By means of a crank or similar device, illustrated in common foot-power machinery like the turning-lathe, scroll-saw, or sewing-machine, the alternating rectilinear motion of the piston-rod is changed into a continuous rotary motion. A circular shaft is thus given a revolution for every to-and-fro movement of the piston. This shaft generally carries an eccentric for working the sliding-valve rod. The eccentric consists of a circular piece of metal, rigidly attached to the shaft of the engine in such a position that the centre of the piece does not coincide with the centre of the shaft. The eccentric turns within a collar, which is fastened to the frame. Every turn of the shaft moves the eccentric with its collar and the frame, backward and forward into the two positions. The point a may be fastened directly to the sliding-valve rod or through the agency of the bent lever, as the circumstances of the case render more desirable.