5 8 POPULAR LECTURES AND ADDRESSES. 



as Phillips pointed out in the Geological Society 

 of London, are very unsatisfactory. Air circulat- 

 ing through the mines, and water percolating and 

 being pumped out, give rise to disturbances so 

 great, that we cannot say if in a lower level of a 

 mine, we find a colder temperature than in a 

 higher level, the result is due to colder strata. The 

 best ventilated deep mine will be the coolest ; and 

 in passing, I may remark, which is, perhaps, of 

 some interest in the present and prospective state 

 of the question of the supply of coal, that we know 

 no limit of the depth to which coal may be worked, 

 depending on terrestrial temperature. Suppose 

 there was coal, or rather charcoal, where the strata 

 were red hot, it might be gone into and that with 

 perfect ease. All that is necessary is plenty of 

 ventilation. This will keep the temperature cool 

 enough for working, and thus there is no limit 

 whatever to the depth to which the miner may 

 proceed. I do not say it would not be enormously 

 more expensive to bring up coal (gas-coke, or char- 

 coal) from four thousand fathoms, if there is any 

 at so great a depth, than to bring up what we call 



