186 MR W. J. M. RANKINE ON THE 
Whence, the heat converted in power, as calculated directly, is, 
95°-8 K,, 
agreeing with the calculation from the power produced. 
In this example, the heat rendered effective is ae or somewhat more than 
one-sixth of that expended in the boiler. 
(33.) The results of the calculations of maximum possible effect, of which 
examples have just been given, are limits which may be approached in practice 
by Cornish and similar engines, but which cannot be fully realised; and yet it 
has been shewn, that in those theoretical cases only about one-sixth of the heat 
expended in the boiler is rendered effective. In practice, of course, the propor- 
tion of heat rendered effective must be still smaller; and, in fact, in some unex- 
pansive engines, it amounts to only one-twenty-fourth, or even less. 
Dr Lyon Puayrarr, in a memoir on the Evaporating Power of Fuel, has 
taken notice of the great disproportion between the heat expended in the steam- 
engine and the work performed. It has now been shewn that this waste of heat 
is, to a great extent, a necessary consequence of the nature of the machine. It 
can only be reduced by increasing the initial pressure of the steam, and the extent 
of the expansive action; and to both of those resources there are practical limits, 
which have already in some instances been nearly attained. 
APPENDIX TO THE FOURTH SECTION, 
CONTAINING TABLES TO BE USED IN CALCULATING THE PRESSURE, VOLUME, AND MECHANICAL 
ACTION OF STEAM, TREATED AS A PERFECT GAS. 
The object of the first of the annexed tables is to facilitate the calculation of 
the volume of steam of saturation at a given pressure, of the pressure of steam of 
saturation at a given volume, and of its mechanical action at full pressure. 
The pressures are expressed in pounds avoirdupois per square foot, and the 
volumes by the number of cubic feet occupied by one pound avoirdupois of steam, 
when considered as a perfect gas; those denominations being the most convenient 
for mechanical calculations in this country. 
The columns to be used in determining the pressure from the volume, and 
vice versd, are the third, fourth, sixth, and seventh. 
The third column contains the common logarithms of the pressures of steam 
of saturation for every fifth degree of the centigrade thermometer from —30° to 
+ 260°: that is to say, for every ninth degree of Fahrenheit’s thermometer from 
—22° to +500°. 
The fourth column gives the differences of the successive terms of the third 
column. 

