The large Cruquius engine is started manually, but once the condenser vacuum is built up and the engine runs smoothly, the driver can switch to automatic operation. To make the action fully automatic, we need provisions for timing the valve action. This is established by activating the cataracts, these are timers. What they do is at the right moment lift the scoggan levers in order to open the steam, exhaust and equilibrium valve.
The cataract is a cistern filled with water, in which a vertical cylinder (6) stands, with a brass plunger pushed down by a weight. The water in the cylinder can only flow out through a small adjustable opening, a little valve (4), so the plunger will descend only slowly.The cataract plunger is connected to a mechanism of rods and levers which causes a vertical rod (2) to rise as the plunger descends. This rod lifts the exhaust and steam scoggan levers, in that order, after a delay which is determined by the setting of the adjustable little valve on the cataract cylinder. This setting can be adjusted at the central driver's position via the vertical rod (5). During the cycle the cataract plunger must be reset, lifted by a rod (1) to prepare for the next stroke. This is done by an additional slide on the plug rod. Lifting the plunger is easy, because there is also a larger check valve (3) which opens inward to admit water to the cylinder. So the cataract is a timing device, the pauses governed by the cataract can be determined by the setting, so there is an adjustable pause between opening and closing of a valve.
In this way the engine cycle is started automatically, after an adjustable delay. The setting of the delay determines the engine speed, i.e. the number of strokes per minute. But a second adjustable pause was desirable, between the end of the steam stroke and the start of the pump stroke. In Cruquius this was needed to give the large pump clacks time to close. Such a delay was easily provided: just actuate the scoggan lever on the equilibrium arbor with a second cataract.
The cataract has a modern descendant, which many people have one or more examples of in their homes: the automatic door-closer. This uses oil instead of water, and a spring instead of a weight, but that is where the differences end.