
TO THE DETERMINATION OF THE EFFICIENCY OF MACHINERY. 709 
- instead of 1 revolution per second, but maintaining the same indicator diagram 
as in example E, the mean effective pressure being 18°905 lbs. as before. We 
_ might expect this high speed to diminish the efficiency, as in examples B and 
D, but this is not the case. Curves F, and F, are identical with E, and E, and 
are therefore omitted; curves F; and F, are, however, very different in character 
from E, and E, The resistance to acceleration causes the effort to be much 
more uniformly distributed. It greatly diminishes the maximum pressure, and 
largely increases the pressure during the second half of each stroke. The resist- 
ance to acceleration is not, however, so large relatively to the steam pressure as 
to produce negative ordinates, such as were found in cases Band D. The action 
of the reciprocating masses is, therefore, on the whole, beneficial, and we find 
the efficiency of the engine to be ‘917, or somewhat higher than in example E. 
Much care has been taken in the computations to check this rather remark- 
able result, but there seems no reason to doubt its accuracy. The important 
conclusion to be drawn does not, however, depend on trifling differences—such 
as that between ‘917 and ‘913. It is that high speeds do not necessarily entail 
low efficiency, so that by judicious arrangement of the masses we can have 
small engines running rapidly with high expansion which, so far as frictional 
resistances are concerned, may be at least as efficient as the same engines run- 
ning slowly, and necessarily more efficient than larger engines doing the 
same work. 
§ 39. Examples G. and H, figs. 53 and 54, Plates XXXII. and XX XIII.— 
The unexpected results obtained rendered it desirable to investigate the case 
of infinitely slow speed, or that in which the effect of inertia was wholly dis- 
regarded. Curves G, and G, show the efforts with and without friction for the 
practical distribution of steam assumed in cases E and F. The total work done 
by the steam is 30770 inches, the useful work 20120, and the efficiency ‘914— 
aresult hardly differmg sensibly from that obtained with the high speed of 4 
revolutions per second. 
§ 40. Example H.—Lastly, curves H, and H, show the efforts with and 
without friction when the engine is modified so as to make the connecting-rod 
only 28 in. long, or 3} times the stroke. The weight of the new rod is taken 
as 28 lbs., and its centre of gravity 14” from the crank end, and its radius of 
gyration about the cross-head 18°83 inches. The speed assumed was 1 revolution 
per second.* As calculated from the areas of the curves the whole work done 
by the steam is 30740 inch lbs., the useful work 27920 inch lbs., and the 
efficiency 0°91. 
This result is of considerable practical value, showing that the efficiency of 
the engine is hardly diminished by shortening the connecting-rod to this extent. 
* The initial steam pressure, the ratio of expansion, and the back pressure were assumed to be 
the same as in example E. 
