Why an older water-wave forms a wider circle than a younger one
A sharp wave b and a rounder wave a spreading from a submerged object
Beside a schematic sketch of nested wave-arcs radiating in the upper right, Leonardo compares two waves rising from an object hidden beneath the water: b, the sharper wave, and a, the rounder one. He reasons that a wave grows the longer it lasts, so wave a, having been formed earlier than b, has necessarily spread into a larger circle. Its greater distance from its point of origin proves it is more remote in time from its creation than the waves standing before it.
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The older wave a is a larger circle than the younger wave b
Wave b is the sharper and wave a the rounder. A wave arising at b from an object covered by the water grows the longer it persists; since wave a was made earlier than wave b, it is of necessity a greater circle. Its motion, having carried it farther from its birthplace, shows it is the more remote in time from its creation.
Nested arcs of the spreading wave-front
In the upper right a fan of concentric arcs is sketched in pencil, figuring successive wave crests spreading outward, with the crest at a set farther out than the sharper crest at b. The drawing illustrates how each ring lies farther from the source as more time elapses.