348 STEAM ENGINE it caught there on the return of the piston. The trunk engine, in which the connecting rod is attached directly to the piston and vi- hrates within a trunk or cylinder secured to the piston, moving with it, and extending out- side the cylinder, like an immense hollow piston rod, is frequently used in the British navy. It has rarely been adopted in the United States. In nearly all steam vessels which have been built for the merchant service recently, and in some naval vessels, the compound engine has been adopted. Figs. 13 and 14 represent the usual form of this engine. Here A A, A' A' are the small and the large, or the high- pressure and the low-pressure cylinders re- spectively. B B' are the valve chests. COG is the condenser, which is invariably a surface condenser. The condensing water is sometimes directed around the tubes contained within the casing C 0, while the steam is exhausted around them and among them, and sometimes the steam is condensed within the tubes, while the injection water which is sent into the con- denser to produce condensation passes around the exterior of the tubes. In either case, the tubes are usually of small diameter, varying from five eighths to half an inch, and in length from four to seven feet. The extent of heat- ing surface is usually from one half to three fourths that of the heating surface of the boilers. The air and circulating pumps, D D, are placed on the lower part of the conden- ser casting, and are operated by a crank on the main shaft at E' ; or they are sometimes placed as in the style of engine last described, and driven by a beam worked by the cross bide EWaliou. FIGS. 18 and 14. Compound Marine Engine. head. The piston rods are guided by the cross heads X X working in slipper guides T T, and to these cross heads are attached the con- necting rods 1 1, driving the cranks Y Y. The cranks are now usually set at right angles ; in some engines this angle is increased to 120, or even 180. Where it is arranged as here shown, an intermediate reservoir, E R, is placed between the two cylinders to prevent the excessive variations of pressure that would otherwise accompany the varying relative mo- tions of the pistons, as the steam passes from the high-pressure to the low-pressure cylinder. Steam from the boilers enters the high-pres- sure steam chest S, and is admitted by the steam valve alternately above and below the piston as usual. The exhaust steam ig conduct- ed through the exhaust passage around into the reservoir R, whence it is taken by the low-pres- sure cylinder, precisely as the smaller cylinder drew its steam from the boiler. From the large or low-pressure cylinder the steam is exhaust- ed into the condenser. The valve gear is usu- ally a Stephenson link, L, the position ol which is determined, and the reversal of which is accomplished, by a hand wheel U and screw P, which, by the bell crank N M, are attached to the link L L. The Screw. Screw steamers are far more efficient than paddle-wheel ves- sels, not only because the screw is a better in- strument of propulsion, but because it permits the use of more efficient machinery, and espe- cially because it utilizes a large amount of en- ergy entirely wasted with the paddle wheel in putting in motion the water, which latter, com- ing into contact with the hull of the vessel, is set in motion by friction, and the following current is left behind to expend its vis viva by
