STEAM ENGINE 343 tinuing the motion of the piston during that portion of its stroke which is performed after expansion has begun. The direct-acting steam pump is sometimes used as a pumping engine. (See PUMP.) The compound pumping engine has been recently adopted with great success. FIG. 4. Leavitt's Pumping Engine. One of the most efficient forms is that designed by E. D. Leavitt, jr., for the Lynn (Mass.) water works, and shown in fig. 4. The two cylinders, A and B, are placed one on each side the centre of the beam D, and are so inclined that they may be coupled to opposite ends of it, while their lower ends are placed close to- gether. At their upper ends a valve is placed at each end of the connecting steam pipe. At their lower ends a single valve serves as ex- haust valve to the high-pressure and as steam valve to the low-pressure cylinder. The pis- tons move in opposite directions, and steam is exhausted from the high-pressure cylinder A directly into the nearer end of the low-pressure cylinder B. The pump, E, of the " Thames-Dit- ton" or "bucket and plunger" variety, takes a full supply of water on the down stroke, and discharges half when rising and half when de- scending again. The duty of this engine is reported by a board of engineers as 103,923,215 foot pounds for every 100 Ibs. .of coal burned. The duty of a moderately good engine is usual- ly considered to be from 60 to TO millions. This engine has steam cylinders of 17"^ and 36 in. diameter respectively, with a stroke of 7 ft. The pump had a capacity of about 195 gal- lons, and delivered 96 per cent. Steam was carried at a pressure of 75 Ibs. above the at- mosphere, and was expanded about ten times. Plain horizontal tubular boilers were used, evaporating 8'58 Ibs. of water from 98 F. per pound of coal. The stationary steam engine has a great variety of forms. Since compact- ness and lightness are not as essential as in portable, locomotive, and marine engines, the parts are arranged with a view simply to secu- ring efficiency, and the design is determined by circumstances. It was formerly usual to adopt the condensing engine in mills and wherever a stationary ^ engine was required. In Europe generally, "and to some extent in the United States, where a supply of condensing water is obtainable, condensing engines and moderate steam pressures are still employed. But this engine is gradually becoming superseded by the high-pressure condensing engine, with con- siderable expansion, and with an expansion gear in which the point of cut-off is determined by the governor. The best known engine of this class is the Corliss engine, which is very extensively used in the United States, and which has been copied very generally by Euro- pean builders. Fig. 5 represents the Corliss engine as built in the United States by Harris. The horizontal steam cylinder is bolted firmly to the end of the frame, which is so formed as to transmit the strain to the main journal with the greatest directness. The frame carries the guides for the cross head, which are both in the same vertical plane. The valves are four in number, a steam and an exhaust valve be- ing placed at each end of the steam cylinder. Short steam passages are thus secured, and this diminution of clearance is a source of some economy. Both sets of valves are driven by an eccentric operating a disk or wrist plate, which vibrates on a pin projecting from the cylinder. Short links reaching from this wrist plate to the several valves move them with a peculiarly varying motion, opening and closing them rapidly, and moving them quite slowly when the port is either nearly open or almost closed. This effect is ingeniously secured by so placing the pins on the wrist plate that their FIG. 5. Corliss Engine: line of motion becomes nearly transverse to the direction of the valve links when the limit of movement is approached. The links con- necting the wrist plate with the arms moving the steam valves have catches at their extremi- ties, which are disengaged by coming in con-
