i873«l Limits of our Coal Supply. 349 



one effect would be to increase the density of the coal itself, 

 and of its accompanying strata, so as to increase the diffi- 

 culty of excavating," and he concludes by stating that " In the 

 face of these two obstacles — temperature and pressure, ever 

 increasing with the depth — I have considered it Utopian to 

 include in calculations having reference to coal supply any 

 quantity, however considerable, which lies at a greater 

 depth than 4000 feet. Beyond that depth, I do not believe 

 that it will be found practicable to penetrate. Nature rises 

 up, and presents insurmountable barriers."* 



On one point I differ entirely from Mr. Hull, viz., the con- 

 clusion that the increased " density of the coal itself and of 

 its accompanying strata " will offer any serious obstacle. 

 On the contrary, there is good reason to believe that such 

 density is one of the essential conditions for working deep 

 coal. Even at present depths of working, density and 

 hardness of the accompanying strata is one of the most im- 

 portant conditions of easy and cheap coal-getting. With a 

 dense roof and floor the collier works vigorously and fear- 

 lessly ; and he escapes the serious cost of timbering. 

 Those who have never been underground, and only read 

 of colliery disasters, commonly regard the fire-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 accompanying strata 

 at the place of these occurrences. I suspect that it was 

 simply a want of density that allowed the superincumbent 

 pressure 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 anticipates, 

 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 



* The Coal Fields of Great Britain, pp. 447 and 448. 

 VOL. III. (N.S.) 2 Z 



