A great torsion sling-engine driven by worm screws
Twin twisted-cord drums, sizing of the arms and stone, and the screws A and B
A powerful stone-throwing engine dominates the sheet: two great drums, wound by endless (worm) screws, twist skeins of cord to store energy that is loosed through the sling arms. Leonardo captions the operating sequence — turn the wheels to twist the cords, then set the sling-staves — and specifies dimensions, with the crossbow arms opening to twenty braccia, sling-staves of ten braccia and a stone of fifty pounds. A detail below identifies the two worm screws A and B, struck with a mallet, that turn the sling wheels, and he notes that all these parts belong to one and the same engine.
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Twist the cords, then set the sling-staves
Leonardo instructs that once the wheels have been turned to twist the cords, the staves of the slings are put into place. The stored torsion is what will hurl the projectile.
Dimensions of the engine and its shot
A note states that all these parts form one and the same engine, then gives sizes: the crossbow arms open to 20 braccia, the sling-staves measure 10 braccia, and the stone weighs 50 pounds. The figures fix the scale of the throwing machine.
Worm screws A and B turn the sling wheels
A lower detail labels two endless (worm) screws, A and B, and marks where the mallet strikes. Leonardo explains that these two screws turn the wheels of the slings, winding the torsion cords.