REGULATION OF STEAM.

127

pressure of the steam acting on the valve and the pressure impartedb y the springs, which can be adjusted to the greatest nicety. This

From, Boiler

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Figs. 6 g, 70.—Slide Valve by the Author.

valve requires no packing rings, and should priming occur thecylinders are instantly relieved from the water. This roller motionshould work easily and with less friction than the arrangementswith packing rings on the back. Should the boiler pressure, too,become higher than the working pressure, this arrangement willact as a safety valve, blowing the steam through the exhaust intothe condenser or into the atmosphere, as with high-pressure engines.

RELIEVING THE SLIDE VALVE FROM BACK PRESSURE.

The double-ported valve for high-pressure engines differs verylittle from those for the marine engine; in fact, the only differenceconsists in making the exhaust ports at each end of the valvesmaller, as likewise the ports in the cylinder may also be reduced inwidth, and when made very small no packing rings are required,neither is it necessary that marine engines should be so fitted, aswith small valves the pressure is not much feit. However, correctlyspeaking, all valves should be relieved from the back pressure, whetherthey are double-ported, or simply the original arrangement, with only