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This compound mechanism was used to control the valves in steam engines through a link feedback between the rotary motion of the engine flywheel, and the slidiing motion of the valve. The timing of the steam valve opening and closing, controls both the speed and the reversing motion of the engine. The operator control link is the vertical lever on the right. Two eccentrics provide motion to a horizontal lever that in turn drives the vertical motion of the valve rod. This mechanism is named after the railroad pioneer George Stephenson (1781-1848). Another valve control mechanism can be seen in the Redtenbacher model UK087. A kinematic description of this linkage can be found in the 1893 English translation of Reuleaux's "The Constructor".