538 SUBTERRANEAN TEMPERATURE OF COAL MINES. 



more likely to fall below than to exceed the truth, and calculating 

 the ratio of increase beneath this point, I find it to be 1° for 

 62 feet. 



Making like computations in reference to the second table, the 

 results are — 



1st. Estimated from the surface down, 1° to 66 feet. 



2d. Estimated from the invariable plane down, 1° to 59 feet. 



Comparing the numbers deduced from the two classes of ob- 

 servations together, it will be seen that the diminution of tem- 

 perature in descending is slower in the mines in active operation 

 than in the recently opened shafts ; so that, whatever may be the 

 extraneous heating influences affecting the former, they are more 

 than counterbalanced by the permanent cooling due to the de- 

 scent into all parts of the mine of the cooler water from above. 



Comparing the three last observations in table second, the two 

 former of which were made in the same shaft, and the last in 

 one only a few hundred feet removed, there is ground for in- 

 ferring that the rate at which the temperatm*e increases grows 

 less as the depth augments. In descending from 330 feet to 600, 

 that is, through 270 feet, the rise of temperature is 4." 5 ; while in 

 descending from 600 to 780, or through 280 feet, the rise is only 

 2." 5. This difference would, I think, have been less, could I 

 have obtained the temperature at 780 feet free from the cooling 

 influence of the copious drippings from above. Yet even with 

 the most liberal allowance, there would still remain evidence of 

 a diminishing rate of increase with the depth, such as has al- 

 ready been remarked by Mr. Fox and other European observers. 

 Considering the observations in the IVIid Lothian shaft at 330 

 and 600 feet, as the ones most exempt from any known source 

 of error, and deducing the rate of increase from them, I find the 

 result to be almost precisely that inferred from table second, that 

 is 1° for every 60 feet. 



I may therefore, I think, in conclusion, affirm as approximately 

 true, for the region in which these mines are situated, that from 

 the invariable plane downwards for many hundred feet, the tem- 

 perature augments at the rate of V for every 60 feet of depth. 



