COAL MINING. Coal mining differs from metalliferous mining chiefly in the better ventilation required, the extent and regularity of mine workings and the necessity of getting out the mine product with as little dust as possible. The principal gases found in coal mines are carbon dioxide, CO., heavier than air, suffocating, but not inflammable, called choke damp by miners; carbon monoxide, CO, about as heavy as air, poisonous and inflammable, the dreaded white-damp of the miners; carburetted hydrogen, CH., light, not poisonous but in flammable, the chief constituent of fire-damp; also, but less important, sulphuretted hydrogen, poisonous and inflammable, but easily detected by its odor. Of these gases, marsh gas, given off in large quantities in some .mines, is the chief agent in coal-mine explosions. A mine is said to be fiery when the coal-seams give off much fire-damp. Many of the deeper coal mines of Great Britain, France and Germany are very fiery. The most fiery mines in the United States are in the anthracite region of Pennsylvania, the South Wilkes-Barre shaft at Wilkes-Barre being one of the most fiery mines. in the world. A mixture of marsh-gas and air in certain proportions explodes violently on contact with flame. Coal-dust in the air makes a ranch smaller proportion of marsh-gas an explosive mixture.
To enable men to work in places where suf ficient air to carry away the gas does not circu late, or to enable them to work in very fiery mines, safety-lamps are used. In a safety-lamp the flame is enclosed by wire gauze and cannot ignite gas in the air outside the gauze, unless the gauze is heated to the combustion point of the gas. The safety-lamp was invented by Sir Humphry Davy in 1815 and ha.s been improved in vanous ways. Many patterns are in use; one of the latest types used in this country is an electric lamp, to which is attached a lead plate battery, carried on the belt, a cable con necting it with the lamp, which is held in a steel Shell, carried on the cap. A safety-lamp
indicates the presence of fire-damp by the , lengthening of the flame.
Carbon dioxide and carbon monoxide given off by the coal in place are produced in the mined-out areas known as "gob.° These gases are the chief constituents of after-damp, the gases resulting from an explosion of dust or fire-damp. They, rather than the shock, cause the terrible loss of life in mine explosions, since men may be killed by carbon monoxide without knowing they are in danger.
Good ventilation is thus a prime necessity in coal mining. It is sometimes secured by a fur nace over a shaft, the fire producing a suffi ciently strong up-current. But in all fiery mines, and generally in all large mines, large revolving. fans, sometimes 35 feet in diameter, are used to circulate the air. The fans may exhaust or force in air (up-draft or down draf ; various types of mine fans are used, but the majority of those in service in Great Britain and at the larger mines in this country are modifications of the Guibal type.
As most coal-seams worked are a few feet thick, but of considerable extent, and as in this country at least, most coal-beds lie flat or dip at low angles, a coal-mine can be opened in a more regular way than a metalliferous mine. Two systems of mining are used— the pillar and room (uboard and pillar°), and the long wall. The first,. generally used in the United States, consists in taking out various portions of the coal as the work proceeds from the mine-opening, and the remainder in worlcing back toward the opening. The long wall sys tem, used in certain bituminous distncts in the United States, and extensively used in Eng land, consists in taking out all the coal in a long face as the work advances from the mine opening, the roadways and air-passages being protected by packs or walls. It is best suited for thin coal-seams with weak roofs, while the pillar-and-room system is best suited for thick seams with rock roofs. The two systems grade into each other.