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 thick, forming a kind of filter, through which the fine dirt passes to the bottom of the hutch. The cleaned coal is carried by a stream of water to a bucket elevator and delivered to the storage bunkers, or both water and coal may be lifted by a centrifugal pump into a large cylindrical tank, where the water drains away, leaving the coal sufficiently dry for use. Modern screening and washing plants, especially when the small coal forms a considerable proportion of the output, are large and costly, requiring machinery of a capacity of 100 to 150 tons per hour, which absorbs 350 to 400 H.P. In this, as in many other cases, electric motors supplied from a central station are now preferred to separate steam-engines.

Anthracite coal in Pennsylvania is subjected to breaking between toothed rollers and an elaborate system of screening, before it is fit for sale. The largest or lump coal is that which remains upon a riddle having the bars 4 in. apart; the second, or steamboat coal, is above 3 in.; broken coal includes sizes above 2 or 2 in.; egg coal, pieces above 2 in. sq.; large stove coal, 1 in.; small stove, 1 to 1 or 1 in.; chestnut coal, to  in.; pea coal,  in.; and buckwheat coal,  in. The most valuable of these are the egg and stove sizes, which are broken to the proper dimensions for household use, the larger lumps being unfit for burning in open fire-places. In South Wales a somewhat similar treatment is now adopted in the anthracite districts.

With the increased activity of working characteristic of modern coal mining, the depth of the mines has rapidly increased, and at the present time the level of 4000 ft., formerly assumed as the possible limit for working, has been nearly attained. The following list gives the depths reached in the deepest collieries in Europe in 1900, from which it will be seen that the larger number, as well as the deepest, are in Belgium:—

The greatest depth attained in the Westphalian coal is at East Recklinghausen, where there are two shafts 841 metres (2759 ft.) deep.

The subject of the limiting depth of working has been very fully studied in Belgium by Professor Simon Stassart of Mons (“Les Conditions d’exploitation à grande profondeur en Belgique,” Bulletin de la Société de l’Industrie minérale, 3 ser., vol. xiv.), who finds that no special difficulty has been met with in workings above 1100 metres deep from increased temperature or atmospheric pressure. The extreme temperatures in the working faces at 1150 metres were 79° and 86° F., and the maximum in the end of a drift, 100°; and these were quite bearable on account of the energetic ventilation maintained, and the dryness of the air. The yield per man on the working faces was 4·5 tons, and for the whole of the working force underground, 0·846 tons, which is not less than that realized in shallower mines. From the experience of such workings it is considered that 1500 metres would be a possible workable depth, the rock temperature being 132°, and those of the intake and return galleries, 92° and 108° respectively. Under such conditions work would be practically impossible except with very energetic ventilation and dry air. It would be scarcely possible to circulate more than 120,000 to 130,000 cub. ft. per minute under such conditions, and the number of working places would thus be restricted, and consequently the output reduced to about 500 tons per shift of 10 hours, which could be raised by a single engine at the surface without requiring any very different appliances from those in current use.

In the United Kingdom the ownership of coal, like that of other minerals, is in the proprietor of the soil, and passes with it, except when specially reserved in the sale. Coal lying under the sea below low-water mark belongs to the crown, and can only be worked upon payment of royalties, even when it is approached from shafts sunk upon land in private ownership. In the Forest of Dean, which is the property of the crown as a royal forest, there are certain curious rights held by a portion of the inhabitants known as the Free Miners of the Forest, who are entitled to mine for coal and iron ore, under leases, known as gales, granted by the principal agent or gaveller representing the crown, in tracts not otherwise occupied. This is the only instance in Great Britain of the custom of free coal-mining under a government grant or concession, which is the rule in almost every country on the continent of Europe.

The working of collieries in the United Kingdom is subject to the provisions of the Coal Mines Regulation Act 1887, as amended by several minor acts, administered by inspectors appointed by the Home Office, and forming a complete disciplinary code in all matters connected with coal-mining. An important act was passed in 1908, limiting the hours of work below ground of miners. For a detailed account of these various acts see the article.

Coal-mining is unfortunately a dangerous occupation, more than a thousand deaths from accident being reported annually by the inspectors of mines as occurring in the collieries of the United Kingdom.

The number of lives lost during the year 1906 was, according to the inspectors’ returns:—

The principal sources of danger to the collier, as distinguished from other miners, are explosions of fire-damp and falls of roof in getting coal; these together make up about 70% of the whole number of deaths. It will be seen that the former class of accidents, though often attended with great loss of life at one time, are less fatal than the latter.

—The most important new publication on British coal is that of the royal commission on coal supplies appointed in 1901, whose final report was issued in 1905. A convenient digest of the evidence classified according to subjects was published by the Colliery Guardian newspaper in three quarto volumes in 1905–1907, and the leading points bearing on the extension and resources of the different districts were incorporated in the fifth edition (1905) of Professor Edward Hull’s Coal Fields of Great Britain. The Report of the earlier royal commission (1870), however, still remains of great value, and must not be considered to have had its conclusions entirely superseded. In connexion with the re-survey in greater detail of the coalfields by the Geological Survey a series of descriptive memoirs were undertaken, those on the North Staffordshire and Leicestershire fields, and nine parts dealing with that of South Wales, having appeared by the beginning of 1908.

An independent work on the coal resources of Scotland under the title of the Coalfields of Scotland, by R. W. Dixon, was published in 1902.

The Rhenish-Westphalian coalfield was fully described in all details, geological, technical and economic, in a work called Die Entwickelung des niederrheinisch-westfälischen Steinkohlen Bergbaues in der zweiten Hälfte des 19ten Jahrhunderts (also known by the short title of Sammelwerk) in twelve quarto volumes, issued under the auspices of the Westphalian Coal Trade Syndicate (Berlin, 1902–1905).

The coalfields of the Austrian dominions (exclusive of Hungary) are described in Die Mineralkohlen Österreichs, published at Vienna by the Central Union of Austrian mineowners. It continues the table of former official publications in 1870 and 1878, but in much more detail than its predecessors.

Systematic detailed descriptions of the French coalfields appear from time to time under the title of Études sur les gîtes minéraux de la France from the ministry of public works in Paris.

Much important information on American coals will be found in the three volumes of Reports on the Coal Testing Plant at the St Louis Exhibition, published by the United States Geological Survey in 1906. A special work on the Anthracite Coal Industry of the United States, by P. Roberts, was published in 1901.

The most useful general work on coal mining is the Text Book of Coal Mining, by H. W. Hughes, which also contains detailed