86 Mr. C. V. Boys on Apparatus for the 



the two sides of the piston of the engine. Then, at any moment, 

 the cylinder will turn with a speed which is proportional to 

 the rate at which work is being done, and the number of 

 revolutions, as measured by a counter, will be a measure of 

 the work done in foot-pounds or other units during any time. 

 Figs. 5 and 6 are views of an engine-power meter, each partly 

 in section. A, A are two boxes with flexible covers, like the 

 corrugated plate in an aneroid barometer. They may be 

 filled with a mixture of glycerine and water or other liquid, 

 and connected each with one end of the cylinder of the engine. 

 Each diaphragm will feel the pressure, but not the heat, of the 

 steam or gas in the cylinder. The two diaphragms are con- 

 nected by the rod r ; and so the effective force tending to bend 

 the diaphragms is the difference of pressure at the two ends of 

 the cylinder. This is the force acting on the piston of the 

 engine. Inclination is given to the tangent-wheel t by the 

 rod r by a pin working in a radial slot, as is better shown in 

 fig. 7. This arrangement causes the tangent of the inclination 

 of the tangent- wheel to be proportional to the displacement of 

 the rod r, and so to the force acting on the piston. C is the 

 integrating-cylinder, which is capable of sliding along, but of 

 turning with a wire W, which may be grooved or polygonal ; 

 pinion-wire is very suitable. The integrating-cylinder is 

 caused to reciprocate, by means of a yoke Y and lever L, in 

 time with the piston of the engine. The stroke is reduced to 

 a convenient amount by attaching a string from the piston- 

 rod to a suitable part of the lever L. ISow, as the rate at 

 which the cylinder turns is proportional to the longitudinal 

 motion of the integrating-cylinder multiplied by the tangent 

 of the inclination of the tangent-wheel, and as this is propor- 

 tional to the motion of the piston multiplied by the force 

 urging it, i. e. to the work being done, the whole number of 

 the revolutions of the cylinder will measure the whole amount 

 of work done. If the instrument gets out of adjustment so 

 that the tangent-wheel is not parallel to the axis of the cylin- 

 der when there is no force, then whatever error it makes in a 

 forward stroke it will takeoff in the return stroke; so that no 

 accumulating error will be produced. The diaphragms may 

 either be made of steel or highly elastic metal, in which case 

 they form their own springs ; or a softer metal, controlled by 

 an external spring, might be used. If a diagram is required, 

 one may be drawn as described on page 81. But it will not be 

 a diagram such as is drawn by an ordinary indicator, but the 

 integral curve of such a diagram ; so that force, instead of 

 being represented by the length of an ordinate, will be repre- 

 sented by steepness. Instead of diaphragms as described. 



