332 APPLIED SCIENCE 



and 18 was much reduced from this cause. In curves C 3 and 

 C 4 we have this useful result of the mass of the moving parts 

 without the reversal of the stresses which brings down the effi- 

 ciency in B 3 and B 4 . Similarly, in curves D 3 and D 4 the effect 

 of the high initial pressure is to prevent the negative parts of 

 the curve from falling so low as in Example B. The extremely 

 low efficiency of A 4 , B 4 , and C 4 illustrates the evil effect of using 

 a large and heavy engine running with small mean pressures of 

 steam. We not unfrequently see large engines ordered for fac- 

 tories, mines, or water-works to allow for subsequent extension, 

 or for large variations in the work required at different times. 

 The above cases show how very serious the loss may be when an 

 engine is habitually worked much below its power. The cases 

 are no doubt extreme, but they show the tendency of the practice. 



37. Example E, Figs. 50 and 51. The four cases hitherto 

 analysed are not, strictly speaking, practical cases : they were 

 chosen so as to bring into prominence the effects of friction 

 and high speed, separately and combined. These effects are 

 obviously most marked when the engine is running lightly 

 loaded. We will now consider a more practical example. The 

 curves E correspond to a speed of 1 revolution per second, 

 and to an indicator diagram drawn as follows : Initial inten- 

 sity of pressure, 50 Ibs. per square inch, r= 5*93 ; back pres- 

 sure, 3 Ibs. per square inch; mean effective pressure, 18'095 

 Ibs. per square inch ; work done by steam, 30,640 inch Ibs. 

 per revolution. 



Ej is the effort curve, neglecting mass, weight, and friction. 



E 2 is the effort curve, taking the friction into account which 

 results from effort and resistance, but neglecting that due to 

 weight and mass ; the area of E, is 30,590 inch Ibs., the error 

 being 50 inch Ibs. The area of E 2 is 28,400 inch Ibs., making 

 the efficiency 0'928 on this hypothesis. 



E 3 is the effort curve, taking the mass and weight into 

 account, but neglecting friction. The area is 30,700, the error 

 being 110 inch Ibs. in the drawings. 



E 4 is the effort curve, taking mass, weight, and friction into 

 account in other words, the true effort curve. Its area is 

 28,030 inch Ibs., giving an efficiency of 0-913, or a little less 

 than that calculated for E t . Now that the steam pressures are 



