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. 137 and 138 represent the usual form of this engine. Here _A A_, _B B_ are the small and the large, or the high-pressure and the low-pressure cylinders respectively. _C C_ are the valve-chests. _G G_ is the condenser, which is invariably a surface-condenser. The condensing water is sometimes directed around the tubes contained within the casing, _G G_, 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 condenser 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 heating-surface is usually from one-half to three-fourths that of the heating-surface of the boilers. The air and circulating pumps are placed on the lower part of the condenser-casting, and are operated by a crank on the main shaft at _N_; or they are sometimes placed as in the style of engine last described, and driven by a beam worked by the cross-head. The piston-rods, _T S_, are guided by the cross-heads, _V V_, working in slipper-guides, and to these cross-heads are attached the connecting-rods, _X X_, driving the cranks, _M M_. 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, _P O_, is placed between the two cylinders to prevent the excessive variations of pressure that would otherwise accompany the varying relative motions of the pistons, as the steam passes from the high-pressure to the low-pressure cylinder. Steam from the boilers enters the high-pressure steam-chest, _X_, and is admitted by the steam-valve alternately above and below the piston as usual. The exhaust steam is conducted through the exhaust passage around into the reservoir, _P_, whence it it is taken by the low-pressure cylinder, precisely as the smaller cylinder drew its steam from the boiler. From the large or low-pressure cylinder the steam is exhausted into the condenser. The valve-gear is usually a Stephenson link, _g e_, the position of which is determined, and the reversal of which is accomplished, by a hand-wheel, _o_, and screw, _m n p_, which, by the bell-crank, _k i_, are attached to the link, _g e_. The "box-framing" forms also the hot-well. The surface-condenser is cleared by a single-acting air-pump, inside the frame, at _T_. The feed-pump and the bilge-pumps are driven from the cross-head of the air-pump. [Illustration: John Elder.]