This mechanism is a special form of the four-bar linkage (model C-1) used by James Watt to convert the rotary motion of a steam engine balancer arm into approximate straight-line motion of the pump piston in the late 18th century. Watt is often cited for his invention of the separate steam condenser that dramatically increased the efficiency of his engine over the Newcommen steam engine. However, he was very proud of his invention of the straight-line linkage and a painting of him in the National Gallery in London shows him contemplating a drawing of this linkage. Reuleaux’s use of the term 'lemniscate' for this mechanism refers to a special case of the four-bar linkage that will trace out one of the forms of a lemniscate curve, a double-loop curve with a flat, approximate straight-line section.

In addition to Watt's linkage mechanism that created an approximate straight line motion, he also invented a planetary gear mechanism to convert linear motion of the cylinder to rotary motion of the flywheel and a rotating ball governor that controlled the speed of the engine.

Francis Moon 2004-07-01