326 MINING FOR COAL 



driving tho level. When the level is driven correctly, with tho proper depth of water, 

 it is said to have dead water at the face. In this operation, therefore, the miner pays 

 no regard to the backs or cutters of the coal ; but is guided in his line of direction 

 entirely by the water-level, which he must attend to solely, without regard to slips or 

 dislocations of the strata throwing the coal up or down. In the last figure, the coal-field 

 is a portion of a basin ; so that if the shape be uniform and unbroken, and if any point 

 be assumed on the dip of the crop, as D, the level lines from that point will be parallel to 

 the line of crop, as D E, D F, and the levels from any point, whatever the dip or incli- 

 nation of strata, will be also parallel to these ; and hence, were the coal-field an entire 

 elliptical basin, the dip-head levels carried from any point would be elliptical, and parallel 

 to the crop. If, as is more commonly the case, the coal-field be merely a portion of a 

 basin, formed by a slip of the strata, as represented in 

 jig. 1503, where a, a, a is the crop and A B, a slip of great 

 magnitude, forming another coal-field on the side c, then 

 the crop not only meets the alluvial cover, but is cut off 

 by the slip at A and at B. Should any point, therefore, 

 be assigned for an engine-pit, the levels from it will 

 proceed in a line parallel to the crop, as D d, DC; and the level on both sides of the 

 engine-pit will be also cut off by the slip A B. In this figure, the part included be- 

 tween the two curve lines is the breadth or breast of coal-field won by the engine-pit 

 ; what is not included is termed the under-clip coal, and can be worked only by one 

 or more new winnings towards the dip, according to circumstances. 



In British practice, there are four different systems of working coal-mines. 



1. Working with pillars and rooms or boards, styled post and stall, where the 

 pillars left bear such proportion to the coal excavated as is just adequate to the sup- 

 port of the incumbent strata. 



2. Working with post and stall, where the pillars are left of an extra size, and 

 stronger than may be requisite for bearing the superior strata, with the intention ot 

 removing a considerable portion of each massive pillar, whenever the regular working 

 of post and stall has been finished in the colliery. 



3. Working with post and stall, or with comparatively narrow rooms or boards, 

 whereby an uncommonly large proportion of coal is left, with the view of working back 

 towards tho pits, whenever the colliery is worked in this manner to the extent of the 

 coal-field, and then taking away every pillar completely, if possible, and allowing the 

 whole superincumbent strata to crush down, and follow the miners in their retreat. 



4> Working the long way, being the Shropshire and Derbyshire method ; which leaves 

 no pillars, but takes out all the coal progressively as the workings advance. On this 

 plan tho incumbent strata crush down, creeping very close to the heads of the miners. 



The post-and-stall system is practised with coals of every thickness. The long-work 

 method is adopted generally with thin coals ; for when the thickness exceeds 6 or 7 

 feet, and there is only little refuse made in excavating the coal to cart into the exca- 

 vated part, this mode has been found impracticable. 



The following considerations must be had in view in establishing a coal-mine : 



1. The lowest coal stratum of the winning should be worked in such a manner as 

 not to injure the working or the value of the upper coals of the field; but, if this 

 cannot be done, the upper coals should be worked in the first place. There are, how- 

 ever, cases where an upper seam of coal can be worked more advantageously by 

 working a lower seam first on the long-wall method. 



2. The coals must bo examined as to texture, hardness, softness, the number and 

 openness of the backs and cutters. 



3. The nature of tho pavement of the coal-seam, particularly as to hardness and 

 softness ; and if soft, to what depth it may be so. 



4. The nature of the roof of the coal-seam, whether compact, firm, and strong ; or 

 weak, and liable to fall ; as also tho nature of the siipcrincumbent strata. 



5. The nature of the alluvial cover of the ground, as to water, quicksands, &<?. 



. 6. The situation of rivers, lakes, or marshes ; particularly if any be near tho out- 

 crop of the coal-strata. 



7. The situation of towns, villages, and mansion-houses, upon a coal-field as to 

 chance of their being injured by any particular mode of mining the coal. 



Mr. Bald gives the following general rules for determining the best mode of wo 

 ing coal by post and stall : 



' 1. If the coal, pavement^ and roof, are of ordinary hardness, the pillars and 

 rooms may be proportioned to each other, corresponding to the depth of the super- 

 incumbent strata, providing all the coal proposed to bo wrought is taken away by tin; 

 first working, as in the first system ; but, if the pillars are to be winged, or partially 

 worked afterwards, they must be left of an extra strength, as in the second system. 



' 2. If the pavement is soft, and the coal and roof strong, pillars of an extra size 



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