Cataract (beam engine)

A cataract was a speed governing device used for single-acting beam engines, particularly (though not exclusively) Cornish engines. The earlier atmospheric engines also used cataracts, but these worked on a different principle.

The earlier type of cataract was a form of water clock: a 'tumbling cup' was gradually filled with water. When it reached a certain level, the water caused the cup to tip over, which (via a chain link to a lever) opened the injection cock to start the working cycle. After each return cycle, the piston would remain at the top of the cylinder until the cup had refilled (the number of strokes per minute could be adjusted by varying the flow of water into the cup).

In its later form, the cataract was 'a kind of hydraulic governor, consisting of a plunger-pump with a reservoir attached': the plunger gradually descended, which (by way of a release-mechanism) delayed the opening of the steam and exhaust valves to the main cylinder; in this way, the cataract controlled the interval between successive strokes of the engine.

In either case, the cataract is distinctly different from the centrifugal governor, in that it does not control the speed of the engine's stroke, but rather the timing between strokes.