On the Weights of Pulley-Tackles
Forces in cords over fixed and movable pulleys; the mover's excess and the war sling
Under the heading "On the weights of pulley-tackles," Leonardo analyzes the forces carried by cords running over fixed and movable pulleys, illustrated by lettered diagrams in the right margin (a-c-b; a c b - d e; a b - c d - e). He argues that a mover prevails over its load only by the amount by which it exceeds it, so the cord joining mover and load feels twice the weight of the load, growing without limit as impetus increases. A closing example cites war sling-staves (cacciafrusti) whose cords snap from the furious impetus of the thrown stone, which he attributes to air condensing in front of the projectile; a marginal note continues this thought.
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A mover prevails only by its excess; the cord feels twice the load
Two equal weights hung at opposite ends of a balance resist each other's descent equally. If one moves the other, it is the more powerful, and its cord is the more strained. Leonardo generalizes that no power avails over its resistance except by the part by which it exceeds it, so the cord joining mover to load feels twice the load's weight, and more as impetus grows.
Lettered pulley diagrams: fixed and movable blocks
The right margin carries a sequence of small drawings of a weight held by a cord, a fixed pulley with two weights, and a movable pulley, keyed by letters a, c, b and d, e. The suspended heavy body is described as wholly present along the whole length of its support and in every part of it.
War slings whose cords break from the stone's impetus
As proof that no converse action relieves a cord, Leonardo cites the cacciafrusti, sling-staves used in war to hurl stones at the enemy by hand. Their cord often breaks from the furious impetus of the staff, because the air condenses more in front of the stone than at the staff or cord and resists. A marginal note adds that such condensation can break any very thick cord.