196 SCIENCE IN SHORT CHAPTERS. 



We may safely apply the same reasoning to the limits of 

 depth. The 4000 feet limit of the Royal Commissioners 

 is at present unattainable, simply because the immediately 

 prospective price of coal would not coyer the cost of such 

 deep sinking and working; but as prices go up, pits will 

 go down, deeper and deeper still. 



The obstacles which are assumed to determine the 4000 

 feet limit are increasing density due to greater pressure, 

 and the elevation of temperature which proceeds as we go 

 downwards. The first of these difficulties has, I suspect, 

 been very much overstated, if not altogether misunder- 

 stood; though it is but fair to add that Mr. Hull, who 

 most prominently dwells upon it, does so with all just and 

 philosophic caution. He says that "it is impossible to 

 speak with certainty of the effect of the accumulative weight 

 of 3000 or 4000 feet of strata on mining operations. In 

 all probability one effect would be to increase the density 

 of the coal itself, and of its accompanying strata, so as to 

 increase the difficulty of excavating," and he concludes by 

 stating that "in the face of these two obstacles tempera- 

 ture and pressure, ever increasing with the depth I have 

 considered it Utopian to include in calculations having ref- 

 erence 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 

 conclusion 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 aids to easy and cheap coal-getting. "With a dense 

 roof and floor the collier works vigorously and fearlessly, 

 and he escapes the serious cost of timbering. 



h The Coal Fields of Great Britain," pp. 447, 448. 



