Friction, counterweight and the elements of machines
Extended justification of the lever-friction rule and a note toward a treatise on mechanical elements
The verso continues the analysis of friction (confregazione) and the counterweight needed to balance it on a lever, restating and defending the rule of multiplying by four, subtracting one, and restoring the unit. Leonardo answers an adversary who proposes simply taking a quarter of the weight, showing that this leads to an infinite regress and that his own rule stands. He defines friction as an accidental power with no body of its own, born of violent motion, and opens a heading on 'mechanical elements' promising a treatise on weight proportioned to the moving power and the resistance of the medium. Small balance-and-pulley diagrams with weights fill the right margin.
On this page
Why the 'subtract one, restore the unit' rule holds
Multiply the lever's length by 4 as often as it holds the counter-lever, since friction resists at a quarter of the rubbed weight, then subtract one; keep only the name of the unit, not its value, and divide by the remaining 3. Against the adversary who would take a plain quarter, this only raises the weight and friction toward infinity, so the first rule stands.
Friction as an accidental power without body
The rubbed bodies are marked a b. Friction, though applied to them, has in itself no body but is an accidental power born of the violent motion of a movable body upon a stable or movable one. Its unit is divided by 3 to clothe that third and make it the quarter of the weight that clothes it.
Elementi machinali: weight, power and the resisting medium
Under the heading 'Mechanical elements', Leonardo notes that for a weight proportioned to the power that moves it one must consider the resistance of the medium in which it is moved, of which a treatise will be made; against itself a power moves a body in a given time.