THE INDICATOR DIAGRAM.

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proportionately less also; consequently this reduction is obviated byusing the 15 1b. spring to suit the pressure in the cylinder of theengine. For compound engines varying springs are necessary; butit is considered that for ordinary marine engines, and in all engineswhere the Variation of the steam pressure is not great, that onescale, and spring to suit, is quite sufficient, as a variety only createsconfusion. To show this more fully, take two diagrams from marineengines of the compound type. The full figure shows the behaviourof the steam in the high-pressure cylinder, and the lesser diagramsteam in the low-pressure cylinder. The same scale is used forboth (the diagrams being reduced from the original). It will be seenthat the diagram for the low-pressure cylinder is very lean, whilethat for the high-pressure cylinder is well defined; and to make theformer bolder it is evident that a different spring and scale mustbe adopted. This would improve the appearance of the low-pressurediagram, but were the same scale and spring adopted for the high-pressure diagram it would make the figure too large. The readingof the high-pressure diagram is somewhat different from ordinaryhigh-pressure engines. The diagram in such cases would show thepressure,, commencing from the atmospheric line, while in theexample before us there is a slight back pressure. This is due tothe steam expanding into the large cylinder instead of into theatmosphere, as with ordinary high-pressure engines.

We give examples in which both the high and low pressure cylin-ders have diagrams taken from them. Both of the figures arewell defined, the scale for the high-pressure diagram being doublethat for the low-pressure diagram, or the spring of double the power.Thus it will be seen that it is quite necessary to have two sets ofsprings for combined engines, so that there may not be so great adifference in the diagrams, or that the figure be not too minute inthe one nor too bold in the other. When the operator is takingdiagrams off an engine he generally takes them for both ends on thesame paper, provision being made on the cylinder for doing so, thesmall steam pipes fitted being in communication with both ends ofthe cylinder. Thus the double figures are represented, one for the topor OUT stroke, and another for the IN stroke of the piston. Caremust be taken that the area of the small pipe connecting both endsof the cylinder is of sufficient size, not less than Y inoh > n diameter,so that the full pressure may be conveyed instantly to the piston ofthe indicator. This pipe must be fitted with a hand-tap for each end