THE INDICATOR.

234. Q.—­You have already stated that the actual power of an engine is ascertained by an instrument called the indicator, which consists of a small cylinder with a piston moving against a spring, and compressing it to an extent answerable to the pressure of the steam.  Will you explain further the structure and mode of using that instrument?

[Illustration:  Fig. 36]

A.—­The structure of the common form of indicator will be most readily apprehended by a reference to fig. 36, which is a McNaught’s indicator.  Upon a movable barrel A, a piece of paper is wound, the ends of which are secured by the slight brass clamps shown in the drawing.  The barrel is supported by the bracket b, proceeding from the body of the indicator, and at the bottom of the barrel a watch spring is coiled with one end attached to the barrel and the other end to the bracket, so that when the barrel is drawn round by a string wound upon its lower end like a roller blind, the spring returns the barrel to its original position, when the string is relaxed.  The string is attached to some suitable part of the engine, and at every stroke the string is drawn out, turning round the barrel, and the barrel is returned again by the spring on the return stroke.

235. Q—­But in what way can these reciprocations of the barrel determine the power of the engine?

A.—­They do not determine it of themselves, but are only part of the operation.  In the inside of the cylinder c there is a small piston moving steam tight in a cylinder of which d is the piston rod, and e a spiral spring of steel, which the piston, when forced upwards by the steam or sucked downwards by the vacuum, either compresses or extends; f is a cock attached to the cylinder of the indicator, and which is screwed into the cylinder cover.  It is obvious that, so soon as this cock is opened, the piston will be forced up when the space above the piston of the engine is opened to the boiler, and sucked down when that space is opened to the condenser—­in each case to an extent proportionate to the pressure of the steam or the perfection of the vacuum, the top of the piston c being open to the atmosphere.  A pencil, p, with a knife hinge, is inserted into the piston rod, at e, and the point of the pencil bears upon the surface of the paper wound upon the drum A. If the drum A did not revolve, this pencil would merely trace on the paper a vertical line; but as the drum A moves round and back again every stroke of the engine, and as the pencil moves up and down again every stroke of the engine, the combined movements trace upon the paper a species of rectangle, which is called an indicator diagram; and the nature of this diagram determines the nature of the engine’s performance.

236. Q.—­How does it do this?