62 



THE GEOLOGIST. 



All this is only preliminary : there must be a door to your house 

 if you are to get into it, and the shaft is the door to a coal mine. 

 This is the first thing to be completed. It must go the whole depth 

 of the mine, in order that they may get rid of the water that 

 soaks through the strata. This is man's great enemy when he is 

 mining — at least the first one — for bad air is at least as great an 

 enemy afterwards. 



The shaft then is sunk to the "dip" end of the main, or lowest level 

 of the floor, in order that all the water which may percolate into the 

 workings may eventually flow down into the " sump" or cistern that 

 lies at the bottom of this " engine shaft." 



Fig. 3.— Diagram of Shafts, 

 a b, engine shaft, — b, the "sump," or cistern; e, upcast shaft, — d, its furnace. 



It is no light work to sink a shaft — eleven or twelve feet wide — 

 to a depth of perhaps one thousand feet. The materials are sometimes 

 very soft, as shale ; but this has the disadvantage that you must line 

 it with brick or wood throughout. Sometimes the rock is hard 

 enough to stand alone ; then the matrix is a tough rock and very difficult 

 to cut. Oftentimes the leaky state of the bed makes it necessary to 

 line it with wooden " tubbing" throughout ; and this is an old custom. 

 More recently it has been found advantageous to use iron cylinders 

 the whole way ! A shaft a thousand feet deep will ordinarily cost 

 about three thousand pounds ; and if a two hundred and fifty horse- 

 power engine be required, there is another five thousand pounds to 

 begin with ; and while on the subject of expense, it may be well to 

 say at once that fifty thousand to two hundred thousand pounds are 

 no uncommon sums required to set a colliery fairly going, before 

 a bucket of coal is drawn. But then if it yields — and it ought to 

 yield — twelve per cent., it is no bad speculation after all. 



They generally find too they must sink two shafts ; and the pump- 

 ing engine will not do for drawing also. The two shafts are required 

 for ventilation ; and they serve also to prevent the mischief of letting 

 everything down and drawing everything up the engine pit. We will 

 leave the ventilation alone just now ; and only say with reference to 

 the engine that the quantity of water required to be removed is often 

 enormous. 



The way in which water finds its way into a coal pit is the same 

 as that in which it finds its way into Artesian wells. The water comes 



