On wind in caverns and the whirling motion of water
How humid wind breeds water in caves; why a water-vortex spins fastest at its core.
A leaf densely covered in mirror-script, with two small diagrams in the right margin: a horn or trumpet shape above, and lower down a spiral seen from above joined to a downward-pointing cone. The right-margin note observes that a twisted trumpet gathers water inside it from the breath blown through it, while the main column argues that humid wind moving through winding caverns can generate water, and that the whirling motion of a liquid is swiftest nearest the centre of its revolution. Leonardo contrasts this with a rotating wheel, works out why the water keeps equal motion and why its central hollow is not permanent, and closes by distinguishing the columnar upward whirl of air from the pyramidal downward whirl of water.
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The whirling motion of water is swiftest near its centre
Leonardo states that the helical or whirling motion of any liquid is the faster the nearer it is to the centre of its revolution. He calls this a case worthy of admiration, since it is the opposite of a turning wheel, whose parts move more slowly toward the centre.
Equal motion and the doubled lateral weight of the vortex
He argues the water keeps the same motion through every whole revolution because the inner circle is as much less oblique as the outer is more oblique. Because the lateral weight of the whirl is double, the hollow it forms has no permanent motion, and its sides finally lean and collapse onto the air that filled it.
Humid wind breeds water in caverns
Humid wind passing through caverns that have both an entrance and an exit can generate water, especially where the sides are winding and irregular. The water is not permanent: when the wind fails, or a contrary wind enters the opposite mouth, the moisture on the walls is vaporised and turned back into air.
The two motions of air within air
Leonardo notes that the motions of air within air are two: a straight, columnar rise combined with a whirling motion. Water, by contrast, makes such a motion downward and makes it pyramidal, moving the faster the more acute the pyramid becomes.
Wind through the twisted trumpet fills it with water
The wind blown through a straight mouth-trumpet wets its inside far less than the wind driven through a twisted trumpet, which the player must empty of the water that collects within it. The moisture gathers by means of the wind passing through the coiled bore.
Marginal diagrams of the vortex and funnel
At the right edge a small spiral seen from above is joined to a downward-pointing cone, illustrating the whirling plan and the pyramidal descent of the water-vortex described in the text. A small horn or trumpet shape is drawn higher up the margin.