472 
rxARFORTH : DEEP MIXING, 
scarcity several important inventions were introdnced wliereby a 
nearer approach to perfect combustion of fuel was obtained, and a 
permanent reduction and saving obtained in the consumption of 
coal. From this fact may it not be argued that if coal were situate 
within easier reach of the surface it might not be preserved and used 
with the same care as it probably will be in the future, after being 
raised from depths exceeding one thousand yards. 
The question for the future is not : How long will the coal in 
this country afford a supply? but rather, at what price can coal 
be raised to enable the manufacturer in this country to successfully 
compete with foreign productions ? 
The chief obstacle in working coal at great depths is the increase 
of temperature which accompanies increase of depth. From obser- 
vations which have been made, it appears the temperature of the 
earth is constant at a depth of about 50 feet, and at that depth it is 
usually 50 degrees Fahr. In 1871 the Commission assumed the rate 
of increase to be one degree for every 60 feet in depth. Recent observa- 
tions taken at Ashton ^loss Colliery, which is the deepest shaft in 
England, at a depth of 2,850 feet show the temperature to be 85 
degrees. This proves the rate of increase to be one degeee for every 
7 6 '9 feet. At another colliery a few miles distant, one degree for every 
79 feet. In this district large quantities of water have been found in 
the strata, whicli may account for the slowness of the rate of increase 
shown by the foregoing observations, as compared with that fixed by 
tlie Commission. Water cannot be taken as a guide to temperature 
as it may come from above or below. It has been hoped future deep 
sinkings might prove the increase of temperature to be in a diminish- 
ing ratio ; but when the primary cause which has led to the increase 
already proved, is considered, it is hardly reasonable to expect such 
will be the case. High temperature of the strata operates as an 
impediment to deep working by heating the air circulating through 
the passages of the mine. This heating process is most rapid at first 
when the difference of temperature between the air and the strata is 
greatest, gradually diminishing as the length of the passage is 
extended, and never ceasing until complete assimilation is effected. 
The progress towards this assimilation is much more rapid when the 
