STATIONARY ENGINES.

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(2) The cut-off must be rapid. (3) The back pressure must be aminimum. (4) The steam must be admitted into the cylinder atits full boiler pressure until the point of cut-off is reached. In theCorliss diagrams these conditions are strictly fulfilled. The admis-sion of steam is indicated by a nearly perpendicular line, Figs. 198and 199, and the cut-off must, with the means employed, be practi-cally instantaneous. The diagrams exhibit a remarkably small backpressure; this result, along with the constancy of pressure main-tained until the point of cut-off is reached, is accounted for by thelarge area that can be given to the steam and exhaust passages, asthe valves employed are of the whole breadth of the cylinder.

In ordinary engines a large expenditure of power is required tomove the valves; this loss of power is saved in the Corliss engine,as one man with an ordinary starting bar can move the valves ofa IOOO horse-power engine against the full pressure of steam. Asevery part of the engine is readily open to inspection, no difficultyis experienced in examination, and repair of any of the partsrequiring it; but in practice the wear is found to be very slight.The Corliss engine is economical in the matter of fuel, its con-sumption being at the rateof 2^ lbs. per horse power per hour, asproved by experiment,—a result that must go far to recommend itto the favourable notice of manufacturers requiring steam-power.

HIGH AND LOW PRESSURE COMBINED BEAM

ENGINE.

The high and low pressure combined beam engine is much usedwhere great regularity of motion is required, more especially fordriving spinning machinery. This regularity of motion is due tothe steam expanding from the top of the high-pressure to thebottom of the low-pressure cylinders, and vice versa, by which thejerk at the commencement of the stroke of the piston is not somuch feit as in ordinary engines receiving the full force of thesteam on one side of the piston. The example illustrated, Fig. 200,consists of a pair of engines, coupled at right angles, for driving themachinery at the Royal Gun Factory at Woolwich. The diameterof the fly wheel is 22 feet at the pitch line, the breadth of the teeth12 inches, and the pitch 3 inches, gearing into a pinion 4 feet6 inches in diameter.