THE INDICATOR DIAGRAM.

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theoretical curve as when the valves are suddenly shut off with acam motion. To ascertain the pressure of the steam in the cylin-der, as likewise how the valve acts in its admission, recourse mustbe had to a very simple contrivance, termed theindicator, or miniature cylinder and piston, similarto that of the engine itself. This instrument, inits original form, has a small cylinder, fitted witha piston and rod. On the top of the cylinder alight spiral Steel spring was placed, fixed to thecylinder at one end and to the piston rod at theother end, a pencil fastened to the piston rodmoving along with it. The steam pressure raisesthe piston above a line termed the atmosphericline, and when there is a vacuum in the cylinderthe piston is depressed below the atmosphericline. Thus the rising and falling of the pistondenotes in the first instance the steam pressureabove the atmosphere, and secondly the vacuumbelow it A roller is placed alongside, fittedwith a pulley, having a cord attached to it; bypulling the cord the roller rotates, and by slack-ening the cord it returns to its original position,being moved by a spring. The cord is fastenedto some reciprocating part of the engine, andby a reducing lever motion is imparted to the A,Roiier. Puiiey and cord,roller; thus the full stroke of the piston is taken pendi.' ypiug-tap.in miniature, the motion being simply changedfrom reciprocating action to that of a rotary motion. A roll ofpaper is fastened round the roller, and secured with a clip. Thepencil fastened to the piston rod is made to press on the paperwith a slight spring. The cord is moved by hand, and the pencilmarks a straight line on the paper, termed the atmospheric line.When the engine is in full working order this line never varies untilthe steam is admitted by a hand tap to the under side of thepiston, which instantly rises, distending the spiral spring accord-ing to the pressure of the steam. The roller being in motion, afigure is traced on the paper with the pencil, delineating the pres-sure on the piston of the engine, above the atmospheric line, aslikewise, on the return stroke, marking the vacuum in the cylinderbelow the line, the spring being compressed by the pressure of the

Fig. 75,—M'Naught’sIndicator,