Two devices for slow reciprocating motion
A half-toothed wheel drives a rack back and forth; a second mechanism does the same along a screw.
Under the heading of movements made with slowness of time, Leonardo draws two mechanisms that turn continuous rotation into back-and-forth motion. In the first, turning the handle o p in one direction alone makes the rack m n travel now one way, now the other, because the wheel's middle ring is toothed for only half its circumference (6 teeth) and engages a pinion of 6 spindles. The second device produces the same reciprocation by sliding a tube o p along the length of a screw f t, and he notes that the two motions are equal in both power and movement.
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Half-toothed wheel driving the rack m n
Turning the handle o p in one direction only makes the rack m n go now this way and now that. The reversal occurs because the middle circle of the toothed wheel is toothed for only half its circumference, with 6 teeth, engaging a pinion of 6 spindles; the pinion works better shorter and not halved.
Second device: tube o p travelling along the screw f t
A second mechanism performs the same office: the tube o p moves this way and that along the length of the screw f t, just as the rack m n does above. Leonardo notes that the two motions are equal in power as in movement.