Force and motion of the lever and screw
A lever's power stands to its counter-lever inversely as their motions; the force of the screw.
On f. 94v Leonardo demonstrates that the force of a lever stands to that of its counter-lever as their motions do: since a n is a fifth of the lever n m, the motion a g is a fifth of f m and the power f m is fivefold the power a g. He states the general rule that the powers of lever and counter-lever stand together inversely to their motions, and applies it to the screw, comparing the advance of its nut in one turn with the motion of the lever. Diagrams carry number-series (a doubling ladder and a series running 3, 6, 18, 36, 108, 216, 648, 1296, 3888, 7776, 23328) and a screw-and-lever is drawn at the lower left.
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Power fivefold where motion is one fifth
Since a n is a fifth part of the lever n m, the motion a g is a fifth of the motion f m, and so the power f m is five times the power a g. The circles drawn represent the motions of the ends of the members that move.
General rule: powers inverse to motions
For any lever working with its counter-lever, the ratio of the length of motion of the counter-lever to that of the lever equals the ratio of their motions, so the powers stand together inversely. From this one can read the screw's force by how far it carries its nut in one whole turn.
Number-series along the stepped lever
A stepped lever diagram is labelled with a rising series of numbers (3, 6, 18, 36, 108, 216, 648, 1296, 3888, 7776, 23328) figuring the multiplying forces or motions. A second figure carries a doubling ladder (18 18 9 6, 3 3 1 1/2 1).
Reason of the force of the screw
The power of the lever against the screw's thread stands in the same proportion as the height of that thread has to the length of the lever that moves it. A screw with its lever is drawn at the lower left to illustrate the rule.