Gas-Engine Indicator-Diagram. 69 



we may regard q as the rate (per foot-travel of the piston) at 

 which heat is received by the fluid, just in the same sense as p 

 the pressure is the rate at which work is done by the fluid ; 

 and a comparison of q and p shows at once the comparison 

 between the rates at which heat is being received and work is 

 being done. 



Now, since we consider the fluid to behave like a perfect 

 gas, we know from thermodynamics that 



*-Fl(w+I$> (6) 



where 7=1*37 (see §§3 and 4) ; and it is obvious that the 

 relation of p to q is not altered in any way by altering the 

 scale to which I or p is represented in the indicator-diagram. 

 We have taken three methods of drawing the curve whose 

 ordinate is q. In the first method a list of the values of 

 p was made out from careful measurements of the curve 

 ABCD (fig. 1), for values of I, 0*889, 0'9, 0-911, 0*922, 

 0*933, 0*944, &c. The observed increment of p divided by 



the increment of I was taken to represent the value of -^- for 



the mean value of I. In this laborious way q was obtained 

 for many values of I, and the curve EFGfl (fig. 4) repre- 

 sents our result. It is obvious that the rate at which the 

 fluid receives heat is greatest at the very beginning of the 

 stroke, falling off during the ignition period, much more 

 rapidly at the end of the ignition period ; and in G H we see 

 that the fluid is losing heat during what we call the ex- 

 pansion part of the stroke. In the same figure the actual 

 indicator-diagram is shown in ABCD, the pressure being 

 shown to the same scale as the ordinate of the heat-diagram. 



The area between EFGrH, the line OX, and any two ordi- 

 nates, shows in foot-pounds the heat given to the fluid between 

 the two positions, to the same scale as that to which the area 

 of the indicator-diagram represents work done. 



Another method which we have taken is this. To find q 

 corresponding to a point S (fig. 5) on the indicator-curve. 

 Draw a tangent S R to the indicator-curve, meeting the line 

 O P in R. O P and O L are the lines O P and O L of fig. 1. 

 Draw from R a line R Q parallel to O L, meeting the ordinate 

 from S in the point Q. Then the distance S Q, or rather 

 TS-TQ, represents 



dp 



l TV 

 Measuring S Q, therefore, paying attention to the iact that it 



