The systems of working the coal-seams after access is attained to them by the means de scribed are two, known as the room-and-pillar and the long-wall systems. The room-and-pillar method—also known as the pillar-and-chamber or board-and-pillar method, which may in clude the pillar- and - stall system—is the oldest of the systems, and the one very generally used in the United States. By this system, coal is first mined from a number of comparatively small places, called rooms, chambers, stalls, boards, etc., which are driven either square from or at an angle to the haulageway. Pillars are left to support the roof. In the long-wall meth od the whole face of the coal-team is taken out, leaving no coal behind, and the roof is allowed to settle behind as the excavation progresses, care being taken to preserve Imulageways through the falling material. Both the room-and-pillar and the long-wall methods are employed in vari ous modifications. for the details of which spe cial treatises on coal-mines should be consulted. The coal is cut from the seam by hand or by some form of coal-cutting machine. In America machine cutting is used extensively. There are four general types of machines in general use: Pick machines, chain-cutter machines. cutter-bar machines. and long-wall machines; the machines most used in America are pick machines and chain-cutter machines. Both compressed air and electricity are used for operating coal-cutting machines. Pick machines are very similar to a rock - drill; chain - cutter machines consist of a low metal bed - frame upon which is mounted a motor that rotates a chain to which suitable cutting teeth are attached. The ventilation of the workings, owing to the presence of gases, is a very important feature of coal-mining, and great care is taken to lay out the workings so as to facilitate ventilation. Mechanical ventila tion by means of fans and blowers (see BLowrivo MACHINES) is usually employed. Hoisting in mines is accomplished by means of cages run ning up and down the shafts, and operated by large hoisting engines on the surface. There are two general systems of hoisting in use- hoisting without attempt to balance the load, in which the cage and its load are hoisted by the engine and lowered by gravity, and hoisting in balance, in which the descending cage or a spe cial counter-balance assists the engine to hoist the loaded ascending cage. Haulage in mines is accomplished by animal power or by steam hoisting engines operating a system of rope haul age or by mine locomotives operated by steam, electricity, compressed air, or gasoline.
The preparation of mined coal for the market consists in screening the coal over bars and through revolving or over shaking screens, to gether with breaking it with rolls to produce the required market size. The large lumps of slate or other impurities are separated by hand, while the smaller portions are picked out by auto matic pickers or by hand by boys or old men seated along the chutes leading to the shipping pockets or bins. When coal contains much sul phur, this is frequently removed by washing it with water in special washing plants.
BinuoGRAPHY. Lesley, Manual of Coal and Its Topography (Philadelphia,• 1850)—a good work, but difficult to find; Chance, "Coal-Min ing," in Second Geological Surrey of Pennsyl vania, Report AC (Harrisburg, 1883) ; Hughes, A Textbook of Coal-Mining (London, 1899) ; Peel, Elementary Textbook of Coal-Mining (Lon don, 1901) ; Macfarlane, The Coal Regions of America, Their Topography, Geology, and De velopment (New York, 1875) ; Nicolls, The Story of American Coals (Philadelphia, 1s97) ; Lesley and others, "Reports on the Coal-Fields of Penn sylvania," in various publications of the Second Geological Survey of Pennsylvania (Harrisburg). Numerous scattered papers have been published in the following annuals and periodicals: Trans actions of the American Institute of Mining Engineers (New York) ; The Mineral Industry (New York) ; The Engineering and Mining Jour nal (New York) ; Mines and Minerals (Scran ton, Pa.) ; "Mineral Resources of the United States," United States Geological Surrey (Wash ington). For foreign coal deposits, consult: ilemoirs of the Geological Survey of Great Brit ain (London) ; Reports of Progress of the Geo logical Surrey of the United Kingdom (Lon don) ; Annales de la geologique de Bel pique (Liege, 1874 et seq.) ; Bulletin de la so cif'te beige de geologic, de paieontologie of d'hydrologie (Brussels, 1877 et seq.) ; Annales des mines ( Paris, 1816 et seq.) ; Bulletin de la societe geologique de France (Paris, 1896 et seq.) : Loze. Les ekarbons britannigues et lour epuiscinent (Paris, 1900) : Zcitsehrift fiir prak tische Gcologie (Berlin, 1893 et seq.). See AN THRACITE; BITCH INOUS COAL; CARBONIFEROUS SYSTEM : COKE ; CULM CRETACEOUS SYSTEM :• PEAT; TERTIARY SYSTEM; GRAPHITE; CARBON; FIRE-CLAY; and the articles on the different States and countries in which coal has been found.