Locomotive

Locomotive. The form of steam engine best adapted for actual locomotion is not the most efficient from the point of view of the economy of coal. The whole machinery, including furnace, boiler, and engines, must be compressed into small space, and must be arranged so that high speeds shall not endanger its stability. The ordinary type of locomotive has not altered much in general appearance since the general adoption of the "narrow" gauge for railways. The boiler is cylindrical and multitubular, of wrought-iron or steel, terminated at one end by the fire-box and at the other end by the smoke-box. The fire-box is of rectangular section, and holds the grate. It is made of copper, and is so arranged that the water of the boiler can circulate round a considerable portion of its surface. To support- the great pressure that this incurs, the box is heavily stayed with ribs at its crown, the copper tubes acting as stays for the front. The chimney is short, and leads direct from the smoke box. The steam cylinders are arranged in the front either underneath the fire-box or else at each side. The compound-cylinder principle has been successfully employed with locomotives, but is not yet general. The steam pipe gathers steam from near the top of a dome, where the steam is fairly dry; it passes down to the cylinders and ultimately to the exhaust in the smoke-box. The exhaust steam blows out in jets, which force a draught through the furnace and thus compensate for the shortness of the chimney. The slide-valves are regulated by a link motion (q.v.), which serves as a reversing gear. The power of the engine is also modified by a. regulator valve in the steam pipe worked by hand, as is also the link motion. Goods engines used for heavy traffic have the driving-wheels coupled up with two other pairs to avoid slipping on the rails. They are adapted for slow motion, and the driving-wheels are small. Passenger engines have large driving-wheels, and may have another pair of wheels coupled on. Driving-wheels vary in diameter from about five to seven feet. The tender carries fuel and water, and is also constructed with the most powerful brake on the train. [Steam-Engine.]