[Illustration: Courtesy of U. S. Bureau of Mines AN “ENTRY” IN A COAL MINE Showing timbered roof] [Illustration: Courtesy of U. S. Bureau of Mines COATING WALLS OF A MINE WITH CEMENT To prevent coal-dust explosions] [Illustration: DRILLING IN COAL FOR BLASTING] “Anthracite” is the hardest of coals. It was formed from bituminous coal under the crushing pressure due to the upheaval of mountains or by the intense heat of adjacent molten rocks. Most American anthracite is mined in eastern Pennsylvania. The largest deposits in the world are found in China. Anthracite burns slowly, with little smoke. It is well adapted for domestic use on account of its cleanness, but is not an economical fuel for steam-raising or general manufacturing. [Illustration: Press Illustrating Service MODERN ELECTRIC LOCOMOTIVE Used for hauling coal from the mines] “Semi-anthracite” also ranks as a hard coal, though it is less hard than anthracite. Very little is mined in this country, and it is generally sold as anthracite. “Semi-bituminous” coal is a softer coal, which, when properly burned, gives off but little smoke. The best semi-bituminous coal ranks highest among the coals in heating value. It is the most valuable fuel for manufacturing purposes; also for steamships, as it requires less bunker space per unit of heat than any other coal. “Bituminous” coal, or ordinary “soft coal,” burns readily, with a smoky flame, and is the coal most commonly used for manufacturing purposes; in fact, the bulk of the coal mined throughout the world belongs to this rank. It includes a good many varieties, some of which are extensively used in making coke, while others, such as “cannel” coal, have been in great demand for use in gas-works. Nowadays, however, the widespread introduction of “water-gas,”[2] which does not require any particular kind of coal, has diminished the demand for “gas coals.” [2] Made by forcing steam over glowing coal or coke. See Monograph No. 4. “Sub-bituminous” coal, or “black lignite,” is common in some of our western coal fields. It is a clean and useful domestic fuel when used near the mines, but is not very satisfactory for shipment, as it shrinks and crumbles under the effects of “weathering” and is liable to spontaneous combustion. “Lignite” is the least valuable of coals, and is the form of coal which is the least altered from the original peat. The Geological Survey applies this name only to those coals which are distinctly brown and either markedly woody or claylike in appearance. Lignite, as it comes from the mine, contains from thirty to forty per cent. of moisture, and it “slacks” or falls to pieces much more rapidly than sub-bituminous coal when exposed to the air. It is hardly suitable for transportation.