ON GEOLOGICAL TIME. 59 



coal from a depth of one hundred or two hundred 

 fathoms, but that it could be got at and brought 

 out, notwithstanding even a red hot temperature of 

 the surrounding strata is quite certain. Plenty of 

 ventilation, conducted on proper thermodynamic 

 principles, 1 will give quite a satisfactory tempera- 

 ture for the workers in the mines. 



1 That is to say, the air must be compressed and cooled at the 

 upper surface, or at some convenient place in the mine or shaft, at 

 no great depth below the surface. This cold dense air must be 

 conducted to the lowest levels through a strong enough pipe, and 

 allowed to expand into the mine through an engine, or engines, 

 like a common high-pressure engine working expansively. A 

 great part of the work of this engine must be spent otherwise 

 than in generating heat in the mine ; for instance, it may be used for 

 working the gear to raise the mineral. A portion of its work 

 may be spent in cutting out the minerals, as is sometimes done 

 at present by compressed-air engines ; but it must be remembered 

 that the full dynamical equivalent of this part of the work of the 

 engine is developed in heat in the mine. Probably best prac- 

 tical plan for working very deep mines will be to employ the engine 

 power used at the surface all in compressing air ; the compressed 

 air to be cooled, either by water, if there is a sufficient cold water 

 supply, or by radiation to the sky, and by atmospheric convection. 

 This condensed air should be used for working the engine or engines 

 in proper places at the great depths required to work the gear for 

 raising the minerals, etc. , and small cutting engines in various parts 

 of the workings. Thus a sufficient supply of cool air may be 

 distributed through the mine. If the ordinary method of ventilation 

 by drawing otit air, whether by an air pump, or by a fire burning at 



