Oblique motion still grows in arithmetical proportion (prism figure)
A prism (b, c, a, m, n, o, p) shows velocity increasing by equal steps even when the motion runs oblique.
Folio 42v shows a triangular prism seen in three dimensions: a triangular face with base b–c and apex a, a parallel m–n drawn at mid-height, and lines carried through the solid to the far face (apex c), cutting the half-base plane at o and p. Beside it Leonardo notes that even when motion is oblique it observes, at every degree, the increase of motion and velocity in arithmetical proportion. The solid serves as a figure for how speed accumulates in equal increments during accelerated descent.
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Oblique motion accelerates in equal degrees
Leonardo states that even though the motion is oblique, it will observe, in each of its degrees, the increase of motion and velocity in arithmetical proportion. Obliquity does not change the rule that speed grows by equal successive steps.
A prism with base bc, apex a, cut at m n and o p
The figure is an isosceles triangular prism drawn in three dimensions, with a triangular face b–c–a, a line m–n parallel to the base at mid-height, and edges carried through the solid to the far apex, intersecting the half-base plane at o and p. It gives geometric form to the arithmetical growth of velocity described in the note.