APPLICATION OF GRAPHIC METHODS 331 



first admitted at a constant pressure of 10 Ibs. per square inch ; 

 that /-, the reciprocal of the fraction of the stroke during which 

 steam enters at a constant pressure, was 12-8 ; that the steam 

 expanded, according to an adiabatic curve, and was suddenly 

 released at the end of the stroke, and that during the return 

 stroke the back pressure was 0-516. These data give a total 

 effective initial pressure of 476- 7 Ibs. ; a total effective final 

 pressure of 3'7 Ibs. ; and a mean effective intensity of pressure 

 of 2 Ibs. per square inch, as in Examples A and B. The work 

 done by the steam per stroke is, therefore, as before, 3,212 inch 

 Ibs. The speed of the engine is taken, as for curve A, at 1 

 revolution per second. Curves C 3 and C 4 give the effects with 

 and without friction ; the area of C 3 is 3.223 inch Ibs., showing a 

 very small error of execution; the area of C 4 is 2.640; the 

 ratio of these values gives 0*819 as the efficiency of the engine 

 worked in this way. This efficiency is actually a little higher 

 than that of curve A, which is repeated in this figure to allow 

 it to be more readily compared with C. A similar result is to 

 be observed in curve D, Fig. 49, constructed from the same data, 

 but at the high speed of 4 revolutions per second. The area 

 of D 3 is 3,270 inch Ibs., that of D 4 1,965 inch Ibs., giving 

 an efficiency of 0-601 a value sensibly higher than that de- 

 rived from curve B when the steam was admitted throughout 

 the stroke. The errors due to imperfect drawing have generally 

 the result of slightly increasing the effort, and the error in curve 

 D 3 for this particular drawing (3.270 over 3.216) is nearly 1-7 

 per cent. The liability to error is much increased when, as 

 here, a large portion of the area is negative. If this percentage 

 were reckoned on the arithmetical sntn of the areas, instead of 

 on their difference, it would be insignificant. Notwithstanding- 

 this inevitable imperfection, there is every reason to expect thafc 

 the errors in curves D 3 and D 4 resemble one another ; and we 

 have the less reason to suspect the accuracy of the conclusion, 

 because we can see that since the tendency of resistance to 

 acceleration during the beginning of each stroke is to diminish 

 the effort, while that of a large initial pressure is to increase it, 

 the two tendencies counteract one another without causing 

 pressure on the main bearings or crank pin. Thus, in curves 

 B 3 and B 4 we found that the loss due to friction at positions 5 



