshapes its channel. Leonardo describes how a river consumes its softer support, hollows out eddies, undercuts banks, carries off the lightest earth to drop it in quieter places, raises its bed, and lays bare buried stones that deflect its impetus to the opposite shore. He then explains foam as air dragged under by falling water that surges back to the surface clothed thinly in moisture as round bubbles. A final passage, with a small marginal sketch of a cascade, explains why gently falling water digs a round pit without widening it — the trapped air cannot escape and rises again in bubbles near the point of impact — whereas a river swollen by new rains strikes united and forceful down to the hard bottom, baring and shifting the stones and splitting into two contrary courses.
On this page
How a river changes its bed
The moving water always tends to consume its support; the softer part yields and departs, leaving hollows where the water turns on itself in eddies, widening the voids and striking the banks. It carries the lightest earth away to drop it in quieter places, raises the bed, undermines and erodes banks, and uncovers buried stones whose size resists the current and throws it back to the opposite shore.
What the foam of water is
Water falling into other water encloses a quantity of air that is driven under by the blow and, with swift motion, surges back to the surface clothed thinly in moisture as a spherical body, leaving the place of first percussion in a circle. The falling water breaks into branching, forked, hollow streams that interweave, and when they strike, the air has no time to escape and is submerged.
Why falling water digs a round pit
Gently falling water digs a round pit without widening it, because the air caught in its disunited fall cannot escape quickly and is pressed under, then rises in bubbles near the impact, sparing the banks. But an impetuous river swollen with new rains falls united and forceful to the hard bottom, baring and removing the buried stones, making a new barrier of the pit's sides, and dividing into two contrary courses at the percussion.