On rivers.

The water falls along some line of the crest of its wave, and moves the more swiftly where that fall is less oblique, and breaks the more into foam where it finds more resistance. Therefore, by what has been said, the waves break against the current of the river and never in the direction of its course, because water falling upon the current cannot generate a rebound upon the thing that flees and receives no percussion. But from the opposite fall, toward the course of the waters, the water falling from the wave against the river's current finds no water that flees such percussion, but comes to meet that fall; whence, the fall of the wave being of 4 degrees of velocity and the water coming to meet it of another 4 degrees, the percussion of the wave comes to be made at 8 degrees of velocity; and therefore the waves of rivers break against their current, and that of the sea breaks against the water that flees back from the shore where it struck, and not against the wind that drives it.

Here the 6 sectors made by the semicircle will be equal to the single sector b c. Therefore the greater is six times one of these lesser ones, and consequently the circle from which the sector b c is taken is six times greater than that from which the lesser sectors come.

Painters are often deceived in making waters, in which they make the water see what the man sees; but the water sees the object from one side and the man sees it from the other; and it often happens that the painter will see a thing from above and the water sees it from below; and thus one and the same body is seen in front and behind, above and below, because the water shows it the image of the object in one way and the eye sees it in another.

b g - r d o - c a p - f a - e n

Every motion of water makes flux and reflux in every part of the river where the velocity of its course is retarded. It is proved because where the course of the water is more steep it is swifter, and where it is more level it is more slowed. Therefore the level pool receives more water than it discharges, for which reason the water of such a pool must rise to such a height that its weight overcomes the water that drives it. And then such driven water descends from its height around the base of the aforesaid hill, and that part which descends against the said current engorges that current in such a way that the upper water of the same current is retarded until the succeeding water superabounds and overcomes the reflux and generates a new flux.

8 7. The percussion given upon an object that flees before the striker is of little force. And if it is given upon an object that moves against the striker, it will be of great strength. But when such percussion is given upon an object that does not move, then that object receives a blow of middling power. The blow given upon a swift wheel will be of little force, and especially if the motion of the striker and of the wheel are equal.