Why water in a straight river runs an oblique, bank-to-bank course
A chain of propositions on incident and reflected motion in rivers
The main passage, headed 'Water,' argues that a river descending straight always moves obliquely from mid-channel to the opposite banks and back, and supports this by citing its own earlier propositions, the seventh through the twelfth. These hold that straight rivers never run in a straight line, that flow is fastest farthest from the bank, and that incident motion turns to reflected motion only by striking the bed or bank. The margin adds that the lateral obliquity of the water grows or shrinks with its speed. It also notes that percussion lifts earth from the bed and banks, the bed rising under incident motion and recovering under reflected motion.
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Oblique flow between opposite banks
Water in a straight river always moves obliquely from the middle to the opposite banks and back to the middle. Straight rivers never run in a straight line, because the water is faster the farther it is from the bank, its impediment; where incident motion is impeded, reflected motion is born.
Scour and rebuilding of the river bed
Where the water strikes the bed or bank, earth is lifted from it. Under the incident motion the bed of the river rises, and under the reflected motion the bed restores its height.
Obliquity scales with velocity
The lateral obliquity of waters moving through straight rivers is of so much greater or lesser obliquity as those waters are of lesser or greater velocity of course.