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.

The typical installation of a house-built beam engine spanned four floors. The cylinder and the engine driver's usual working position were located in the 'bottom chamber', approximately at ground level. Above this were the 'middle chamber', with the cylinder top cover and 'top nozzle' (the upper valve chest), and above that the 'top chamber' or beam chamber. The earlier type of cataracts, used on atmospheric engines, were located on the middle floor, above the valve gear and connected to it via a chain. The later type, used with Cornish engines, were positioned at a lower level and connected to the valve gear via rods and underfloor linkages.

The valve gear (or 'working gear') of a Cornish engine is based on the plug rod. This is a vertical rod, hung from the beam, and moving in parallel to the piston. Adjustable tappets are attached to this rod. These tappets strike long curved iron levers or 'horns' that are carried on three horizontal shafts or 'arbors'. Each arbor works one of the engine's valves. For the Cornish cycles, these valves are the upper steam inlet to the top of the cylinder, the equilibrium valve that links upper and lower portions of the cylinder, and the lower exhaust and condensing water injection valves, which share an arbor.

By means of a cataract, these engines could be made to run intermittently: making a single stroke, then stopping and waiting for the valves to be released (by the cataract) to begin the next stroke. The speed of each power stroke was a feature of the engine and was not easily varied, but there was no need for the engines to run continuously, stroke after stroke. This was a direct contrast to the rotative beam engine, and the rotary nature of almost all other steam engines.

Use of a cataract could allow an engine to be operated at only a third of its ungoverned speed.^{[failed verification]} When pumping load was variable, cataracts could also be connected and disconnected as required, allowing the engine to work at full speed for a period and then stopped in between.^{[failed verification]}