OF STEAM-ENG^INES WITH DEY SATUKATED STEAM. 
189 
by means of which, when the work of one pound of coal is known, its available heat 
can be computed, and vice versa, as with the exact formula. 
Tables of the ratios given by the equations from 25 to 29 for various ratios of expansion 
have been computed, and are in course of being printed. The following are examples 
of the results contained in them : — 
Expansion r. 
Effective cut-off 1-4-r. 
Mean gross pressure -4- initial 
pressure, (w+p^-^py 
20 
0-05 
0-186 
10 
010 
0-314 
0-15 
0-417 
5 
0-20 
0-505 
4 
0-25 
0-582 
0-30 
0-648 
2f 
0-35 
0-707 
2i 
0-40 
0-756 
0-45 
0-800 
2 
0-50 
0-840 
1 1 
0-55 
0-874 
-*■ 3 
0-60 
0-900 
1 3 
0-65 
0-929 
7 
0-70 
0-945 
li 
0-75 
0-960 
li 
0-80 
0-976 
Comparison of Theory with Experiment. 
In comparing the results of formulae for the expansive working of steam with those 
of the indicator-diagrams of engines, it is not to be expected that the indicated pressures 
corresponding to particular volumes during, or at the end of, the expansion, will closely 
agree with those given by calculation ; because considerable deviations of the line 
marked on the diagram, alternately upwards and downwards, arise from the friction of 
the indicator, from elastic vibration of the indicator-spring, and from oscillations of the 
steam itself. In the course of a complete stroke, however, those deviations neutralize 
each other, so that the indicated mean effective pressure, if the theory is sound, ought 
to agree with that given by theory within the limit of errors of observation. 
About half a pound on the square inch, or 72 lbs. on the square foot, may be con- 
sidered as an ordinary limit of error in indicator-diagrams. 
Two examples of the application of the exact formulae, and four of the application of 
the approximate formulae, to actual engines, are annexed ; and the results of the formulae 
are compared with those of experiment. 
