242

MODERN STEAM PRACTICE.

of the weight off the piston on the end glands, the piston rod actingas a round beam loaded at the middle when the piston is at halfstroke; by this means the action of the piston has not so muchtendency to wear the cylinder oval. Trunks or hollow pipes havebeen introduced in some classes of blowing engines to remedy thisevil, inherent in all horizontal engines of very large size; and whenproperly proportioned they have given good results. The cylinderis cast with brackets ori the bottom half for bolting it down on thebed plates; of course these palms or brackets should be nearly in aline with the strain, or a little below the centre line of the cylinder.Joggles are cast on the bed plate to embrace the brackets, and bythis means end keys are fitted and driven in tightly, thus takingthe shearing strain off the bolts. By attention to these details secureand firm work is obtained, more especially for high-pressure engines,where the succession of shocks from the high steam pressure usedhas a great tendency to shake the cylinder and loosen the fittingsif not properly joggled to the bed plate. Indeed, for fast-goingengines of the high-pressure type, the repeated shocks received onthe end of the cylinder necessitates the use of wrought-iron staysto bind the cylinder and bed plate firmly together; some makerseven casting the cylinder along with the bed plate, which effectuallysecures this object.

The steam valve is an ordinary D one, and should be fitted withpacking rings on the back, bearing on the valve-casing cover. Somemakers, however, prefer a small piston working in a short cylinderplaced on the valve-casing cover, the piston being connected to thevalve by means of a vibrating link. Some such contrivance isabsolutely necessary to take the back pressure off the valve; andthe former method does so by reducing the area on the back ofthe valve—that is to say, the rings are made steam tight, and thesurface exposed to the steam in the casing is reduced; while by thelatter method the valve is drawn, as it were, off the face with acertain force applied by means of the piston, and which beingreceived on the pins of the vibrating link, renders it comparativelyeasy to move the valves by hand, just as any heavy weight is easilymoved when suspended by a chain. The valve casing is sometimescast on the cylinder; but many prefer it to be separate, and securedwith bolts, as in this case the facing for the valve is more readilyplaned, and afterwards scraped to a true surface. The usualstuffing box is cast along with the casing, with brass bush and