Pushing versus pulling in cords, water, wedges and screws (ff. 91v-81r)
Why screws pull more strongly than they push, drawn beside a large timber machine
A transmitted-light spread whose transcribed note on folio 81r argues that pushing and pulling are equal in power but that different media resist them differently: cords can only pull, water in its course can only push, while wood and iron can do both, as seen in wedges and screws. Leonardo adds that a screw is stronger in pulling than in pushing, because pulling straightens it while pushing curves and can break it, especially a thin one. The facing leaf and margins carry much further mirror-script and figures that the bundle does not transcribe, and a large timber-frame machine with a graduated fan-shaped sector is drawn at lower right. Circular percussion-and-rebound figures appear at lower left.
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Pushing and pulling are equal but the medium differs
Pushing and pulling are of equal power, but the media in which they act resist differently. Cords are apt only to pull, waters in their courses only to push, while wood and iron can both push and pull.
A screw is stronger pulling than pushing
The power of wedges and screws shows wood and iron pushing and pulling, but a screw has greater force in pulling than pushing: pulling makes it straighter, while pushing curves it more and often breaks it, especially when it is not of good thickness.