MINING, COAL. Although it would be impracticable to enter upon the various modifications of mining processes which are required in the working of all tine varied natural depositaries of mineral wealth, the peculiarities of coal-mining are such as to require a brief notice, important and extensive as are the coal-mines of this country, and the arts and manufactures supported by thew.
The nature and distribution of our carboniferous deposits have been already fully described In the article Csete-FtrAns, in NAT. MIST. Div. In working mines of coal, or indeed of iron-atone or rock-salt, and other minerals which are found in beds, the object to be attained is the effectual opening and extraction of a tolerably uniform mass of mall thickness, but of great horizontal extension. The process thus differs considerably front the working of mineral veins, where the object of research la fluctuating and irregular in its produce, and either vertical or highly inclined in its position. The thickness of beds of coal may be said generally to vary from three or four to eight or nine feet, Although sometime., when several seams come together without any Intervening layers of rock, they may expand to twenty or thirty feet, of which we have an example in the " ten aril eel " of Staffordshire. It) every coal-field there are marry seams of coal at greater or leas Intervals one below another, of which as many as three or fur are frequently worked in the saute untie ; and inteneratilled with the rock which divides them there are in many teabfielde extremely productive beds of Ironstone, which arc wrought at the same time, and in a similar nuumer with the coal.
The mode of working coal-mines will be must clearly understood by tracing the works from their commencement, In the same manner as in the former article. Fronts the geological indication of coal and the known extension of its bola over very considerable areas, it Is pretty certain to be found In the trial's which rum insulin for it, if carried on with prayer judgment Tho probable existence of beds of coal in depth having been ascertained, and perhaps the beds themselves dis covered by boring, the feet process is to sink a perpendicular shaft from the surface so as to intersect the various strata containing the coal, and of course as navy of the beet,' of coal as are couaidered to be worth working. The shaft is generally circular, and may vary accord
ing to circumstances from six or seven to eight or ton feet in diameter;; the upper portion, as far down as the solid rock, is either bricked or walled, and where the ground is weak this casing may be continued throughout. On reaching the first workable scant of coal, the sinking of the pit is for a time suspended, and a broad straight isassage termed " Lord" or " gate " is driven from it upon the seam of coal, in opposite directions. The breadth of the passage is usually twelve or fourteen feet, and it is formed the whole height of the scats of coal, so as to expose the stratum above, which is called the " roof," and the one below, which is termed the "thin," and its direction is alwaye arranged so as to follow the cleavage of the coal which forms its sides, which thus present a clean uniform surface. When the Lord or " mother gate" has proceeded some distance on both shies of the nit, narrow passages, termed " headways," are driven from it at regular intervals, and exactly at right angles; and when these have proceeded eight or ten yards, they are made to communicate with another Lord, which is opened parallel to the first and on each nide of it. It will be seen that by continuing this system of excavation the bed of coal will be en tirely laid open and intersected by a eeriest of broad parallel leassages or bords of about eight yortle apart, communicating with each other by narrower passages or " liteelways," which cross them at right angles, and likewise traverse the whole extent of the mines ; immense square or rectangular pillars of coal being left standing between the two. A coal-mine in this state is not in fact unlike a regularly-built town ; the bards and headways may be compared to the principal streets and the narrower streets which cross them ; while the pillars of coal may be said to resemble the intermediate masses of buildings.