Eddies and the erosion of riverbanks
Why one stone makes many bumps, and how water forced into slow water whirls
Folio 60r pursues the motion of water past obstacles and the formation of eddies. Leonardo explains why a single stone on a flat bed makes many diminishing bumps downstream, comparing the water to a ball that bounces repeatedly with ever-smaller leaps, and a river-section diagram shows the surface rising and falling behind a round rock. A longer passage, illustrated by an overhead view of water forced through a gate into slow open water, treats the cause of eddies: the faster arriving water, resisted by the slower, turns back in circular revolutions that lengthen, erode the banks, and bring them to ruin. He adds that no water surface can naturally lie below that of the sea.
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Why a single stone makes many bumps downstream
The water striking the stone flows down behind it and forms a small pit, into which it flows, rising and falling again, repeating many times — like a ball struck against the ground that bounces many times before stopping, each bounce smaller than the last.
What causes eddies where swift water meets slow
Water pouring furiously through a narrow mouth into a great slow expanse cannot give way quickly enough; its percussion made, it turns back in circular eddies, finishing its desire at the bottom. This revolving motion lengthens because it meets nothing opposing it but itself, and it erodes the banks until they collapse to ruin.
All things seek to maintain their nature and completed motion
Wind is entirely the same as water in its movement, and all things universally desire to be kept in their nature. So a moving course of water tries to maintain the power of its cause, and if it meets opposition it completes the measure of its begun course with circular, eddying movements.
No water can lie below the sphere of the sea
No surface of water can by its nature be lower than that of the sea, and no free water can remain outside the sphere of its element without motion.