177 
The most serious difficulty that has been, and will con- 
tinue to be felt, is the cost which must be increased the 
deeper we go, by reason of the greater outlay in sinking and 
plant, &c., but that will not be so serious a charge upon cost 
as is sometimes estimated. 
The most difficult obstacle to engineers in deep mining 
is the increase of the temperature of the superincumbent 
strata ; this has been estimated by numerous experiments, 
and the following conclusions arrived at : — 
First, that the depth of the stratum of invariable 
temperature, i.e., the depth at which the temperature of the 
rock is unaffected by atmospheric influences in England is 
50 feet, at which the temperature is 50° 
Second, that there is a possibility that the rate of increase 
may diminish as greater depths are reached ; and that at 
a depth of 2,000 feet, the average rate of increase may 
possibly not exceed 1° Fahrenheit, for every 68 feet, con- 
tinuing beyond that depth at that rate, or it may be in a 
diminishing ratio. 
The latter probability is supported by the evidence of 
Monsieur Lambert, Government Inspector of Mines for 
Belgium, who informed the writer, when on a visit to that 
country, that at the Simon Lambert Colliery, at a depth of 
3,489 feet, the temperature was only 78°, or an increase of 
l"* in 119 feet. Calculations made in accordance with this 
hypothesis, give the results tabulated below. 
AT 50 FEET TEMPERATURE = 50° CONSTANT. 
From 
50 ft. to 2000 ft. an increase of T in 68 ft. = 78° at 2000 ft 
2000 „ 4000,, „ „1°„75„ =104°,, 4000,, 
4000 „ 6000 „ „ „ 1° „ 80 „ = 129° „ 6000 „ 
6000 „ 8000 „ „ „ 1^ „ 85 „ = 152° „ 8000 „ 
8000 „ 10000 „ „ „ r „ 90 „ = 174° „ 10000,, 
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