88 Appendix to Report on Trials of Steam-Engines, 8fc. 
from the coal stores. It is probable, therefore, that the relative 
value assigned to this coal is rather under than over the mark, 
as samples for experiment are too often picked. 
It is very convenient to express the gross calorific power of 
fuel, in the weight of water at the boiling point, which an unit 
of weight of that fuel would evaporate at the same temperature, 
in an absolutely perfect boiler. The same figures, then, express 
the power under all standards of weights and measures. 
Reduced to this standard of comparison " Llangennech," com- 
pared with those above referred to and other fuels, stands as 
follows : — 
Theoretical Evaporative Povjer in lbs. of Boiling Wafer evaporated per lb. of 
Fuel. 
Llangennech Coal (R.A.S.E. sample) 15-24 
Ebbw Vale Coal (Admiralty Experiments) .. .. 16-8 
Powell's DufFryn „ .... 16-3 
Best Aberdare supplied in London 15'9 
Best Newcastle Steam Coal, about 15'0 
■ Coke 13-0 
Dry Peat 10-0 
Dry Wood 7'5 
Petroleum, about 22-0 
These figures become useful for considering the efficiency of 
boilers, by comparing the water at 212 ^ actually evaporated into 
steam per lb. of coal with the amount, according to the above 
figures, which would be evaporated in a theoretically perfect 
boiler ; for it is not fair to judge of a boiler by the water 
evaporated, unless the quality of the coal is also considered. 
As an example, the average weight of cold water evaporated 
in the ploughing engine boilers per lb. of coal during the trials 
at Wolverhampton was 7 2 lbs., which is equivalent to 8'6 lbs. 
of water at 212°, evaporated per lb, of coal. The theoretical 
power of Llangennech coal is 15'3, consequently the average 
8'6 
efficiency of the ploughing engine boilers was = "56; in 
l0'24: 
other words, the furnaces and boilers rendered available 'oG of 
the absolute power latent in the fuel. 
Similarly as regards the traction engine boilers, those of the 
9 
locomotive tvpe showed an efficiency = — =: '59, while the 
10-24 
7'G 
Thomson pot boilers gave efficiency 24. ~ 
