THE FIRE-HARDENED ROCKS 



usual in ordinary mines and coal-pits. It would be sunk 

 in stages each of about half a mile in depth, and at each 

 stage there would be placed the hauling and other 

 machinery for dealing with the next stage below. This 

 machinery, in order to economise space and limit the 

 heat of the workings, would be electrical. Even so there 

 would have to be special arrangements for cooling ; and 

 the depth of each stage in the boring would be restricted 

 to half a mile in order to avoid great cost in the hauling 

 arrangements, great weight of rope, and the great cost 

 of keeping the machinery and workings cool. At each 

 second or third mile down there would be air-locks to 

 prevent the air-pressure from becoming excessive, owing 

 to the weight of the superincumbent air. For when we 

 got between two and three miles down below the surface 

 of the earth the atmospheric pressure there would be 

 double what it is at the earth's surface, or, therefore, 

 about thirty pounds to the square inch. It would not be 

 easy to work under greater air-pressure than that, firstly 

 because of the strain on the workmen, and secondly 

 because of the rise of temperature which this increased 

 air-pressure would cause. Therefore special chambers 

 would have to be constructed to relieve the pressure, as 

 well as special pumps to provide ventilation, and other 

 machinery to carry the superfluous heat to the surface. 

 This last-named machinery would be of the nature of 

 brine-filled pipes, in which a freezing mixture would 

 always be kept circulating. (The arrangements suggested 

 by Mr. Parsons for keeping the underground workings 

 cool are rather too complicated for description here ; but 



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