Pushing versus pulling; the force of screws and wedges (f. 81r)
With perspective sketches of wooden framed devices.
The left page (folio 91v) is blank; the right page (folio 81r) opens with a paragraph on pushing and pulling as equal powers acting through mediums of different resistance. Leonardo observes that cords can only pull, water in its courses can only push, while wood and iron can do both, as in wedges and screws. He argues that a screw is stronger in pulling than in pushing, because pulling straightens it while pushing bends and may break it. Below the text are perspective sketches of wooden framed devices - trestle and screw-fitted structures, one with a large studded diagonal panel.
On this page
Pushing and pulling are equal powers
Pushing and pulling are of equal power, but the mediums in which they act have different resistances. Cords are apt only to pull and never to push with great force; waters in their simple courses push but do not pull. Wood and iron can do both.
A screw pulls more strongly than it pushes
The power of wedges and screws shows how wood and iron both push and pull. A screw is stronger in pulling than in pushing: pulled, the more it is drawn the straighter it grows; pushed, the more it is driven the more it curves and often breaks - which never happens when pulling, especially in a screw of good thickness.
Perspective studies of wooden framed devices
Beneath the text are pen sketches of built wooden structures seen in perspective: trestle frames, a threaded screw-fitted assembly, and a large braced panel with a studded diagonal face. They set the argument about screws and wedges against real carpentered mechanisms. Further untranscribed annotation appears among the drawings.