Water Striking Obstacles, and a Refutation of Epicurus on the Sun
Matching a fall to a wheel's power, rebounding currents, and the true size of sun and moon
A large pen-and-red-chalk drawing dominates the sheet, showing turbulent water and vortices flowing past a vertical wheel and channel structure at the right. The first note asks how to find the height of a waterfall needed to make its power equal to that of the wheel it drives, given the water's thickness and obliquity. The second observes that water striking obstacles rebounds much, little, or descends according to the obstacles' width, the descent before them, and the current's force; it then turns to astronomy, refuting Epicurus's claim that the sun is only as large as it appears by reasoning from solar and lunar eclipses that the luminous body casting a pyramidal shadow must be larger than the opaque body that causes it.
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Matching a waterfall to the power of its wheel
Given the thickness and obliquity of the water's fall, together with the power of the wheel set as its object, Leonardo seeks the height of fall that would make the water equal in power to the wheel. The wheel and its channel are drawn at the right of the sheet.
Why water rebounds from obstacles
Water striking objects sometimes rebounds greatly, sometimes little, and sometimes descends. This depends on whether the obstacles are narrow or wide, on the greater or lesser descent in front of them, and on how powerful the striking current is.
Refuting Epicurus on the size of the sun
Epicurus held the sun to be only as large as it appears, about a foot across. Leonardo shows the absurdity through eclipses: since the earth casts a pyramidal shadow onto the moon, the luminous body causing that shadow-pyramid must be larger than the opaque body, so the sun cannot be so small.