325 



1500 



These segments of the iron cylinder arc dfet true to the radius of the pit ; and every 

 horizontal and perpendicular joint is made tight with a layer of sheeting deal. A 

 wedging crib is fixed at the bottom, and the segments are built up regularly with 

 joints like ashlar-work. This kind of tubbing can be carried to any height, till the 

 water finds an outlet at the surface, or till strata containing water can be tubbed off, as 

 by the modes of tubbing already described. A shaft finished in this manner presents 

 a smooth lining-wall of iron, the flanges being turned towards the outside of the 

 cylinders. In this iron tubbing, no screw bolts are needed for joining the segments 

 together ; as they are packed hard within the pit, like the staves of a cask. 



The weight of the hydrostatic column is not the only pressure to which the tubbing 

 is exposed. There is the pressure from accumulated carburetted hydrogen gas, 

 which considerably exceeds the water-pressure. If the tubbing in deep shafts was 

 put in without pressure pipes, it would be liable to be fractured by great pressure 

 from gas. The pressure pipes are usually fixed to each length of tubbing ; strong 

 taps or cocks are first screwed into the tubbing, and malleable iron pipes of from 1 to 

 2 inches in diameter are fixed to the tops and carried up to the surface ; and in many 

 cases a continual overflow of gas and water issues. By these means the tubbing is 

 only subject to the pressure due to the hydrostatic column. 



Before tubbing a shaft, it is necessary to ascertain whether the strata containing 

 water is likely to be dislocated, so as to let down the water by working the coal 

 away ; in such a case, tubbing the shaft is unnecessary. The judgment of the mining 

 engineer must decide about this. 



When a porous thin bed or parting betwixt two impervious strata gives out much 

 water, or when the fissures of the strata, called cutters, are very leaky, the water can 

 be completely stopped up by the improved process of wedging. The fissure 

 is cut open with chisels, to a width of 2, and a depth of 7 inches, as 

 represented in fig. 1500. The lips being rounded off about an inch and a 

 half, pieces of clean deal are then driven in, whose face projects no further 

 than the contour of the lips, when the whole is firmly wedged, till the water 

 is entirely stopped. By sloping back the edges of the fissures, and wedging 

 back from the face of the stone, it is not liable to burst or crack off in the operation, 

 as took place in the old way, of driving in the wedge directly. 



Messrs. Kind and Chaudron's improved methods of sinking shafts through water- 

 bearing strata and beds of quicksand, successfully practised in many of the continental 

 coal-fields, are fully described in the article BORING. 



Working of Coal. A stratum, bed, or seam of coal, is not a solid mass, of uniform 

 texture, nor always of homogeneous quality. It is 

 often divided and intersected, with its concomitant 

 strata, by what are named partings, backs, cutters, 

 reeds, or ends. Besides the chief partings at the 

 roof and pavement of the coal-seam, there are sub- 

 ordinate lines of parting in the coal mass, parallel 

 to these, of variable dimensions. These divisions 

 are delineated in./?*?. 1501, where A, B, c, D, E, F, o, 

 represent a portion of a bed of coal ; the pajal- 

 lelogram A B D c the parting at the roof, and E F G 

 the parting at the pavement ; a b, b c, d e, and ef, are the subordinate or intermediate 

 partings ; (jh,ik,l m, the backs ; o p, p q, r s, s t, u v, and v w, the cutters. It is 

 thus manifest that a bed of coal, according to the number of these natural divisions, is 

 subdivided into solid figures of various dimensions, and of a cubical or rhomboidal shape. 



When the engine-pit is sunk, and the lodgement formed, a heading or drift is then 

 made in tho coal to the rise of the field, or a cropping from the engine-pit to the se- 

 cond pit. This heading may be 6 or 8 feet wide, and carried either in a line directly 

 to the pit bottom, or at right angles to the backs or web of the coal, until it is on a line 

 with the pit, where the heading is set off, upon one side, to the pit bottom. This 

 heading is carried as nearly parallel to the backs as possible, till the pit is gained. 

 Fig. 1502 represents this mining operation. A, is the 

 engine-pit. B, the second or bye-pit, A c, the gallery or 

 heading driven at right angles to the bocks, c B, the 

 gallery set off to the right hand, parallel to the backs. 

 The next step is to drive the main levels from tho engine- 

 pit bottom. In this business the best colliers are always 

 employed, as the object is to drive the gallery in a truly 

 level direction, independently of all sinkings or risings of 

 the pavement. For coal-seams of ordinary thickness, this 

 gallery is usually not more than 6 feet wide ; observing to have on the dip side of the 

 level a small quantity of water, like that of a gutter, so us to guide the workmen in 



1501 



1502 



