Wind speeds up as it condenses; winds on mountains and valleys
Air forced through a narrowing mouth gains sixteenfold velocity, and how wind strikes and bends over peaks
Folio 74 verso studies the behaviour of moving air. Leonardo states that wind grows faster the denser it becomes, and denser as a great mass is squeezed through a small space: air passing through the hopper a b c d issues sixteen times faster through the smaller mouth n m o p, as shown by converging lines drawn into a nozzle. He then argues that wind is far stronger on high mountain summits and struck slopes than in valleys, where reflected gusts multiply, the mass bending up to the crest, condensing, and dispersing down the far side. A drawn lantern-like turret with streaming lines below and a spreading plume illustrate wind flowing past an elevated body; a further note begins on dense winds gathered in enclosed valley mouths.
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Air through the hopper a b c d issues sixteen times faster
Wind driven through the hopper a b c d comes out sixteen times faster at the narrow mouth n m o p than it enters, because its exit fits sixteen times into its entrance. The general rule is that wind becomes faster the denser it is made, and it condenses the more when its great mass passes through a small place.
Why wind is stronger on mountain summits and struck slopes
Wind is much more powerful on high summits and struck slopes than in valleys, because there it condenses and its reflected gusts multiply. On the struck side the whole mass bends and drives up to the crest, where it converges condensed, then disperses and bends toward the base of the opposite side, so the two flanks carry different motions even under a single wind.