THE LIMITS OF OUR COAL SUPPLY. 197 



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 den- 

 sity, and that this difficulty will occur in about the same 

 proportion of cases as at present, 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 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 in- 

 creasing 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 jn 

 reference to this question. 



Mr. Hull has collected a considerable amount of data 



