The name coal was originally given to any substance used for fuel, and the use of mineral coal, or, as it was formerly termed, _sea-coal_, from the fact of its being brought by sea, has not prevailed for longer than 200 years. Coal is found in this country in extensive deposits called coal-fields, the most important of which are:—1. Those of the great northern district, including the coal-fields north of the river Trent. 2. Those of the central district, including the Leicester, Warwick, Stafford, and Shropshire coal-fields. 3. Those of the western districts in North Wales, South Wales, and Gloucester. The coal occurs in a number of layers, or beds, termed _seams_, and these are separated from each other by layers of slatey clay, called _shale_, and coarse, hard sandstone, known as _grit_. The seams of coal are mostly comparatively thin, but varying from a few inches to six or eight feet in thickness, and underneath the layers is usually found a bed of mountain limestone extending beyond the coal-field, and rising to the surface of the ground around it. The first process for finding coal is to bore with iron tools a perpendicular hole in the ground; then if coal is discovered, a shaft, or pit, from ten to fifteen feet in diameter, is sunk, and lined with brick, cast iron, or wood, to prevent the admission of water and the falling in of the sides. When the shaft reaches a seam of coal, passages are made, twelve to fourteen feet wide, and varying in height with the coal; from these proceed smaller ones, which are again crossed by large ones, enormous blocks of coal being left to support the earth above. As the mine becomes larger, a second shaft is sunk at some distance from the first, in order that air may be supplied to the mine, and that the gas that escapes from the coal may be carried away. A current of air is caused by burning a large fire in one shaft, and the draught is directed to the part of the mine where it is required, by doors, which close up some of the passages and leave others open. There are several varieties of coal found in this country. The most important are: the common, or bituminous coal, which soils the fingers on handling; the cannel, or candle coal, which burns with a bright flame, but does not soil; the anthracite, or culm coal, which burns without flame, does not form cinder when half consumed, and is most useful for furnaces, in consequence of its intense heat and the absence of smoke. Although often classed amongst the minerals, coal is evidently of vegetable origin; many sorts of it being distinctly fibrous, and showing the grain of the wood from which it is composed. Upon examination with the microscope, coal shows vegetable remains; and these are frequently so perfect that they prove its formation from such plants as ferns and fir-trees, of a kind somewhat different from those which now exist.