The Percussion of Water and a Bladed Mill Wheel
Making falling water strike a dense body, and keeping its weight on the wheel after the blow
The text explains that a blow of water is weak when it strikes other water, so to increase its power the falling water should strike a dense body, which is how water drives mills. Leonardo wants the water S to strike the wooden blade n o m and, after the blow, be trapped on the wheel and led into the basin a o b so both the percussion and the weight of the retained water turn the wheel with great speed and power. The large drawing shows a bladed wheel in its housing fed by a channel; smaller sketches below show drum-like rollers, two of which are meant to run tangent, while others are marked as having no effect.
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Strengthening a water blow and retaining the water's weight on the wheel
A percussion is weak when the thing struck resists little, so water striking water has small power; make the falling water strike a dense body instead, as water does at mills. If the water S strikes the blade n o m and is then kept on the wheel rather than falling off, both its blow and its retained weight give the wheel great velocity and power.
A bladed wheel shaped so the water does not splash out
The wheel must be finished on this side so that the percussion of the water does not bounce back out. The large drawing shows the cupped blades set radially in a housing, fed from the channel at S, so the struck water is caught and led into the basin a o b rather than escaping.
Rollers running tangent, and forms that do not work
The two little wheels (rollers) are meant always to stay tangent to one another in their circular motion. Neighbouring drum-like sketches are dismissed with the note that these have no effect, recording trials of paired rollers rejected as unworkable.