350 Limits of our Coal Supply. July* 



proportion of cases as at present, but will operate more 

 seriously at the greater depths. 



A very interesting subject for investigation is hereby sug- 

 gested. Do rocks of given composition and formation 

 increase in density as they dip "downwards, and if so, does 

 this increase of density follow any law by which we may 

 determine whether their power of resisting superincumbent 

 pressure increases in any approach to the ratio of the 

 increasing pressure to which they are naturally subjected ? 

 If the increasing density and power of resistance reaches 

 or exceeds this ratio, deep mining has nothing to fear from 

 pressure. If they fall short of it, the difficulties arising 

 from pressure may be serious. Friability, viscosity, and 

 power of resisting a crushing strain must be considered in 

 reference to this question. 



Mr. Hull has collected a considerable amount of data 

 bearing upon the rate of increase of temperature with 

 depth. His conclusions give a greater rate of increase than 

 is generally stated by geologists ; but for the present argu- 

 ment I will accept, without prejudice, as the lawyers say, 

 his basis of a range of i° F. for 60 feet. According to this, 

 the rocks will reach 90/6 2 , a little above blood-heat, at 3000 

 feet, and ii6'3° at the supposed limit of 4000 feet. It is 

 assumed by Mr. Hull, by the Commissioners, and most 

 other authorities, that this rock temperature of u6° will 

 limit the possibilities of coal-mining. At the average 

 prices of the last three years, or the prospective prices 

 of the next three years, this temperature may be, like 

 difficulties of the thin seams, an insurmountable barrier ; 

 but I contend that at higher prices we may work coal 

 at this, and even far higher, rock temperatures ; that 

 it matters not how high the thermometer rises as we 

 descend, we shall still go lower and still get coal so long 

 as prices rise with the mercury. Given this condition, 

 and I have no doubt that coal may be worked where the 

 rock temperature shall reach or even exceed 212°. I do not 

 say that w T e shall actually work coal at such depths ; but if 

 we do not, the reason will be, not that the thermometer is 

 too high, but that prices are too low : in other words, value, 

 not temperature, will determine the working limits. 



Mr. Leifchild, in the last number of the " Edinburgh 

 Review," in discussing this question, tells us that "thenormal 

 heat of our blood is g8~, and fever heat commences at ioo = , and 

 the extreme limit of fever heat may be taken at H2\ Dr. 

 Thudicum, a physician who has specially investigated this 

 subject, has concluded from experiments on his own body at 



