1872.] The Coal Commisstoner’s Report. 43 
depth the temperature is 50° F. © The rate of increase 
of the temperature of the strata in the coal districts of 
England is in general about 1° of Fahrenheit for every 
60 feet of depth. The depth at which the temperature of 
the earth would amount to blood heat, or 98°, is about 
3000 feet. Under the long wall system of working, a dif- 
ference of about 7° appears to exist between the temperature 
of the air and that of the strata at the working faces, and 
this difference represents a further depth of 420 feet; so 
that the depth at which the temperature of the air would, 
under present conditions, become equal to the heat of the 
blood, would be about 3420 feet. Beyond this point the 
considerations affecting increase of depth and temperature 
become so speculative as to render it necessary to leave the 
question in uncertainty; but, looking to possible expedients 
which the future may elicit for reducing the temperature, it 
is considered that it may fairly be assumed that a depth of 
at least 4000 feet might be reached. 
Before considering the supplies of coal which still remain 
for consumption, within a workable depth, it is necessary to 
refer briefly to the waste that now occurs in working, and 
through imperfect combustion. 
The theoretical value of 1 lb. of coal is 14,000 units of 
heat, which, if properly applied, should be equal to the 
power of lifting 10,800,000 lbs. 1 foot high, whilst the 
highest practical result which has been realised is 1,200,000 
lbs., or less than one-eighth of the theoretic value, and this 
without counting the impurities of ordinary coal, which 
cannot be taken at lessthan 10 per cent. Theoretically, 1 |b. 
of pure coal should evaporate about 13 Ibs. of water; 
practically, 1 lb. of ordinary coal does not evaporate 4 lbs. 
The best results are stated to have been obtained in the 
boilers of Cornish engines, or in boilers constructed upon 
tneemodel af the, “Cornish boiler”. ‘he “duty.” of the 
best Cornish engines since 1814 shows that, up to a certain 
point, there was a gradual increase in the number of pounds 
lifted 1 foot high by the combustion of 1 bushel (94 lbs.) in 
the earlier tables, and of 1 cwt. (112 lbs.) in the latter ones; 
and that, after the maximum had been obtained, there was 
a steady decline in the effective power obtained; but the 
highest recorded duty is about 98,300,000, in 1857. Upon 
the general question of coal consumption, the conclusion 
arrived at by the Commission is, that ‘‘ for some time past, 
in our manufactures, there have been constant and perse- 
vering efforts to economise coal, by the application of 
improved appliances,” and there is reason to believe that, 
