MINING, the processes whereby min erals are obtained from their natural lo calities beneath the surface of the earth, and the subsequent operations by which many of them must be prepared for the purposes of the metallurgist. The art has been practiced from the remotest times. The coal mines of England are mentioned as early as 853 and the dis covery of the Zwickau coal mines was made in the 10th century. In the 12th century the coal mines at Liittich began to be exploited and in the 13th century the coal deposits at Newcastle, Wales, and Scotland were discovered. In the course of the 19th century, after the in vention of the steam engine and the smelting of iron by means of coal and coke, mining was marvellously developed. Coal has been discovered in Australia, New Zealand, Borneo, China, and Japan. Great Britain has larger deposits than any other country in Europe. Next to these in amount of output are the coal mines of Belgium and France. In Ger many those along the Rhine and in West phalia, and in Saxony, Bohemia, and Si lesia, are of great importance.
Various machines have been invented with a view to lessen the labor and ex pense of under-cutting coal seams. They work with compressed air or electricity, some on the principle of the reciprocat ing rock drill, while others have the cut ters arranged on the periphery of a rotating disk, or on a traveling pitch chain. The coal, when broken down, is placed in wagons and drawn by horses, mules, traction rope, locomotive, or trol ley, to the bottom of the shaft and raised to the surface.
The actual mode of working the coal varies greatly in every district. By the post-and-stall, or bord-and-pillar, or room-and-pillar, or (in Scotland) stoop and-room, method the first stage of ex cavation is accomplished with the roof sustained by coal; in the long wall method the whole of the coal is allowed to settle behind the miners, no sustaining pillars of coal being left. This when well planned is the safer both as regards fa cility of ventilation and liability to acci dents from falls. In Pennsylvania anthra cite beds, which are mostly highly in clined, the workmen often work on the coal broken down in driving the "breast" upward; this broken-down coal being drawn off from below at intervals to give the workmen room to work. When the
breast has been extended upward to its full length the broken-down coal is all removed. Later when the breasts in one section of the mine are all worked out the sustaining pillars between them are "robbed" or removed, allowing the roof to fall.
In working metalliferous veins horizon tal galleries termed "levels" are driven by the lode usually 50 to 100 feet apart. They are rarely perpendicularly above one another, as they follow the incli nation of the vein. The levels are con nected by means of small shafts, termed "winzes." Represented on a vertical plane, the vein will thus be seen to be cut up into pillars which are worked by the method of "stoping." In underhand stoping, the ore is gradually worked away downward from the floor of one level, the ore and worthless mineral be ing taken out through the level next be low. This method is especially adapted for working any valuable ores, as the loss is small. The overhand method, in which the miners stand on timber plat forms and break down the mineral above them, is more economical, so far as cost of excavation is concerned; but the loss of ore is greater than by under hand stoping, and hence it is best adapted for the working of low-grade ores.
Placer Gold-Mining.--Many deposits of gold ores, as those first worked in Cali fornia and those of the Klondike, are in the gravel or sand in the present or ancient stream beds. Such deposits are called "placers." In working these de posits in the beds of existing streams, the course of the stream may be de flected by a dam and sluice, and the water of the stream utilized for separat ing the gold from the gravel which may be dug from the stream bed. In the case of larger streams, mechanical dredges re move the material from the bottom, which after separation of the gold is re turned to the stream bed. In the old placers which are abandoned stream courses, generally above water level, hydraulic mining is resorted to. A stream of water from 6 to 12 inches in diameter and under the pressure caused by a head of several hundred feet is di rected on the hillside where the old placer outcrops and rapidly washes it away. The material thus removed is sorted by the same water and the gold separated.