The Supply and Waste of Coal. 321 



4. The roof and floor. When a continuous bed, or stratum 

 of coal is removed from the mass of the earth, it is as certain 

 that the rest will come together as that the ceiling of a room 

 would fall if the four walls were taken away. By the ordinary 

 mode of mining this is checked for a time, by leaving walls and 

 props of coal, stone, or timber; but the check is only tem- 

 porary. It is clear that the amount of fracture of the over- 

 lying strata will depend on their plasticity and thickness. 

 Thus, a very thick, hard roof of sandstone may, perhaps, 

 squeeze up a soft floor of clay into the spaces between the 

 walls, and any beds of coal in the overlying strata may not be 

 crushed. If, however, there is the smallest irregularity in the 

 sinking, the coal above will be crushed to powder and de- 

 stroyed. When the roof and floor are both good, the works of 

 a mine may also be carried on more regularly and more pro- 

 fitably than when they are indifferent; and, as in many 

 cases the most valuable seam of a series lies below two or 

 three others that are somewhat inferior, it may happen that the 

 question of the preservation or destruction of these may de- 

 pend on the nature of the associated beds. 



5. Water. In almost every coal district a certain quantity 

 of water, draining naturally to the deepest point, will ac- 

 cumulate in the mines, and require removal by artificial means. 

 Every stratum of the coal series that allows water to percolate 

 will act as a channel; and faults, though sometimes they are 

 "close," and cut off the water, in many instances act as conduits, 

 and help to bring into the mine all the rain-fall of a district. 



It is evident that, the deeper the mines, the more will they 

 be subjected to this cause of trouble and expense ; but, as all 

 the strata cut through in sinking the shafts can be effectually 

 stopped from entering by a water-tight iron casing to the 

 shaft, it is only the quantity brought in by the coal strata 

 themselves that is troublesome. Perhaps it may safely be 

 assumed that improvements in pumping machinery will render 

 this part of the work of mining as easy hereafter in deeper 

 mines as it is now in those already worked. 



6. Faults and the dip of strata. The main faults in some of 

 the most important British coal-fields are tolerably well known ; 

 but there are many, affecting parts of the coal not yet reached, 

 that may prove very serious. The dip of the coal may also 

 render more difficult, though it may make much easier, the 

 working of the deep coal under the New Hed Sandstone. There 

 is reason to believe that none of the coal-fields of Europe are 

 really very extensive, and even the adjoining districts in our own 

 island, though but a few score of square miles in extent, are not 

 capable of any exact comparison with one another. On the other 

 hand, the American coal district of Western Virginia is certainly 



