Jack teeth, dragged loads, and crossbow springs
Strength of forged teeth; centre of gravity in traction; 200-pound springs
Folio 85v gathers several mechanical notes. One explains that teeth cut on the face (end) of an iron rod are far stronger than those cut crosswise, so the teeth of good jacks (martinetti) are forged that way. Another returns to traction, arguing that a dragged rope or body only matches its mover's speed once its central line and centre of gravity are aligned with the line of force. Lower diagrams treat springs bent into arcs over pulleys: a spring holding two cords of 100 pounds can raise 200 with both sides but only 100 if one side is fixed, and the same reasoning is applied to a crossbow's spring, whose force is shown to make an impossible motion 'by the 5th of the 2nd.' Small figures are annotated 'in motion' and 'beginning of motion.'
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Teeth of jacks are stronger cut on the rod's face
A tooth made on the end (face) of an iron rod is much stronger than one made across it, so the teeth of good jacks are taken from the face of the rod, fixed to a crosswise rod, and only then cut. The rods gain this strength at the forge where they are first made.
Dragging a rope: aligning the centre of gravity
A dragged rope or body never moves uniformly with its mover until it is first straightened to the line of the one who pulls, setting the centre of its gravity by the shortest distance to its mover. Only then does the dragged thing move equally with its mover.
A spring raising 200 pounds from its two sides
The cord a holds 100 pounds and the cord b another 100, of the 200 hung. In extending itself the spring can raise 200 pounds with its two sides; but if one side is fixed, the other raises no more than 100.
The force of a crossbow spring and an impossible motion
The cords c and d each hold 100; a crossbow force of 200 pounds moves 200 pounds one arm's length. Were arm c fixed, arm d would move those 200 pounds two arms' lengths — which, by the 5th of the 2nd, is impossible.