The blow: turbid water and the rebound of colliding spheres
Equal spheres striking at a right angle, and the angle of rebound equal to that of percussion
The page moves from the percussion of water to the mechanics of impact between balls, headed 'Moto d'acqua' and 'Colpo'. Turbid water, being heavier than clear, strikes harder against whatever opposes it, and water falling on gravel scours a deeper hollow than in soft mud. Labelled collision diagrams then state that two equal spheres meeting at a right angle depart on a tangent (a m), that the angle of percussion always equals the angle of rebound (K n p), and that a ball rebounds off a wall or off a second ball just as light reflects (s from b to a; t from c to d).
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Turbid water strikes harder than clear water
Turbid water gives far greater percussion against whatever opposes its course than clear water, because it is heavier. Falling among gravel mixed with sand and earth it makes a deeper, prompter hollow than in soft mud, taking up a swift, powerful bounce that gnaws and lifts at its first opposition.
Equal spheres colliding at a right angle depart on a tangent (a m)
If two spherical bodies of equal material, weight, size, and course strike at a right angle, they leave the struck place on a tangent course, their contact not separating until the end of their run, as shown in a m.
The angle of percussion equals the angle of rebound (K n p)
When two equal spheres meet at a right angle to the lines of their course, the line from the centre to their contact and the line of their rebound form a right angle; K n p is a right angle. The general rule: the angle of percussion of equal spheres always equals that of the rebound.
Rebound off a wall or a second ball (s from b to a)
If e f were a wall, ball s leaving b and striking it would rebound to a; likewise ball t leaving c rebounds to d after bouncing off ball S. The impact obeys the same equal-angle reflection whether the obstacle is a wall or another sphere.