Studies on Hair and the Treatise on Painting

On the percussion of water and air

Why air or water, fleeing the site of percussion original: "percussione" — the point of impact where a fluid hits a solid object, does not bend? in an equal? line? but? curves?...

That part of the air which, striking in the middle of the surface of a flat and narrow board, divides into two parts; and why, if such a board be oblique tilted or at an angle, it does not divide into equal parts; and why, if such a board be mobile free to move and, upon the air striking it, it flees, it bends and twists in itself just as a banner does...

If water strikes in the middle of a flat board, the part that strikes it divides into two equal parts, and these parts direct themselves toward the ends of that board, and afterwards they bend back and create two eddies term: revertigini (vortices or eddies). Leonardo uses this to describe the circular, turbulent back-flow of fluids, which turn one toward the other, and each of these eddies shall be of a round shape.

The reflected motion of the water shall be so much more powerful as the incident motion the incoming flow before impact is shorter and straighter; and so, conversely, it shall be so much weaker as it is longer and more oblique...

The water that strikes in the middle of the surface of an immobile board divides into 2 parts, and these 2 parts divide into another 4 parts, of which 2 bend toward the beginning of the incident motion, and 2 bend toward the end of that motion...

If it happens that the water striking the board finds it mobile, and that this board bends or turns in any of its parts, then the water does not divide into equal parts...

The water that strikes an oblique board does not divide into equal parts, but that part which strikes the board closer to its edge shall be shorter and straighter, and that which strikes closer to the middle shall be longer and more curved...