a f, 8 32, m b, 32, q 32, 32 8, p 32 n

When you turn the wheel q, which has 32 teeth, the two wheels b and p will also turn, each of which likewise has 32 teeth on its head [rim] and 32 on the flat of the half-wheel. And because the 32 [teeth] above, when moved by the wheel q, turn — let us say to the left — both wheels turn likewise to the left. And when the 32 teeth of the lower wheel come into contact with the wheel q, these two wheels turn in the contrary course, that is to the right. Now because the two wheels pivoted at a f turn to the left, it is necessary that they keep the impetus they began with, and this they always follow. But the pinions m n turn on their poles and go back, and let the wheels moved by them run on to the course begun, together with their poles. And they go back with the wheels p b, by which they are now pushed and now drawn back.

The weight that is given to the screw must be attached to that screw in such a way that it loads it equally, that is, so that the center of the weight is beneath the center of the force, or rather the center of the power above the center of the resistance.

But if the center of the screw is outside the center of the weight moved by that screw, the whole part of the screw that lies in its nut will do as the figure below, that is, from the middle of the nut upward the screw will press toward the weight. This force will press so much into its nut that its motion will become very difficult. And from the middle of that nut downward the screw will press on the side opposite the weight, which force will make the motion no less laborious than it would be from the middle upward.

This is demonstrated in the 20th [proposition] of the second, how the center of every weight desires to stay beneath the center of its support. Therefore the weight hung at c would wish to be at m, and truly in such a position it would move, were it not that the part from the pole upward presses at b, and from the pole downward presses contrariwise, that is at a.

If the screw is loaded at the middle of its thickness and is not supported in its nut at that center, its motion will become difficult, as is demonstrated in the pole r S, which is outside the center of the nut.

b, a, c, m

r, s