Rustic military bridge of lashed branches and an endless screw
A self-tightening field bridge for an army, and a worm gear whose tooth rises and dips
The upper drawing shows an arched field bridge built for an army from the trees found on riverbanks, its lashed framework forming the "bones" that are then covered with timber, branches, brushwood and clods of earth. Leonardo notes that the more it is loaded the more it binds together, needing no strong abutments. Below, a coiled cylindrical device illustrates an endless screw whose tooth m, driven by the handle n turning the carriage S, is thrust now outside and now within the screw.
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Self-tightening rustic bridge for an army
The framework or "bones" of the bridge is made from riverbank trees, then covered with closer timber, branches, brushwood and clods of earth. The more weight it bears, the more tightly it binds together. Unlike ordinary bridges it needs no strong abutments.
Endless screw with rising-and-falling tooth
Turning the screw by the handle n makes the carriage S revolve, so the tooth m is thrust now outside the screw and now descends within it. As the screw turns continuously the tooth keeps travelling around, alternately inside and outside. It is Leonardo's demonstration of a worm-gear action.