Weights descending three inclined planes meeting at b (c, e, d)
The steeper (shorter) the plane, the faster the fall: descent along bc is slower than along bd as cb is longer.
On this recto three spherical weights rest on three inclined planes, c, e and d, whose upper ends all meet at the point b in the top-left corner of the framing rectangle (the opposite top corner is a). Leonardo argues that a body descends as much more slowly along the longer, gentler line bc than along bd as cb exceeds bd in length — assuming the bodies are equally heavy, equally round and in equal motion, a condition he calls impossible in practice. He adds that descent is slower the nearer the point of contact lies to the moving centre of gravity.
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Descent is slower on the longer, gentler incline
Leonardo states that a body will descend along line bc as much more slowly than along bd as cb is longer than bd, provided the bodies are of equal heaviness, roundness and motion — a condition he calls impossible. Steeper, shorter planes therefore give a quicker descent.
Contact near the centre of gravity slows the fall
He adds that a rolling weight descends more slowly in proportion as the point where it touches the plane is closer to the moving centre of gravity. The geometry of contact, and not the slope alone, governs the speed of descent.