When you weigh the fall of water to make a rule, weigh it at different heights.

m

a b c d

I find among the Lombards a certain kind of mill which has about 3 braccia of fall. And these mills are placed beneath one and the same line of embankment, at different distances from it, according as necessity constrains them to be set one after another, within the walls of the mill buildings, as you see in the drawing. The first mill, a, runs stronger than any; the slowest is d, it being one and the same water. Reason requires that if the obliquity of the channel of a were in the middle of the embankment, where the water pushes most, that water would drive stronger than any of the others.

I believe these would be better. I have not tested it, but according to opinion.

If the water falls a height of 3 braccia and then strikes the vanes of the wheel, arrange that the exit of the water after that impact does not submerge that part of the wheel which, beyond its axle, must rise out of the water.

Of the figure m. These Lombards make the mouths of their mill water-falls very wide, and it keeps narrowing until it strikes the paddles of the wheels, so that being one braccio and a half at the mouth, it will be less than half a braccio at the foot. And it happens that, the water being a quarter of a braccio high at the mouth, it is about 3/4 high at the foot, so it comes to beat much toward the axle of the wheels of the first motion. It is true they could allege that the water, being thicker, weighs more. But it is quite true that the surface of such a descent of water is less oblique than its bottom. So we may say that the great weight falling little obliquely weighs as much as the small weight that falls very obliquely. Then there is the said disadvantage, namely that the great weight arrives near the axle, where it avails little, because it does not give much force. 4 [continues in the margin]

4 So that if the mouth is one braccio and a half, make its end likewise, that the paddles be also one braccio and wide, so the height of the water will be equal and it will strike the wheel far from its axle.