708 PROFESSOR FLEEMING JENKIN’S APPLICATION OF GRAPHIC METHODS 
ately 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 intensity of pressure 50 Ibs. 
per square inch, 7=5'93; back pressure, 3 lbs. per square inch ; mean effective 
pressure, 18'905 lbs. per square inch; work done by steam, 30640 inch lbs. 
- per revolution. 
EF, is the effort curve, neglecting mass, weight, and friction. 
E, the effort curve, taking the friction into account which results from effect 
and resistance, but neglecting that due to weight and mass; the area of E, is 
30590 inch Ibs, the error being 50 inch Ibs. The area of E, is 28400 inch lbs., 
making the efficiency 0°928 on this hypothesis. 
FE; is the effort curve, taking the effort mass and weight into account, but 
neglecting friction. The area is 30700, the error being 110 inch Ibs. in the 
drawings. 
E, is the effort curve, taking mass, weight, and friction into account—in 
other words, the true effort curve. Its area is 28030 inch lbs., giving an effi- 
ciency of 0°913, or alittle less than that calculated for E,. Now that the steam 
pressures are large, the loss due to the weight and mass of the parts is at this 
speed comparatively insignificant. The total inevitable loss due to friction in the 
machine alone, even excluding the accidental friction due to tightness of piston 
and glands, and the power required to work the valve gear, is nearly 9 per cent. 
of the whole indicated horse-power. ‘This example shows the complete fallacy 
of the experimental method sometimes adopted with the object of testing the 
efficiency of an engine. The engine is run with no resistance, and the indicated 
H.P. observed. This is assumed approximately to represent the loss due to 
the engine itself when doing useful work against a large resistance ; but from 
example A we see that the power required to overcome the friction in the 
engine when a small resistance was being overcome was only 3210-2974 inch 
lbs., or 236 inch lbs.; whereas in example E the power required is 1190 inch 
lbs., or nearly five times as much. This ratio would be somewhat diminished 
by the constant accidental resistances in glands, &c., and by the power required 
for valves and pumps. It must, however, always remain very large. 
In E, and E, the discontinuity of the curves at positions 0 and 12 is very 
marked. There must also be a slight break near positions 6 and 18, due to the 
change in the bearing-points on the crosshead pin; but the frictional loss at 
this point is so insignificant that the break in the curve is not sensible. There 
is very little difference in general character between curves E, E, and E; Ey 
Mass and weight play a very small part in the general result. 
§ 38. Example F, fig. 52, Plate XXXI.—Example F is important and instruc- 
tive, showing the result of running the engine at 4 revolutions per second 

