The Law of the Block-and-Tackle
How the count of cords sets the ratio of moving weight to load lifted
Leonardo states the rule of the pulley-tackle: the ratio of the cords that pull the load to those that support it fixes the ratio of the moving weight to the weight moved. He adds that the number of sheaves in the blocks tells how many times the mover resists itself when it acts from one side only. A block-and-tackle is drawn at the upper right, its cords running over three sheaves to a weight marked 1 4 1.
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Cords that pull versus cords that sustain
The proportion the number of pulling cords bears to the sustaining cords is the same proportion the moving weight bears to the weight moved. Likewise the cords passing through the load's own block set the ratio of the suspended load to the weight that holds it.
Sheaves of the tackle and self-resistance of the mover
As many as are the wheels of the tackle-blocks, so many times does the mover resist itself when it works from one single side. The drawing shows the tackle in action, its cords led over three sheaves at the top to a weight below marked 1 4 1.