189 
The best coal for steam purposes should unite all these 
qualities in itself, but a coal possessing the first and not 
the fifth may, for ordinary purposes, be equally efficacious. 
For upwards of twelve months I was enabled almost 
daily to inspect the experiments of the Admiralty coal 
investigation, conducted by Dr. Playfair and Sir H. De 
la Beebe. It would, however, be tedious to enter here 
into the details of these extensive experiments, but I think 
it may be profitable to state that the relative evaporative 
value of each coal was tested by three independent experi- 
ments, beneath a steam boiler specially erected for the 
purpose, and furnished with every contrivance for obviating 
the perturbating influence of external causes. 
The following particulars were attended to and results 
obtained. For each coal was marked, the time required 
to get up the steam ; the weight of wood therein employed ; 
the initial temperature of the water in the boiler and the 
tanks ; the state of the barometer and dewpoint ; as well as 
the extremes of external and internal temperature ; the 
area of damper open ; weight of coals consumed ; weight of 
ashes, cinder, and soot left, and their respective propor- 
tions of combustible matter ~ and clinker; weight of water 
evaporated from 212® by the whole, and each pound of 
coal per foot of grate surface ; together with the duration 
of the experiment, the specific gravity, mean weight of a 
cubic foot, cohesive force, and economic weight of each 
variety of coal operated on. 
The general result of these experiments, which were con- 
ducted with scrupulous accuracy, was strongly corrobora- 
tive of the results obtained theoretically ; and the following 
formula, by which the practical results were calculated, may 
be taken as of very considerable accuracy for coal : — 
Let W = the weight of water let down from the tanks 
during the experiment. 
VOL. III. N 
