For commercial purposes coal is also classified according to size. The coal as it comes out of the mine, without any sorting into sizes, is known as “run of mine,” and the semi-bituminous coals are commonly shipped in this form. Most coals, however, are passed over bars or gratings, which constitute screens of different degrees of fineness; each screen permits all the lumps below a certain size to fall through, and thus the coal is divided into the different standard sizes. The sizes of anthracite, from the smallest to the largest, are: rice, buckwheat, pea, chestnut (or nut), stove, egg, broken (or grate), steamboat, and lump. Bituminous coal is divided into slack, nut and lump (the largest size). A mixture of lump and nut is called three-quarter coal. _The Modern History of Coal_ [Illustration: Press Illustrating Service MODERN MINING MACHINE for undercutting coal. The “cutter bar” is shown in front, filled with “cutting teeth” set in a chain that travels around. See illustration opposite] [Illustration: Press Illustrating Service SHAKER SCREENS IN A TIPPLE HOUSE The coal passing over screens is graded according to size] [Illustration: “CUTTER BAR” AT WORK The bar, with its cutting chain of teeth, makes a horizontal cut deep into the bottom of the seam of coal. Blasting then does the rest] The age of the steam-engine is also the age in which the use of coal has become widespread, and the output of coal is a faithful index of industrial progress. Although the Greek writer Theophrastus (about 300 B. C.) mentions the use of coal as a fuel, and its use was also known to the ancient Britons and the Chinese, it was virtually unknown throughout the Middle Ages. The first record of coal mining in England is of the year 1180 A. D., and coal was first shipped to London in the year 1240. It was long known as stone-coal, pit-coal, etc., to distinguish it from charcoal; also as sea-coal, on account of being carried to London by sea. Bituminous coal was first mined in America in 1750, near Richmond, Virginia. Anthracite was discovered in Rhode Island in 1760, and in Pennsylvania in 1766, but for many years its value was not recognized. As late as the year 1812 Colonel George Shoemaker, of Pottsville, was treated as an impostor and threatened with arrest for attempting to sell a few wagon-loads of anthracite in Philadelphia; methods of burning it were not understood, and it was declared to be merely “black stone.” In the year 1820 only 365 tons of anthracite were sold in this country, as compared with the present annual output of about 90,000,000 tons.