Pt. II. Chap. II.] collieries — mode of working. 163 



thickness of the seam mined, being one of the numerous modifications 



of the system, known in England as <c post and stall," or e ' pillar 



«-d . j A n.j * and board." The coal is extracted in galleries, 

 " Post and stall" work- & 



in & s - crossing each other at right angles, square 



"posts" or " pillars" of coal being left to support the roof. The size 



of the pillars and galleries varies in different collieries, depending 



upon the firmness of the roof, or stratum overlying the coal, and, to 



some extent, upon the thickness of the coal itself. Where the roof is 



good, and the coal seam of moderate thickness, the size of the pillars 



is smaller, and vice versa. Of course the smaller the " pillars," and 



the broader the galleries between, the greater will be the quantity of 



coal extracted from any given area ; since, although as much coal as 



possible is robbed or cut away from the pillars, before abandoning the 



mine, only a small proportion of the mass can be thus extracted, the 



major part of what is left at first being inevitably lost, and a further 



advantage in widening the galleries or " boards," is the additional 



space given to the workmen to use their tools freely. 



The following are the sizes of the galleries and 

 Sizes of pillars. . . . 



pillars m a few ot the principal mines around 



Raniganj : — 

 In Raniganj mine, the pillars are 15 feet square, the galleries 15 feet broad. 



In Sirsol mine, 



ditto 



15 



ditto 



ditto 



12 



ditto. 



In Tapassi mine, 



ditto 



12 



ditto 



ditto 



12 



ditto. 



In Chokidanga mine, 



ditto 



15 



ditto 



ditto 



14 



ditto. 



In Harispur mine, 



ditto 



18 



ditto 



ditto 



14 



ditto, 



In Rogonathchuk mine 



, ditto 



18 



ditto 



ditto 



12 



ditto. 



It will be easily seen, that where the " pillars" and " boards" or 

 galleries are equal in breadth, three-quarters of the coal is removed 

 in the first instance. This is the most favorable case, and exists in 

 Raniganj and Tapassi collieries. Allowing, on the one hand, for the 

 quantity of coal which it may be found practicable to " rob" from the 



