REGULATION OF STEAM.

IO9

the reversing link. The forward and backward movement of thelink is actuated by a starting-wheel working a worm-wheel and sector,the shaft for carrying the sector-wheel being fitted with levers androds for connecting to it. Although the link motion and gear for theoscillating engine is somewhat complicated, the action of the doubleeccentrics and link is similar to the locomotive; each must be setidentically the same in relation to the main crank of the engine;the only difference necessarily existing is the sector for communi-cating motion to the valves, and at the same time accommodatingitself to the Vibration of the cylinder.

For direct-acting horizontal marine engines the valve now gen-erally adopted is of the multiple-ported type, having the portsdouble or in some cases in triplicate. This valve was introducedso that a large opening of port by valve could be obtained, witha moderate throw of eccentric, thus reducing the size of the eccen-trics and gear into as small a compass as possible. The valveis usually placed on its edge, so that it is worked directly from theengine shaft by double eccentrics and the link motion. Someengineers place the valves on the top of the cylinder, working themwith a System of wheel-gearing similar to the back motion of aturning lathe; the valve spindles are connected by suitable rodsto a revolving crank shaft, then by a series of wheels driven off themain shaft of the engine, so by shifting the position of the two inter-mediate wheels the relative position for the forward and backwardmovement is obtained in relation to the main crank of the engine.This motion, somewhat modified, is considered by some authoritiesas perfect a motion for actuating the slide-valve as can be conceived,although the wheel-gearing is very objectionable, and certainly thelink motion and double eccentrics is better calculated for modernmarine engines. As the valves for large engines are of considerablesize, and consequently the gearing heavy, and although only a portionof the dead weight of the eccentric rods, link, &c., has to be lifted,that, along with the friction, is considerable; and in all cases wherematter is to be actuated by hand, time must be had, and conse-quently power, to do so,—it is therefore necessary to arrange propermechanical appliances for the handling of the valve mechanism ofthe marine engine. The usual hand-wheel, with worm-wheel andsector-wheel, lifting levers and rods, is by far the best plan, as thelink can be held up in any position, so as to work expansively ifrequired; but this is rarely resorted to, as it is a much better arrange-