The man-treading wheel: body weight versus force in hoists
Walking outside the wheel at c lifts far more: 200 lb raises 1200 lb at a
A large man-treading wheel is drawn with a weight hanging from it, illustrated as the hauling device in which a man most uses his own body. Leonardo praises the architects who chose to raise loads by a man's own weight rather than by his force, but argues it would be far better to have men walk on the outside of the wheel rather than inside, since at the outer point c the leverage is much greater than at the inner point d. Working the proportions, he finds that 200 pounds (a man's weight) at d lifts 600 at a, while the same 200 pounds placed at c would raise or resist 1200 pounds at a.
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Why a man's own weight lifts more than his force
Many architects chose to pull their loads by the man's own weight rather than his force, judging well: it is far easier for a man to move himself on his own feet than, in equal time and motion, to move half a man by force, and the load is more durable and more secure. Leonardo endorses the choice while proposing to improve it.
Walk outside the wheel at c: 200 lb resists 1200 lb at a
If a man walks inside the wheel his weight acts at d, where d b is triple to b a, so 200 pounds at d lifts 600 at a. But if he walks outside at c, whose distance from the pole is 7 times b a, then 200 pounds at c will raise or resist 1200 pounds at a, showing how great the advantage of the outer track is. The reasoning cites the 20th of the 2nd book.
The hauling device that most uses the man's own body
The large treadwheel is introduced as the one that, among pulling devices of equal motion and time, most makes a man avail himself of his own person rather than of any of the others.
Treadwheel hoists as used by builders to raise loads
The whole discussion is framed around the practice of architects who built man-powered wheels to lift building loads, contrasting men made to walk inside the wheel with the more advantageous arrangement of walking on the outside.