706 PROFESSOR FLEEMING JENKIN’S APPLICATION OF GRAPHIC METHODS 






















the resistance of the masses to acceleration is strikingly shown by the curves B, 
and B,, being the effort curves of the same engine, with the same pressure in) 
the cylinder, but running four times faster. When making one revolution 
per second, the resistances to acceleration are smaller than the weights of the 
elements in motion, as may be seen in Table I. of the Appendix, where these 
forces are given for each position. When the speed is increased to four revolu- . 
tions per second, these forces are multiplied by sixteen, and their effect is then 
much greater than that of the weight of the parts and suffices completely to 
change the character of the effort curve. The negative portion of the curve lasts_ 
for nearly half the revolution of the crank, so that for nearly half of each revolu- 
tion the fly-wheel would have to pull the engine round. This is true both for B; 
and B, for the curves without and with friction, and is simply due to the fact 
that during the first half of each stroke the reciprocating masses are being posi- _ 
tively accelerated. The positive ordinates during the period when the engine — 
is driving of course exceed those during which the engine is being driven ; so” 
that for curve B,; the balance of positive area ought to be 3216 inch lbs. It 
actually is, on the drawing, 3256, the excess being due to small errors in draw- 
ing and computation. The area of B, is only 1744 inch lbs. : 
The efficiency, therefore, has sunk to 0°536; almost half the power of thal 
engine is taken up in driving itself; the pressures on the joints caused by 
resistance to acceleration have at this high speed greatly increased the loss due 
to friction. The inequality between back and front strokes is also very great, 4 
the area of the front branch being 953 inch lbs., that of the back branch only 
791. The loss due to friction sinks, however, almost to nothing at one point 
of the front stroke, very near the place where, in example A,, it was a maximum, 
This may serve as a warning against hasty generalisations. In curve B, there 
are sensible sudden changes of efficiency at points 0 and 12, due to a sudden 
change in the position of the points where the elements bear on one another. 
§ 36. Example C, fig. 48, Plate XX VI.—Example C was selected with the 
object of ascertaining how far the efficiency is affected by using the steam expan- 
sively instead of admitting it throughout the stroke. There is a very general idea 
that the sudden shock, as it is called, of admitting steam at a much higher pres- 
sure for a short time at the beginning of the stroke must diminish the efficiency of 
an engine. This is not so in the present example. An imaginary indicator 
diagram was selected for example C, drawn on the supposition that the steam 
was first admitted at a constant pressure of 10 lbs. per square inch ; that 7, the 
reciprocal of the fraction of the stroke during which steam enters at a constant 
pressure, was 128; 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 0516. These data give a total effective initi 
pressure of 476°7 lbs.; a total effective final pressure of 3-7 lbs.; and a meal 

