On a Steam-Engine Indicator for High Speeds. 63 



Appendix. 

 From the original equations, 



, b/3 + bfi-e,y 



C — ■ — , 



r 



»_ by + cfl — bfi 

 c - - —p . 



Hence with y=/3(1 + S) it follows 



c '- c » =z -2 b S + 2(b-c l )-8(b l + c i ); 



the third term on the right-hand side is of the second order 

 of small quantities ; and in any symmetrical switchboard the 

 second term will be exceedingly small, so we have nearly 



c"-c r 



8= 



2b 



The ratio S of the nearly equal arms ft and <y can thus be 

 readily tested. 



X. A Steam-Engine Indicator for High Speeds, 

 By Prof. John Perky, F.R.S* 



MEMBERS who are not practically acquainted with the 

 errors of the ordinary steam- or gas-engine indicator are 

 referred to a paper in the ' Proceedings of the Institution of 

 Civil Engineers ' (vol. lxxxiii. 1885) by Professor Osborne 

 Reynolds, and an exhaustive discussion of the subject. 



Whether we do or do not share Prof. Reynolds's view that 

 even at low speeds of engines the errors in calculating horse- 

 power are very considerable, because of the friction of the 

 paper barrel and the stretching of the cord or wire which 

 gives to the paper barrel a miniature motion of the engine 

 piston, it is obvious that there will be more confidence in an 

 indicator in which the motion -copying mechanism gives a 

 very short stroke, is opposed by very small forces, and requires 

 no string or wire. I do not pretend to get rid of the error 

 in a spring which is due to change of temperature, but where 

 great accuracy is required it is always worth while, when 

 using this or any other indicator, to have a " boiler-pressure " 

 line drawn upon the diagram, as well as an " atmospheric 

 line ;■' and these two lines, with a pressure-gauge on the 

 boiler, will give the scale to which the indicator at the time 



* Communicated by the Physical Society : read May 22, 1891. 



