DIAGRAMS. 659 



If a sheet of paper is made to move, say horizontally, with a constant 

 known velocity, while a tracing point is made to move in a vertical straight 

 line, the height varying as the value of any given physical quantity, the point 

 will trace out a curve on the paper from which the value of that quantity 

 at any given time may be determined. 



This principle is applied to the automatic registration of phenomena of all 

 kinds, from those of meteorology and terrestrial magnetism to the velocity of 

 cannon-shot, the vibrations of sounding bodies, the motions of animals, voluntary 

 and involuntary, and the currents in electric telegraphs. 



Indicator-Diagram. 



In Watt's indicator for steam-engines the paper does not move with a 

 constant velocity, but its displacement is proportional to that of the piston of 

 the engine, while that of the tracing point is proportional to the pressure of 

 the steam. Hence the co-ordinates of a point of the curve traced on the 

 diagram represent the volume and the pressure of the steam in the cylinder. 

 The indicator-diagram not only supplies a record of the pressure of the steam 

 at each stage of the stroke of the engine, but indicates the work done by the 

 steam in each stroke by the area inclosed by the curve traced on the diagram. 



The indicator-diagram was invented by James Watt as a method of estimating 

 the work done by an engine. It was afterwards used by Clapeyron to illustrate 

 the theory of heat, and this use of it was greatly developed by Kankine in 

 his work on the steam-engine. 



The use of diagrams in thermodynamics has been very completely illustrated 

 by Prof. J. Willard Gibbs (Connecticut Acad. Sci., Vol. in.), but though his 

 methods throw much light on the general theory of diagrams as a method of 

 study, they belong rather to thermodynamics than to the present subject. 



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