A contrivance for transmitting rotary motion to a movable pulley. The pulley at the bottom of the figure is the movable one ; if this pulley were raised or depressed, the belt would be slackened or tightened accordingly. In order to keep a uniform tension on the belt, a pulley, A, carried in a frame sliding between guides (not shown), hangs from a rope passing over the two guide-pulleys, B, B, and is acted upon by the balance weight, C, in such manner as to produce the desired result.