Cat′a-ract.
（Steam-engine.) A regulator invented by Smeaton for single-acting steam-engines. The plug-tree in its ascent draws upon a cord, and lifts a piston in a vertical pump-barrel whose foot is submerged in water; a valve at the foot of the barrel admits the water thereto. The up-stroke having ceased, the piston rests upon the water, and a discharge-valve opens. The rate of discharge is regulated by the load on the piston or the size of the aperture. When the plunger passes a certain point, it makes the changes which readmit the steam, the plug-rod having no effect in so doing, as its connection is flexible. By the means of this device, called a cataract, the time of admitting steam is regulated by the flow of a certain quantity of water through an opening, and is entirely independent of the engineer, of the pressure of the steam and other contingencies, provided that sufficient pressure is maintained to run the engine at all. If the boiler steam be at an unusual tension, the stroke may be made faster, but the interval between strokes depends upon the hydraulic device described. See Cornish steam-engine. A modification of this is introduced into marine engines for softening the fall of the expansionvalves. A brass cylinder is filled with water or oil, and fitted with a solid piston connected by a crosshead with the valve-spindle. The fall of the valve is checked and regulated by the escape of the water or oil through a small hole bored for that purpose in the side of the cylinder, the piston of the cataract descending according as the liquid is forced out from before it by the pressure due to the weight of the expansion-valve. See cut-off. [503]
