and are carried away in an inclined trough, while the rest of the coal slides on to the next segment. Here the next smallest particles fall through and are carried away, and the process is continued until the lower end of the screen is reached, out of which end all the coal that was too large to pass through the mesh of the last segment is now poured. It will be seen that by this means the different sizes of coal have been separated from each other and can be carried by separate shutes to the loading place. This is the principle of the rolls and screens which are the main features of every coal breaker, though each breaker usually contains two or more sets of rolls and from eight to twelve screens. The Woodward breaker recently erected near Kingston, Pennsylvania, has six pairs of rollers and twenty screens. Some of these screens are double; that is, they have a larger outside screen surrounding the smaller one, and the coal that passes through the inner screen is caught by the outer one and again divided by means of a smaller mesh. Before the days of breakers and screens coal was sent to market in the lump, as it came from the mine, and it was generally broken and prepared for use by the consumer. But when the separation of coal in the breaker became reduced to a system, the four smaller sizes than lump coal were soon graded. They were known as steamboat, egg, stove, and chestnut. It was thought at the time that no finer grade of coal than chestnut could be burned to advantage. But it was not long before a smaller size, known as pea coal, was separated, placed on the market, and readily sold; and now, within recent years, another still smaller size called buckwheat has been saved from the refuse and has come into general use. Everything smaller than this is culm and goes to the waste pile. The names of the different sizes of marketable coal and the spaces over and through which they pass in the process of separation are given in the following table, taken from Saward’s “Coal Trade Annual,” for 1888:―― ───────────────────┬──────────┬────────── │ OVER. │ THROUGH. │ Inches. │ Inches. ───────────────────┼──────────┼────────── Lump coal bars │ 4½ to 9 │ Steamboat “ │ 3½ to 5 │ 7 Broken mesh │ 2⅜ to 2⅞ │ 3¼ to 4½ Egg “ │ 1¾ to 2¼ │ 2⅜ to 2⅞ Large stove “ │ 1¼ to 1⅞ │ 1¾ to 2¼ Small stove “ │ 1 to 1¼ │ 1¼ to 1½ Chestnut “ │ ⅝ to ¾ │ 1 to 1¼ Pea “ │ ⅜ to ⅝ │ ⅝ to ⅞ Buckwheat “ │³∕₁₆ to ⅜ │ ⅜ to ⅝ Dirt “ │ │³∕₁₆ to ⅜ ───────────────────┴──────────┴──────────