THE INDICATOR DIAGRAM.

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and rounds off the diagram, rendering it difficult to define theexact point of cut-off. To illustrate this more fully a diagram isgiven from an engine fitted with Corlis’s valve gear. This species

of gear shuts off the steam fromthe cylinder very quickly. Thesteam pressure in this example was50 lbs. per square inch, the cylin-ders had a diameter of 38 inches,while the speed of the piston was

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Fig. 77.—Indicator Diagram, from Corlis’s Valve Gear.

500 feet per minute. With such high pressure and piston speedthe diagram approaches more closely to the theoretical figure thanthat obtained from a valve actuated by the common eccentric.All manufacturers strive to obtain a diagram from their engines asnear the theoretical curve as possible, not that the engine gives outmore power or indicated measure, but simply that the valve gearis quick and effective. But as the power given off is measured bythe steam pressure and vacuum as taken from the diagram, no onewill dispute that a full figure in the diagram indicates less powerthan a fine figure; on the contrary, more power must be developed,the speed of piston being identical.

The indicator diagram is of great importance to the engineer, asfrom it he can at once teil the steam pressure in the cylinder ascompared with that in the boiler, whether to ascertain the pressureat the cominencement of the stroke, or to discover at what partof the stroke the steam is cut off,—to notice if “ wire-drawing”occurs, or a sharp cut-off, at what part of the stroke the steampressure falls to the atmospheric line, and whether the vacuum isquickly and effectually maintained until the point of compression isreached. By comparing the boiler and cylinder pressures, too, hecan teil what amount of condensation takes place in the pipes, andadopt means to prevent it. In short—and in this lies the greatvalue of the indicator—by a proper diagram taken off the engine he