STEAM-ENGINE

A machine in which the me. chanical power developed in the evaporation of water is rendered available as a moving power. Steam engines vary much in magnitude, form, and proportions, as well as in the details of the machinery by which the power of the steam is adapted as a prime mover. We shall first explain those forms of steam-engine most generally used in the arts and manufactures, and shall afterwards give a succinct account of the history of the invention and progre ive improvement of this machine. The forms of engine, and the mode of its application to transport by land and water, have been consigned to separate articles. See Locomotive Engine, Railv^ays, and Steam Navigation. Double-acting Condensing Steam Engine. — This form of engine is that which is almost invariably used as a moving power in all manufactures in this country. It consists of a cylinder represented in section at C, in which a moveable piston P is driven upwards and downwards by the force of steam supplied by a boiler placed near the engine. See Steam Boiler. (1.) JThis piston gives motion to a working beam H/, which, by means of a heavy bar O, called a connecting rod, moves a.Jly wheel and crank, from which the machinery to be worked directly receives its motion. Steam is supplied from the boiler to. the cylinder by the steam pipe S. The throttle-valve T in that pipe, near the cylinder, is regulated by a system of levers connected with the governor Q. This governor is an apparatus consisting of two heavy balls attached to the ends of rods which are kept revolving on a vertical shaft by a cord or band, or by a train of cogged wheels connected witli the fly-wheel. The velocity with which the balls of the governor revolve is therefore always proportional to that of the flywheel, and of the machinery driven by it. If by reason of too rapid a supply of steam an undue speed be imparted to the fly-wheel, the balls are whirled round with a corresponding velocity; and by reason of their centrifugal force they recede from the vertical spindle round which they turn, and acting thereby on the system of levers which connect them witli the throttle-valve T, they partially close the latter, check or diminish the supply of steam to the cylinder, and moderate the velocity of the machine. If, on the other hand, the motion of the engine be slower than is requisite, owing to a deficient sup  [s. 1173]