A lever turns about its pivot. With the pivot between its two ends they move opposite ways, and with the pivot at one side they move the same way. An end twice as far from the pivot moves twice as far, and pushes with half the force. A rod passes a movement on unchanged, and a bell crank turns it through a right angle. On a wheel the crank pin sits some distance from the centre, and the piston travels twice that distance from one end of its stroke to the other. A stroke is one way along the guide, so a turn of the wheel is two strokes. A cam raises its follower by the longest radius less the shortest. Rods and levers weigh nothing and have no play.
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