320 



MINING FOR COAL 



For a description of the machine employed for facilitating the ascent and descent 

 of miners, see MAN ENOINE. 



IVKXHrxWG POR CO AXi. Tlio processes of boring, by which it is usual to begin 

 for the purpose of determining the existence and depth of any bed or beds of coal, have 

 been already described. See BOEING. 



Of Winning a Coal-field. In sinking a shaft for working coal, the great obstacle to 

 be encountered is water, particularly in the first opening of a field, which proceeds 

 from the surface of the adjacent country ; for every coal-stratum, however deep it 

 may lie in one part of the basin, always rises till it meets the alluvial cover, or crops 

 out, unless it be met by a slip or dyke. When the basset-edge of the strata is covered 

 with gravel or sand, any body or stream of water will readily percolate downwards 

 through it, and fill up the porous interstices between the coal-measures, till arrested 

 by the face of a slip, which acts as a natural dam, and confines the water to one com- 

 partment of the basin, which may, however, be of considerable area, and require a 

 great power of drainage. 



In reference to water, coal-fields are divided into two kinds: 1, level free coal; 

 2, coal not level free. In the practice of mining, if a coal-field, or portion of it, is so 

 situated above the surface of the ocean that a level can be carried from that plane till 

 it intersects the coal, all the coal above the plane of intersection is said to bo level 

 free ; but if a coal-field, though placed above the surface of the ocean, cannot, on 

 account of the expense, be drained by a level or gallery, such a coal-field is said to be 

 not level free. 



Besides these general levels of drainage, there are subsidiary levels, called off-takes 

 or drifts, which discharge the water of a mine, not at the mouth of a pit, but at some 

 depth beneath the surface, where, from the form of the country, it may be run off level 

 free. From 20 to 30 fathoms off-take is an object of considerable economy in pump- 

 ing ; but even less is often had recourse to ; and when judiciously contrived, may serve 

 to intercept much of the crop water, and prevent it from getting down to the dip part 

 of the coal, where it would become a heavy load on a hydraulic or pumping engine. 



Day-levels were an object of primary importance with the early miners, who had 

 not the gigantic pumping power of the steam-engine at their command. Levels ought 

 to be no less than 4 feet wide, and from 5 feet and a half to 6 feet high : which is large 

 enough for carrying off water, and admitting workmen to make repairs and clear out 

 depositions. When a day-level, however, is to serve the double purpose of drainage, 

 and an outlet for coals, it should be at least 5 feet wide, with its bottom gutter for 

 drainage either covered over or open. In other instances a level not only carries off 

 the water from the colliery, but is converted into a canal for bearing boats loaded with 

 coals for the market. Some subterranean canals are 9 feet wide, and 12 feet high, 

 with 5 feet depth of water. 



If, in the progress of driving a level, workable coals are intersected before reaching 

 the seam which is the main object of the mining adventure, an air-pit may be sunk, 

 of such dimension as to serve for raising the coals. These air-pits do not in general 

 exceed 9 feet in diameter; and they ought to be always cylindrical. Fig. 1490 repre- 

 sents a coal-field where the winning is made by a day-level ; a is the mouth of the 

 gallery on a level with the sea ; b, c, d, e, are intersected coal-seams, to bo drained by 

 the gallery. But the coals beneath this level must obviously be drained by pumping. 

 A represents a coal-pit sunk on the coal e ; and if the gallery be pushed forward the 

 coal-seams, f, g, and any others which lie in that direction, will also be drained, and 

 then worked by the pit A. The chief obstacle to the execution of day-levels, is 

 presented by quicksands in the alluvial cover, near the entrance of the gallery. The 



1490 



1491 



best expedient to be adopted amid this difficulty is the following: Fig. 1491 repre- 

 sents the strata of a coal-field A, with the alluvial earth a, b, containing the bed of 

 quicksand b. The lower part, from which the gallery is required to be carried, is 

 fihown by the line B o. But the quicksand makes it impossible to push forward this 

 day-level directly. The pit B c must therefore be sunk through the quicksand by 

 means of tubbing (to be presently described), and when the pit haa descended a few 



