Local motion of dust: mounds formed on a struck board
Dry grains gathering into hillocks, likened to wind-driven sand waves in Libya and on river sandbanks
Under the heading of the local motion of dry, friable matter such as dust, Leonardo observes that when a board is struck along various lines the dust upon it gathers into hillocks and little mounds. He traces the path of the grains, which descend the hypotenuse of each mound, pass beneath its base, and rise again around the axis to its peak in a right-triangle motion. He instructs himself to note how such mounds begin and to describe them as the wind-driven waves of sand seen in Libya and on the great sandbanks of the Po and Ticino. The page carries several sketches of cylinders struck by lines, with lettered points.
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Dust gathering into hillocks on a struck board
When the board is struck along various lines, the dust lying on it reduces itself into various shapes of hills and little mounds. Leonardo begins to explain the cause but breaks off.
Right-triangle motion of the grains up each mound
The dust that separates into mounds descends the hypotenuse of those mounds, passes beneath their base, and rises again around the axis to the peak, moving with the motion of a right-angled triangle. He also notes that the axis of the incident angle terminates on the bodies along a perpendicular line.
Wind-driven waves of sand, as in Libya and on river sandbanks
Leonardo directs himself to describe the mounds of dry friable matter as the creation of the waves of sand carried by the wind, with their mounds and hills, as happens in Libya. He notes the example can be seen on the great sandbanks of the Po, the Ticino, and other great rivers.