This instrument m n, being toothed over only an eighth part of its circle, takes up less run than if it were toothed over the half part of that circle. Hence with less difficulty it leaves the first motion and takes up another in the opposite direction, provided that it have as many teeth in the eighth part as it would have in the half, and that suffices.

m, 8, 32, 32, 8, 32 32, n

I have learned from the potters how the impetus of their wheels retains in itself something of the nature of that motion which the potters have given them. And for this reason I have judged it possible to bring it about that the water which makes one mill grind may make it grind two. Seeing that, when the 32 teeth of a half wheel have made a pinion of 8 teeth turn round 4 times, that pinion can of itself give 4 more turns, at almost the same speed as when it was driven by its wheel. And in the time that this wheel leaves the one pinion, it takes up the other, as you see in the drawing.

If I had thus made two half-toothed wheels on the spindle, or shaft, m n, as I did on the other sheet, certainly, because of the impetus these wheels take on in giving that half turn, they would have great difficulty in turning back, as happens in the said instrument, and they would stop somewhat before they could take up the second motion. Hence I have thought it better that such wheels give a sixth of a turn rather than a half, because in this sixth motion of the circle the wheel, or lever, takes up less run than in a half circle, whereby it more easily takes up the contrary motion, and it is better.

When you find the millstone at rest, and you give it 3 turns and then let it go, it then continues a motion of a certain speed. But if you return to it, after it has of itself given three turns, to give it another 3, you will give it much greater motion than you gave it before. For at first you found it without any motion, and afterwards you found it already having of itself the motion of the impetus. For which reason the second 3 turns that it gives of itself will be much faster than the first that it gave of itself. And if you return to give it another 3 turns and then let it continue of itself its other 3, these will be as much faster than the second as the second were faster than the first. And so always, up to a certain time, the speed will increase.