REGULATION OF STEAM.

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the arc, or travel of the eccentric centre, equals the opening of valveminus one half of the lead. This may be taken in all cases to bepractically correct when the valve is worked by direct means fromthe eccentric, but when levers and rocking-shafts are interposedbetween the eccentric and the valve, the versed sine will be moreor less, as the case may be; thus supposing the eccentric rod leveris longer than the one on the rocking-shaft for the valve, the versedsine must be greater than for a direct motion, and vice versa; butin all cases the throw of the eccentric, or the circle described bythe eccentric centre or pin, the lap of the valve, &c., must be foundin the first place to suit the cut off in the cylinder, as for directmotion, and the levers proportioned accordingly.

Sometimes the valves for land-engines are made double-ported;this dass of valve is simply adopted to reduce the “ throw” of theeccentric, and secure rapid admission and cut off for the steam;thus with ports of the same length as for single-ported arrange-ments, we can, by having double or more ports, increase the areafor the entrance and exit of the steam,—a matter of importancewhen a high rate of piston-speed is adopted.

When the valve is made large, it is necessary to relieve it fromthe steam-pressure that tends to force it against the cylinder face.There are a variety of plans for doing so: some engineers introducea piston working in a short cylinder, placed in the valve-casingcover, connecting the piston to the valve by means of a vibratinglink; by this plan it is lifted as it were off the face, thereby reduc-ing the friction, as the valve is partly suspended, and consequentlymore easily moved. Others have introduced a flexible plate, Con-necting it to the valve in like manner, the spring of the plate actingin a similar way as the piston arrangement; both are acted on bythe steam in the valve-casing, pulling the valve from the face, ofcourse, according to the amount of area exposed. However, sucharrangements are not to be relied on, and the end in view is attainedby simpler contrivances. The usual method, now in extensive use,is by recessing two rings in the valve-casing cover, and pressingthem against a planed face, on the back of the slide-valve, by anumber of set screws, placed around, central with the recess; theseset bolts press against a ring of iron in the first place, then a plaitedgasket is interposed between this ring and the brass ring, whichpresses on the back of the slide-valve, thus making the area coveredby the ring perfectly free from steam; the valve is by this

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means