The New International Encyclopædia/Coal
COAL (AS. col, OHG. kolo, Ger. Kohle; ultimately connected with Skt. jval, to blaze, and probably with Ir., Gael. gual, coal). A mineral fuel of solid character, found and used in many countries. The name is a word common to all the languages of the Gothic stock, and seems allied to the Latin calere, to be hot; as also ‘to glow,’ and ‘kiln.’ The word ‘coal’ has often prefixed to it some qualifying word, to distinguish different kinds of coal; such as cannel coal, stone coal, pea coal, etc.
Origin. Coal is one of the most important economic minerals, and is of vegetable origin. When vegetable matter accumulates under water it undergoes a slow process of decomposition, gradually giving off its nitrogen, hydrogen, oxygen, and some carbon, the result of which if carried far enough is the formation of a mass of carbon. Peat (q.v.), the material so often found underlying swampy tracts in north temperate zones, represents the first stage in the coal-forming process, and the further stages are obtained by the burial of these vegetable deposits under great loads of sediment, where they become subjected to pressure, and at times to heat also. This effects a series of changes, especially consolidation and loss of oxygen, and gives a series of products, whose nature depends on the degree to which the original vegetable matter has been changed. The products are known as lignite, bituminous coal, and anthracite coal; these three types being connected by all degrees of intermediate stages. In Carboniferous times certain regions were covered by rank and luxuriant vegetation which grew upon swampy land slightly raised above the level of the sea. As the plants died, their remains fell into the water of the swamp, and slowly formed an accumulation of vegetable matter of increasing thickness. By slow subsidence this thick layer of vegetable matter sank below the water, and became gradually covered by sand, mud, or other mineral sediments, washed out from the shore. Successive elevations and depressions, with intervening accumulations, may thus have yielded successive beds. Subsequent elevation, folding of the earth's crust, and accompanying metamorphism, followed by erosion of the surface, has exposed to view the edges of the once deeply buried beds of coal.
Composition. The following analyses of peat, lignite or brown coal, and true coal indicate the changes which vegetable matter undergoes by decay and pressure:
Ultimate Analyses of Peat and Coals
| COMPONENTS | PEAT | LIGNITE | BITUMINOUS COAL | ANTHRACITE | |||||||
| Carbon, Wyo. |
Robertson Co., Texas. |
Whiteside, Tenn. |
Brazil, Ind. |
Spring Mt., Pa. |
Crested Butte, Colo. | ||||||
| % | % | % | % | % | % | % | |||||
| Water | 20.00 | 7.35 | 16.40 | 1.04 | 5.45 | 1.97 | 0.72 | ||||
| Carbon | 47.20 | 63.65 | 54.46 | 78.83 | 76.05 | 91.40 | 82.50 | ||||
| Hydrogen | 4.90 | 4.60 | 4.41 | 5.51 | 5.88 | 2.59 | 5.15 | ||||
| Oxygen |
|
19.44 |
|
4.00 | 8.13 | 0.08 | 4.55 | ||||
| Nitrogen | 1.40 | 1.12 | 1.37 | 0.21 | 1.12 | ||||||
| Sulphur | ........ | 0.76 | 0.96 | 2.61 | 0.80 | 0.71 | 0.85 | ||||
| Ash | 5.00 | 2.80 | 7.70 | 6.89 | 2.32 | 3.04 | 6.04 | ||||
These analyses bring out well the general relations of the different elements, and the increase in carbon toward the anthracite end of the series; still they give but little information concerning the commercial value of the coal. The usual custom in making a commercial analysis is to determine the form in which these elements occur—that is, the amount of water, volatile hydrocarbon, fixed carbon, sulphur, and ash. This proximate analysis is also used as the basis of classification of coals. Thus:
| COMPONENTS | Peat | Lignite | Bituminous coal |
Anthracite |
| % | % | % | % | |
| Moisture | 78.89 | 13.29 | 1.30 | 2.94 |
| Volatile hydrocarbons | 13.84 | 59.86 | 20.87 | 4.29 |
| Fixed carbon | 6.49 | 18.52 | 67.20 | 88.18 |
| Ash | 0.78 | 8.32 | 8.80 | 4.04 |
| Sulphur | ....... | 2.36 | 1.83 | 0.55 |
A proximate analysis like the above is of practical value, since it gives us a better conception of the coal worth. Thus the freedom of burning increases with the amount of volatile hydrocarbons, while the heating power depends on the amount of fixed carbon present. Sulphur is an injurious constituent when the coal is to be used in the manufacture of gas or for metallurgical purposes; while ash is undesirable, since it displaces so much carbon, and if it contains fusible impurities such as iron, lime, or alkalies, it causes clinkering. Moisture retards the heating power of the coal until it is driven off. Since the heating power of coal is its most important property, this is often tested by means of an apparatus known as a ‘calorimeter.’ (See Calorimetry.) The principle of the test depends on the determination of the weight of water which can be converted into steam at 212° F. under atmospheric pressure with one pound of coal. (See also Heat.) In addition to the varieties of coal given above, there may be mentioned semi-bituminous coal and cannel coal. The properties of the different varieties are as follows:
Anthracite contains 84 per cent. or more of fixed carbon, and also little ash, sulphur, and moisture. It has great heating power, and burns with a smokeless flame. Owing to its comparative scarcity, it commands a higher price than the bituminous. Anthracite is dense, has a shining lustre, and usually breaks with a smooth conchoidal fracture. It is estimated by some geologists that about ten inches of peat is required to make one inch of anthracite coal. In the United States anthracite coal is confined chiefly to the eastern edge of the Appalachians in Pennsylvania, where the folding of the rocks has been very intense, and where the coal-seams have been subjected to great pressure. It is also known in Colorado, near Crested Butte, where the bituminous coals have been locally changed to anthracite by the heat of basalt intrusions. It is mined extensively in England, and large quantities are known in China.
Bituminous Coal contains 50 to 75 per cent. of fixed carbon, and 25 to 30 per cent. volatile hydrocarbons. It burns with a rather long and smoky flame, and is also much used for steaming purposes. Many bituminous coals have the property of coking or caking (see Coke) when heated to redness. Most of the Carboniferous and many of the Mesozoic coals of the United States are bituminous.
Semi-Bituminous Coal resembles bituminous coal in appearance, but is intermediate between it and anthracite. It contains from 70 to 84 per cent. of fixed carbon, and is considered of superior value for steaming purposes. This variety is obtained from Pennsylvania, Maryland, and West Virginia.
Cannel Coal is a variety of coal very rich in volatile hydrocarbons, and found sparingly in parts of Kentucky, Ohio, and Indiana. Its chief use is as a gas-enricher, since it yields 8000 to 15,000 cubic feet of gas per ton. Cannel coal is so called because it burns with a bright flame like a candle, and the name parrot coal was given to it in Scotland, from the crackling or chattering noise it makes while burning. It is very compact in texture and may even have an oily look; certain forms found in England admit of being polished, and ornamental articles have been made from them and sold under the name of jet.
Lignite, or Brown Coal, is a partially formed coal, containing much moisture and volatile matter. It often shows the woody structure of peat and burns very easily, but gives off little heat.
History and Use. The value of coal does not seem to have been known to the ancients, nor is it well known at what time it began to be used for fuel. Some say that it was used by the ancient Britons; at all events, it was an article of household consumption to some extent during the Anglo-Saxon period as early as A.D. 852. There seems to be reason for thinking that England was the first European country in which coal was used in any considerable quantities. In America the deposits near Richmond, Va., were discovered in 1701, and mining was begun in 1750, while anthracite was first produced in 1793. Extended coal-mining in the United States did not really begin, however, until about 1820. Since that time up to the present, the increase has been about 3500 per cent. In 1822 the amount of coal mined in Virginia was about 48,000 long tons. Now the production for the United States is about 270,000,000 short tons, or greater than that of any other country of the world.
Coal is used largely for domestic purposes, either as fuel or, in the form of gas, for illumination. Its use for the latter purpose is, however, not so widespread as formerly, water-gas having superseded it to a considerable extent. In the production of steam for motive power it also finds important applications. It is furthermore widely employed in the metallurgical industry in the form of either coal or coke, and in this connection may serve both as a fuel and as a reducing agent. Coke (q.v.) is made only from bituminous coal. Lignite seldom has much value as a fuel, owing to the large percentage of moisture that it contains. Because of this moisture it tends to crack in drying, and must therefore be used soon after mining, and in localities where it does not require long transportation from mine to market. This is true, for instance, of some of the lignite deposits in Colorado which are near the Denver market, and therefore possess commercial value. Lignite has sometimes been successfully used in the manufacture of producer gas, and indeed even peat has been found adaptable for this purpose.
Coal Areas. The leading coal-producing countries of the present day are the United States, Great Britain, Germany, France, Belgium, Austria-Hungary, and Russia. The Russian coal-fields are probably the most extensive in Europe. In the Far East coal is known in India, the Malay Archipelago, Japan, and China. The coal-fields of the last-named country are probably the greatest in the world, and may become a source of European supply. Up to the present time they have not been developed in a systematic manner. Italy, Spain, Sweden, Australia, New Zealand, Borneo, the Philippine Islands, and many countries in Africa also produce coal; while in America deposits are worked in Canada, Mexico, Chile, and Argentina, and are known to occur in Colombia and Peru.
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| COPYRIGHT, 1902, DODD, MEAD & COMPANY. |
United States. The coal-fields of the United
States are especially extensive; indeed, in some
instances the deposits of a single State exceed
those of Germany or France in area. They are
separable into several regions, the divisions being
geographical and not geological. The geological
ages of the coals in 1, 2, 3, 4, and 6 (table on next pagebelow) are all Carboniferous, except small
Triassic areas in Virginia and North Carolina.
Those of 5 are Cretaceous and Tertiary.
By far the most important of these regions is the Appalachian, which takes in portions of Pennsylvania, Ohio, West Virginia, Virginia, Maryland, Eastern Kentucky, Eastern Tennessee, Georgia, North Carolina, and Alabama. It is about 750 miles long, and 70 to 80 miles wide. The coals are all bituminous or semi-bituminous with the exception of those at the northeastern end, in Pennsylvania, where close folding of the rocks has changed the bituminous into anthracite coal. In general, the rocks at the upper or northern end of the Appalachian belt are folded, while those of the lower end, as in Alabama, are often faulted in addition, so that the coal-miner frequently finds the coal-seam suddenly broken off. The Carboniferous section of this region has been described in the article Carboniferous System, from which it may be seen that the coal-beds occupy more or less well-marked stratigraphic positions. The maximum thickness of strata is from 2,500 to 3000 feet; the seams measuring 120 feet near Pottsville, 62 feet at Wilkesbarre, and 25 feet at Pittsburg, showing a gradual diminution in a westward direction. The most persistent coal deposit is the Pittsburg seam, which is known over an area measuring 225 by 100 miles, and has a thickness varying from 2 to 14 feet. In Alabama the deposits are distributed among three districts—the Warrior, Cahawba, and Coosa, named after the rivers that drain them. The anthracite district of Pennsylvania occupies an area of about 470 square miles on the left bank of the Susquehanna. The strata between Pottsville and Wyoming, which belong to the lowest portion of the coal-measures, are probably about 3000 feet thick; but it is difficult to make an exact estimate, because of the numerous folds and contortions. There are from 10 to 12 seams, each over three feet in thickness. The principal one, known as the Mammoth or Baltimore vein, is 29 feet thick at Wilkesbarre, and in some places exceeds even 60 feet. Many of the Appalachian coals, notably those of western Pennsylvania, West Virginia, and Alabama, produce excellent coke. The Ohio coals do not yield good coke. In most of the other coal-fields of the country the coal-fieds lie comparatively flat, and the basins are quite shallow.
Coal Fields of the United States
| Region | State | Area | ||
| Square Miles | ||||
|
Pennsylvania | 10,700 | ||
| Ohio | 10,000 | |||
| Maryland | 550 | |||
| Kentucky | 9,000 | |||
| West Virginia | 16,000 | |||
| Virginia | 365 | |||
| Tennessee | 5,100 | |||
| North Carolina | 2,700 | |||
| Georgia | 200 | |||
| Alabama | 8,660 | |||
| Total | 63,275 | |||
| 2. Northern | Michigan | 6,700 | ||
|
Kentucky | 4,000 | ||
| Indiana | 6,450 | |||
| Illinois | 36,800 | |||
| Total | 47,250 | |||
|
Missouri | 26,700 | ||
| Iowa | 18,000 | |||
| Nebraska | 3,200 | |||
| Kansas | 17,000 | |||
| Arkansas | 9,100 | |||
| Indian Ter. | 20,000 | |||
| Texas | 4,500 | |||
| Total | 98,500 | |||
|
Colorado | Unknown | ||
| New Mexico | “ | |||
| Utah | “ | |||
| Wyoming | “ | |||
| South Dakota | “ | |||
| North Dakota | “ | |||
| Montana | “ | |||
| California | “ | |||
| Oregon | “ | |||
| Washington | “ | |||
| 6. Rhode Island | Rhode Island | Small | ||
The Michigan area is a small one in the lower peninsula of Michigan. It forms a circular basin with a diameter of about 50 miles. The coals are bituminous, non-coking, and are mined chiefly for local use. The seams range from a few inches to three feet in thickness.
The Central area includes parts of western Kentucky, Indiana, and Illinois, and lies chiefly within the latter State. These coals are all bituminous, of Carboniferous age, and are used chiefly for steaming. The thickness of the Carboniferous System varies from 1200 to 1400 feet in southern Illinois, to about 600 feet in Indiana, and the workable coal-seams vary in number from 7 to 12 in Illinois, and their thickness from three to eight feet. The ‘block coal’ of Indiana has quite a reputation. The Western Central area includes Iowa, Missouri, Arkansas, Indian Territory, Kansas, and part of Texas. Here again there is an abundance of bituminous coal, which has been developed chiefly in Iowa and Missouri, while Kansas is now coming into prominence. The coals of this area are chiefly adapted to smithing and steaming purposes, and, so far as tried, Kansas yields the only coking varieties.
CROSS-SECTION OF ANTHRACITE COAL MEASURES (PENNSYLVANIA).
The Cordilleran area comprises the coal regions of Colorado, New Mexico, Utah, Wyoming, South Dakota, North Dakota, Montana, California, Oregon, and Washington. In this field are found many varieties grading between lignite and anthracite. They are all of either Tertiary or Cretaceous age, and their discovery showed the incorrectness of the old classification, which included all post-Carboniferous coals under lignite. Colorado is perhaps the most important producer, having a number of good bituminous seams. Those in the vicinity of Crested Butte have been changed locally to anthracite by the metamorphic action of igneous intrusions. Excellent coking coals are found near Trinidad. The New Mexican coals are in part an extension of the Colorado veins, and bear a good reputation, as do also many of the Wyoming coals. California has little fuel of good quality, and has for many years drawn on Australia for its coal-supply, but in recent years the coals of Oregon, Washington, and British Columbia have become a source of supply.
The rocks of the small Rhode Island area have been so highly metamorphosed that the coal has been altered to graphitic anthracite. It is sold on the market as amorphous graphite, and has little value as a fuel.
Canada. The Acadian field includes deposits in Nova Scotia and New Brunswick, the former being quite important. The coals are bituminous and of good quality. In the mountain ranges of British Columbia extensive coal-seams have been discovered, and they are now under development. A good quality of coke is made from the coal of Crow's Nest Pass, which finds a market at the British Columbian smelters. The most productive mines of the Pacific Coast are located on Vancouver Island, whence large shipments of bituminous coal are made to San Francisco and other ports in the Western United States.
South America. Coal, probably of Carboniferous age, is found in the Brazilian provinces of São Pedro, Rio Grande do Sul. Santa Catharina. also in the neighboring Republic of Uruguay. Very little development work has been done in the fields, and the output is inconsiderable. In Argentina and Chile, where Cretaceous coal occurs, there is more activity; but these countries still depend largely upon Great Britain for their supplies. In Peru both Cretaceous and Carboniferous deposits are found at various points in the interior, the former occupying a position on the first rise of the Andes, while the latter occurs in higher ground and at a greater distance from the coast.
United Kingdom. Next to the coal-fields of the United States, those of the United Kingdom are of the greatest economic importance. Within the limits of England, Scotland, and Wales there are more than twenty areas underlain by seams of anthracite, bituminous, and cannel coal. The largest of these areas is that of South Wales, in Monmouthshire and Pembrokeshire, which has a length of about 50 miles and a width of nearly 20 miles. The coal-measures form an elliptical basin, and are several thousand feet in thickness. Coal is found in three horizons, of which the upper has no less than 82 seams, measuring 180 feet in all. The lowest horizon yields valuable steam and blast-furnace coal. In the north of England the coal-fields of Lancashire, Derbyshire, and Yorkshire are the largest. The Lancashire field is of irregular quadrilateral form, with a width of about 18 miles from north to south, and a length from east to west of more than 50 miles. It includes about 100 feet of coal in workable seams, which dip at a high angle and are much broken by faulting. The Yorkshire and Derbyshire measures occupy a single area that extends for a distance of about 60 miles from Bradford on the north to near Derby on the south, and has a breadth of from 3 to 32 miles. They yield bituminous coal, excellent for steaming and iron-making purposes. North of the Yorkshire field is the large basin of Northumberland and Durham, from which steaming, coking, and house coals are produced. In Scotland the coal-measures are extensively developed in Ayrshire, Lanarkshire, Stirlingshire, and Fifeshire. The productive coal-fields of the United Kingdom belong to the Carboniferous period; brown coal of Jurassic or Tertiary age is known to occur, but the seams are too small to be profitably exploited. The exports of coal from this country are of great importance, amounting in 1900 to 51,638,000 short tons, valued at $193,032,000. Much of the coal goes to Italy, Russia, Holland, and to the European countries that possess small resources of the mineral, while the remainder is exported to the more remote parts of the world.
Further details regarding the distribution of coal will be found under the titles of countries.
Output. The world's annual production at the present time is about 850,000,000 short tons; the output in 1900, according to The Mineral Industry, was distributed as follows:
| COUNTRY | SHORT TONS |
| United States | 268,315,433 |
| Great Britain | 252,176,352 |
| Germany | 164,850,131 |
| Austria-Hungary | 43,020,049 |
| France | 36,673,945 |
| Belgium | 25,863,063 |
| Russia | 16,500,000 |
| Canada | 5,608,636 |
| Japan | 8,189,490 |
| India | 6,852,803 |
| Spain | 2,847,199 |
| Sweden | 278,132 |
| Italy | 528,989 |
| Africa | 546,563 |
| New South Wales | 6,168,337 |
| New Zealand | 1,225,603 |
| Queensland | 656,939 |
| Victoria | 237,052 |
| Tasmania | 56,822 |
| Other Countries | 2,755,750 |
| Total | 843,247,288 |
It is interesting to follow the progress of the United States as a coal-producer. In 1868 Great Britain produced 3.6 times as much coal as the United States, while Germany's product that year was 15 per cent. greater than that of the United States. In 1871 the United States exceeded Germany's output by about 10 per cent., but afterwards fell back to third place until in 1877 she once more sprang forward, and gained on both Germany and Great Britain. In 1899 the United States led the world, and supplied nearly 32 per cent. of its production.
The average price of bituminous coal at the mines in the United States, per short ton, varied between 1893 and 1900 from $0.80 to $1.04; while that of anthracite was between $1.41 and $1.59 for the same period. The total number of laborers employed during 1900 was 449,181, of which number 144,206 were anthracite miners.
During the closing years of the nineteenth century European countries have been confronted with a most serious problem—the exhaustion of their coal-supply. This condition was emphasized in 1899 and 1900 by the occurrence of strikes in the Wales cool regions, by war in South Africa, and by a stimulation of industries in Germany which required much additional coal. Prospecting having shown but little reserve material, the most natural result was to look to the United States, and in 1900 there began a movement of coal to Europe, which may before many years assume large proportions. Ocean freights are the present great drawback.
Mining of Coal. The presence of coal in paying quantities having been determined by prospecting and geological surveys, the next consideration is to extract this coal from seams. No definite rules can be given for the selection of a method of mining that will cover all conditions; each mine furnishes a distinct and separate problem. Every system of mining, however, aims to extract the maximum amount of the deposit in the best marketable shape and at a minimum cost and danger. Speaking broadly, all methods of mining come under the head of either open working or closed working. Open working is employed when the deposits have no overburden of barren rock or earth, or where this overburden is of such small depth that it can be easily and cheaply removed, leaving the coal deposit exposed. The mining of such exposed seams of coal is really a process of excavation or quarrying, and the machines used in making open-pit excavations and in quarrying are applicable to the work. Closed working is adopted when the depth of the overburden is so great that the mining must be conducted underground. The first task in opening up underground coal-seams is to secure access to the seam by means of shafts, slopes, or tunnels. Shafts are vertical openings from the ground surface to the coal-seams. In the United States shafts are usually made square or rectangular in form. This practice is largely due to the fact that timber is used for lining shafts. In Europe round or oval shafts are frequently employed with linings of brick, iron, or masonry.
COAL-MINING
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1. THE OLD WAY—With hand pick.2. THE MODERN WAY—With machine pick.
Generally the shafts are divided into two or more compartments, in each of which is installed an elevator for hoisting the coal-cars to the surface. The number of compartments in a shaft and their arrangements depend upon the particular use to which the shaft is to be put, the number of shafts employed, and their depths. Where the seams are comparatively near the surface, it is usually cheaper to sink a number of two or three compartment shafts than it is to haul all the ore to one large shaft; while, when the shafts are very deep, it is preferable to sink a smaller number of four or six compartment shafts and extend the underground haulage to a single shaft over a great area of the workings. Where timber lining is employed, a stronger construction is obtained by placing the compartments side by side in a long, narrow shaft than by grouping them in a square shaft. In shallow mines separate shafts are often employed for hoisting and for pumping, ventilation and ladder-ways. One of the largest coal-mine shafts in America is situated at Wilkesbarre, Pa.; it is 1039 feet deep, 12 × 52 feet in size, and has five compartments. The methods of sinking mine shafts are essentially the same as those used in sinking shafts for tunnels. (See Tunnel.) Slopes are openings begun at the outcrop of an inclined seam, which they follow down into the earth. Slopes are usually made with three compartments side by side, two of which are used as hoistways and the third for the traveling-way, piping, etc. When the dip of the slope is under 40 degrees the slope is made about seven feet high, but when the dip exceeds 40 degrees cages have to be used and a great height is necessary. Slopes are usually lined with timber. Tunnels are nearly horizontal passageways beginning on the side of a hill or mountain and extending into the earth until they meet the coal-seam; they are built for both haulage and drainage purposes, and are constructed like railway tunnels, except that the cross-section is usually much smaller, and that it is lined with timber instead of with permanent masonry. The forms of timbering used in coal-mining are various, and are of interest chiefly to the practical miner; special treatises should be consulted by those interested in the details. In a general way, it may be said that timber used for underground support in mines should be of a light and elastic variety of wood. Oak, beech, and similar woods are heavy and have great strength, but when they do break it is suddenly and without warning, thus bringing disaster to the miners who might escape if a tough wood were employed which gives warning of rupture by bending and cracking. It is a very common practice to employ preserved timber in mining work. See Forestry.
COAL-MINING
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| 1. | SULLIVAN ELECTRIC CHAIN MACHINE, Making “tight” or corner cut. | 2. | SULLIVAN ELECTRIC CHAIN MACHINE, Cutting across face of room. | |
The systems of working the coal-seams after access is attained to them by the means described are two, known as the room-and-pillar and the long-wall systems. The room-and-pillar method—also known as the pillar-and-chamber or board-and-pillar method, which may include the pillar-and-stall system—is the oldest of the systems, and the one very generally used in the United States. By this system, coal is first mined from a number of comparatively small places, called rooms, chambers, stalls, boards, etc., which are driven either square from or at an angle to the haulageway. Pillars are left to support the roof. In the long-wall method the whole face of the coal-seam is taken out, leaving no coal behind, and the roof is allowed to settle behind as the excavation progresses, care being taken to preserve haulageways through the falling material. Both the room-and-pillar and the long-wall methods are employed in various modifications, for the details of which special treatises on coal-mines should be consulted. The coal is cut from the seam by hand or by some form of coal-cutting machine. In America machine cutting is used extensively. There are four general types of machines in general use: Pick machines, chain-cutter machines, cutter-bar machines, and long-wall machines; the machines most used in America are pick machines and chain-cutter machines. Both compressed air and electricity are used for operating coal-cutting machines. Pick machines are very similar to a rock-drill; chain-cutter machines consist of a low metal bed-frame upon which is mounted a motor that rotates a chain to which suitable cutting teeth are attached. The ventilation of the workings, owing to the presence of gases, is a very important feature of coal-mining, and great care is taken to lay out the workings so as to facilitate ventilation. Mechanical ventilation by means of fans and blowers (see Blowing Machines) is usually employed. Hoisting in mines is accomplished by means of cages running up and down the shafts, and operated by large hoisting engines on the surface. There are two general systems of hoisting in use—hoisting without attempt to balance the load, in which the cage and its load are hoisted by the engine and lowered by gravity, and hoisting in balance, in which the descending cage or a special counter-balance assists the engine to hoist the loaded ascending cage. Haulage in mines is accomplished by animal power or by steam hoisting engines operating a system of rope haulage or by mine locomotives operated by steam, electricity, compressed air, or gasoline.
The preparation of mined coal for the market consists in screening the coal over bars and through revolving or over shaking screens, together with breaking it with rolls to produce the required market size. The large lumps of slate or other impurities are separated by hand, while the smaller portions are picked out by automatic pickers or by hand by boys or old men seated along the chutes leading to the shipping pockets or bins. When coal contains much sulphur, this is frequently removed by washing it with water in special washing plants.
Bibliography. Lesley, Manual of Coal and Its Topography (Philadelphia, 1850)—a good work, but difficult to find; Chance, “Coal-Mining,” in Second Geological Survey of Pennsylvania, Report AC (Harrisburg, 1883); Hughes, A Textbook of Coal-Mining (London, 1899); Peel, Elementary Textbook of Coal-Mining (London, 1901); Macfarlane, The Coal Regions of America, Their Topogrnphy, Geology, and Development (New York, 1875); Nicolls, The Story of American Coals (Philadelphia, 1897); Lesley and others, “Reports on the Coal-Fields of Pennsylvania,” in various publications of the Second Geological Survey of Pennsylvania (Harrisburg). Numerous scattered papers have been published in the following annuals and periodicals: Transactions of the American Institute of Mining Engineers (New York); The Mineral Industry (New York); The Engineering and Mining Journal (New York); Mines and Minerals (Scranton, Pa.); “Mineral Resources of the United States,” United States Geological Survey (Washington). For foreign coal deposits, consult: Memoirs of the Geological Survey of Great Britain (London); Reports of Progress of the Geological Survey of the United Kingdom (London); Annales de la société géologique de Belgique (Liége, 1874 et seq.); Bulletin de la société belge de géologie, de paléontologie et d'hydrologie (Brussels, 1877 et seq.); Annales des mines (Paris, 1816 et seq.); Bulletin de la société géologique de France (Paris, 1896 et seq.); Lozé, Les charbons britanniques et leur épuisement (Paris, 1900); Zeitschrift für praktische Geologie (Berlin, 1893 et seq.). See Anthracite; Bituminous Coal; Carboniferous System; Coke; Culm; Cretaceous System; Peat; Tertiary System; Graphite; Carbon; Fire-Clay; and the articles on the different States and countries in which coal has been found.