The Crossbow: Blow and Motion Given to the Arrow
A proportional study of how a bow's string imparts force to the shaft
Under the heading 'Crossbow', Leonardo examines the difference between giving an arrow a blow, a continuous motion, or both, according to where along the string's travel the arrow is engaged. A small drawing of a bow with its stock and bolt heads the sheet. The lower text turns to proportion, arguing that if 4 moves 2 then doubling to 8 doubles the speed, and reasoning about which quantity is truly the mover.
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Blow, motion, or both imparted to the arrow
Both blow and motion are delivered at the middle of the string's travel; the blow alone acts at the end of that travel; motion alone is given when the string keeps contact with the arrow throughout the whole stroke. The passage compares these three modes of transferring force.
Crossbow drawn at the head of the sheet
A simple crossbow is sketched at the top right, showing the curved bow, the central stock, and the bolt in place. It illustrates the mechanism whose string action the text analyses.
Doubling the mover: 4, 8 and the excess over 2
If 4 moves 2 in a certain proportion of motion, doubling 4 to 8 moves the 2 twice as fast. Leonardo argues the true mover is the excess (2) over the moved body; doubling gives 8, an excess of 6 over 2, so the mover is triple the first.