134

MODERN STEAM PRACTICE.

can teil how it is performing its duty. By means of the indicatornoting the steam pressure and vacuum acting on the piston, as wellas the velocity of the piston, at the time of trial, a true estimate ofthe working of the engine is obtained, and thus steam users aresatisfied and disputes avoided.

Some authorities say Watt invented the indicator, others assertthat M'Naught successfully introduced it, although improvementshave since been made by others to suit modern high-speed engines.The long stroke of the piston and spiral spring causing the pencilto, as it were, “jump,” made the diagram very irregulär. To obviatethis defect the stroke of the piston was reduced, and the ränge ofthe pencil multiplied with a lever parallel motion. Certainly theimprovement is very effective, and fully answers the object in view.To suit the varying steam pressures it is found advisable to supplysprings of different degrees of power. Thus we have springs for6o lbs. to the inch, and others 15 lbs. to the inch; and it will thusbe understood that when the 60 lb. spring is used, and the steam

High-pressure Cylinder.

Steam, 45 lbs. Vacuum, 28% inches. Rcvolutions, 61.Indicated horse-power, 29805 h.p. cylinder.

6o8'93

Low-pressure Cylinder.

Atmespktnc

Figs. 78, 79.—Compound Engine Diagrams.

in the cylinders only 15 lbs., that the steam line on the diagramwill only be % inch from the atmospheric line, and with the vacuum