Trials of Portalle Steam- Engines at Cardiff. 83 
The third, and by far the most important point, to which the 
Judg^es have to call attention, relates to the firing. 
There can be no doubt that a large portion of the great 
success of the engines on this occasion arose from the high 
evaporative duties got out of the boilers. 
The analysis of the Llangennech coal shows that its theoretical 
Fig. 12. 
power of heating amounts to 15*24 lbs. of water, evaporated from 
212°, for each pound of coal burnt (see Appendix). 
That a duty of even so much as 11'83 lbs.* of water should 
have been obtained, proves, not merely that the heat generated 
must have been largely utilized by the boiler, but also that the 
greater part of the coal must have been completely burnt, and 
that this must have been done without the admission of any 
considerable excess of air. 
. The Judges may perhaps be pardoned for reminding some 
of their readers that it is possible so to deal with coal, or coke, 
in a fire, that a large portion of it shall escape unconsumed up 
the chimney, and shall do this without the appearance of 
smoke. 
Assuming a fire to be fed with insufficient air, this is what 
takes place. The air coming in contact with the fuel imme- 
* In the instance of the steam-jacketed cylinders, the steam condensed in their 
jackets passed back direct into tlie boilers, and thus escaped actual measurement ; 
this water of condensation had, however, to be re-evaporated by the boilers, just as 
much as if it had been visibly mingled with the feed water, and thus the evapora- 
tive duties of the boilers of jacketed engines are a trifle in excess of those given in 
Table II. . 
G 2 
