THE LIMITS OF OUR COAL SUPPLY. 225 



good reason to believe that such density is one of the essential con^ 

 ditions for working deep coal. Even at present depths of working, 

 density and hardness of the accompanying strata is one of the most 

 important aids to easy and cheap coal-getting. With a dense roof 

 and tioor the collier works vigorously and fearlessly, and he escapes 

 the serious cost of timbering. 



Those who have never been underground, and only read of colliery 

 disasters, commonly regard the tire-damp and choke-damp as the 

 collier's most deadly enemies, but the collier himself has quite as 

 much dread of a rotten roof as of either of these ; he knows by sad 

 experience how much bruising, and maiming, and crushing of human 

 limbs are due to the friability of the rock above his head. Mr. Hull 

 quotes the case of the Dunkinfield colliery, where, at a depth of 

 about 2500 feet, the pressure is " so resistless as to crush in circular 

 arches of brick four feet thick," and to snap a cast-iron pillar in 

 twain ; but he does not give any account of the density of the accom- 

 panying strata at the place of these occurrences. I suspect that it 

 was simply a want of density that allowed the superincumbent pres- 

 sure to do such mischief. The circular arches of brick four feet 

 thick were but poor substitutes for a roof of solid rock of 40 or 400 

 feet in thickness ; an arch cut in such a rock would be all key-stone : 

 and I may safely venture to affirm that if, in the deep sinkings of the 

 future, we do encounter the increased density which Mr. Hull antici- 

 pates, this will be altogether advantageous. I fear, however, that it 

 will not be so, that the chief difficulty of deep coal-mining will arise 

 from occasional " running in" due to deficient density, and that this 

 difficulty will occur in about the same proportion of cases as at pres- 

 ent, but will operate more seriously at the greater depths. 



A very interesting subject for investigation is hereby suggested. 

 Do rocks of given composition and formation increase in density as 

 they dip downward ; 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 peaches 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 bo 

 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 argument I will accept without prejudice, as the 

 lawyers say, his basis of a range of 1 F. for 60 feet. According to 

 this, the rocks will reach 99-6, a little above blood heat, at 3000 feet, 

 and 116-3 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 116 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 tempera- 

 tures ; that it matters not how high the thermometer rises as we de- 

 scend, we shall still go lower and still get coal so long as prices rise 



