The Nature of Winds and the Weight of Clouds
Wind fills nature's vacuum and moves in straight lines; a cloud's heaviness and lightness; fog into cloud.
Leonardo defines wind as the surrounding air rushing to fill the vacuum left when moisture flees, and notes a contrary wind arising when the sun dissolves a cloud back into air. He argues against an unnamed adversary that winds move in straight, oblique lines rather than curves, citing the need to raise and lower sails and the patchwork ruffling of the sea's surface as proof. Two short sections follow: a cloud is lighter than the air below it and heavier than the air above, and fog struck by winds condenses into cloud. The page bears text only, with no diagrams.
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Wind as air rushing to fill nature's vacuum
Since no vacuum occurs in nature, when moisture flees a place the surrounding air must rush in to fill the void it leaves, and this motion is called wind. When the sun's heat dissolves a cloud back into air, a contrary wind is generated by that destruction and evaporation. Winds thus arise in every part of the air altered by heat or cold.
Winds descend in straight, oblique lines, not curves
Leonardo maintains against the adversary that the motion of winds is straight, not curved. If it were curved, sailors would not need to raise or lower sails to seek the wind high or low, and a sail once struck would be accompanied by that wind continuously. Experience shows the contrary in the sea's skin, struck in different places with short dilating motions, proof that winds descend from various places with various obliquities.
Waves that run on after the wind abandons them
Because the sea has a spherical surface, its waves many times run on without wind, since the risen wind has abandoned them and the wave moves with the impetus already begun. This observation of the ruffled sea-surface serves as evidence for the varied, oblique descent of winds.
A cloud's heaviness and lightness; fog into cloud
A cloud is lighter than the air that lies below it and heavier than the air that lies above it. Fog, struck by various courses of the winds, condenses and becomes cloud with various globosities.