The Threefold Motion by Which Rising Water Scours a Bank
Down, sideways and downstream at once: why an upwelling wears away the foot of the embankment
In a dense theoretical passage Leonardo resolves the erosive action of an upwelling into three simultaneous motions — one downward toward the bottom, one toward the bank, and one along the river's course. Because every stone at the foot of the bank is struck from three different sides at once, no sheltering stone can protect the ground behind it. Where the ground is erodible, he concludes, it must in a short time collapse. The verso is filled entirely with mirror-script and carries no diagram.
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An upwelling falls by a threefold descent of motion
The water spreads as it descends from the upwelling and partly follows the river's course, so it must make three motions: one downward, one toward the bank, one along the current. All three together scour the foot of the bank with the greatest force.
Each stone struck from three sides at once
Because the descent is oblique and follows the river's universal motion, each stone is struck by three different motions and from three different sides. Unlike a lengthwise current, this cannot be blocked by a single protecting stone.
Why the undermined bank must soon collapse
If the ground is of a kind apt to be worn away, the combined scouring means it must ruin in a short time. Leonardo treats the collapse of the bank as the necessary outcome of the threefold attack.