This instrument, by turning the handle f, drives forward and makes return backward the axle m n. Now use it for your own purposes, for it works well. And the belt that moves this axle is fastened between the one lantern-pinion and the other, behind, nailed to the arbor of those pinions.

f, m n

This is the best motion there is among the double motions, because always, in every part of it, it is equal. And know that the large wheel that moves the two pinions is exactly half-toothed, with 12 teeth that move those pinions. And moreover each of these 2 pinions is made of 12 spindles apiece. But take care, you assembler of instruments, that you never make such members in places where they must go too fast, because in taking flight or impetus in one direction, and if that wheel then had suddenly to return backward, it would be in danger of breaking its teeth. And one can also, in place of the axle that goes to and fro, make it with an iron ladder [rack], as is shown here, and it will be more lasting and less variable.