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Coal
1911 Encyclopedia Britannica
or In its most general sense the term " coal " includes all varieties of carbonaceous minerals used as fuel, but it is now usual in England to restrict it to the particular varieties of such minerals occurring in the older Carboniferous formations. On the continent of Europe it is customary to consider coal as divisible into two great classes, depending upon differences of colour, namely, brown coal, corresponding to the term "lignite" used in England and France, and black or stone coal, which is equivalent to coal as understood in England. Stone coal is also a local English term, but with a signification restricted to the substance known by mineralogists as anthracite. In old English writings the terms pit-coal and sea-coal are commonly used. These have reference to the mode in which the mineral is obtained, and the manner in which it is transported to market.
The root kol is common to all the Teutonic nations, while in French and other Romance languages derivatives of the Latin carbo are used, e.g. charbon de terre. In France and Belgium, however, a peculiar word, houille, is generally used to signify mineral coal. This word is supposed to be derived from the Walloon hoie, corresponding to the medieval Latin hullae. Littre suggests that it may be related to the Gothic haurja, coal. Anthracite is from the Greek a vepa , and the term lithanthrax, stone coal, still survives, with the same meaning, in the Italian litantrace. It must be borne in mind that the signification now attached to the word coal is different from that which formerly obtained when wood was the only fuel in general use. Coal then meant the carbonaceous residue obtained in the destructive distillation of wood, or what is known as charcoal, and the name collier was applied indifferently to both coal-miners and charcoal-burners.
The spelling " cole " was generally used up to the middle of the 17th century, when it was gradually superseded by the modern form, " coal." The plural, coals, seems to have been used from a very early period to signify the broken fragments of the mineral as prepared for use.
Coal is an amorphous substance of variable composition, and therefore cannot be as strictly defined as a crystallized or definite mineral can. It varies in colour from a light brown in the newest lignites to a pure black, often with a bluish or yellowish tint in the more compact anthracite of the older formations. It is opaque, except in exceedingly thin slices, such as made for microscopic investigation, which are imperfectly transparent, and of a dark brown colour by transmitted light. The streak is black in anthracite, but more or less brown in the softer varieties. The maximum hardness is from 2.5 to 3 in anthracite and hard bituminous coals, but considerably less in lignites, which are nearly as soft as rotten wood. A greater hardness is due to the presence of earthy impurities. The densest anthracite is of ten of a semi-metallic lustre, resembling somewhat that of graphite. Bright, glance or pitch coal is another brilliant variety, brittle, and breaking into regular fragments of a black colour and pitchy lustre. Lignite and cannel are usually dull and earthy, and of an irregular fracture, the latter being much tougher than the black coal. Some lignites are, however, quite as brilliant as anthracite; cannel and jet may be turned in the lathe, and are susceptible of taking a brilliant polish. The specific gravity is highest in anthracite and lowest in lignite, bituminous coals giving intermediate values (see Table I.). As a rule, the density increases with the amount of carbon, but in some instances a very high specific gravity is due to intermixed earthy matters, which are always denser than even the densest form of coal substance.
Coal is never definitely crystalline, the nearest approach to such a structure being a compound fibrous grouping resembling that of gypsum or arragonite, which occurs in some of the steam coals of South Wales, and is locally known as " cone in cone," but no definite form or arrangement can be made out of the fibres. Usually it occurs in compact beds of alternating bright and dark bands in which impressions of leaves, woody fibre and other vegetable remains are commonly found. There is generally a tendency in coals towards cleaving into cubical or prismatic blocks, but sometimes the cohesion between the particles is so feeble that the mass breaks up into dust when struck. These peculiarities of structure may vary very considerably within small areas; and the position of the divisional planes or cleats with reference to the mass, and the proportion of small coal or slack to the larger fragments when the coal is broken up by cutting-tools, are points of great importance in the working of coal on a large scale.
The divisional planes often contain small films of other minerals, the commonest being calcite, gypsum and iron pyrites, but in some cases zeolitic minerals and galena have been observed. Salt, in the form of brine, is sometimes present in coal. Hydrocarbons, such as petroleum, bitumen, paraffin, &c., are also found occasionally in coal, but more generally in the associated sandstones and limestones of the Carboniferous formation. Gases, consisting principally of light carburetted hydrogen or marsh gas, are of ten present in considerable quantity in coal, in a dissolved or occluded state, and the evolution of these upon exposure to the air, especially when a sudden diminution of atmospheric pressure takes place, constitutes one of the most formidable dangers that the coal miner has to encounter.
The classification of the different kinds of coal may be considered from various points of view, such as their chemical composition, their behaviour when subjected to heat aa s sifica= or when burnt, and their geological position and iron. origin. They all contain carbon, hydrogen, oxygen and nitrogen, forming the carbonaceous or combustible portion, and some quantity of mineral matter, which remains after combustion as a residue or " ash." As the amount of ash varies very considerably in different coals, and stands in no relation to the proportion of the other constituents, it is necessary in forming a chemical classification to compute the results of analysis after deduction of the ash and hygroscopic water. Examples of analyses treated in this manner are furnished in the last column of Table I., from which it will be seen that the nearest approach to pure carbon is furnished by anthracite, which contains above 90%. This class of c coal burns with a very small amount of flame, produc ing intense local heat and no smoke. It is especially used for drying hops and malt, and in blast furnaces where a high temperature is required, but it is not suited for reverberatory furnaces.
The most important class of coals is that generally known as bituminous, from their property of softening or undergoing an apparent fusion when heated to a temperature far below that at which actual combustion takes place.
p This term is founded on a misapprehension of the nature of the occurrence, since, although the softening takes place at a low temperature, still it marks the point at which destructive distillation commences, and hydrocarbons both of a solid and gaseous character are formed. That nothing analogous to bitumen exists in coals is proved by the fact that the ordinary solvents for bituminous substances, such as bisulphide of carbon and benzol, have no effect upon them, as would be the case if they contained bitumen soluble in these re-agents. The term is, however, a convenient one, and one whose use is almost a necessity, from its having an almost universal currency among coal miners. The proportion of carbon in bituminous coals may vary from 80 to 90% the amount being highest as they approach the character of anthracite, and least in those which are nearest to lignites. The amount of hydrogen is from 42 to 6%, while the oxygen may vary within much wider limits, or from about 3 to 14%. These variations in composition are attended with corresponding differences in qualities, which are distinguished by special names. Thus the semi 'anthracitic coals of South Wales are known as " dry " or " steam coals," being especially valuable for use in marine steam-boilers, as they burn more readily than anthracite and with a larger amount of flame, while giving out a great amount of heat, and practically without producing smoke. Coals richer in hydrogen, on the other hand, are more useful for burning in open fires - smiths' forges and furnaces - where a long flame is required.
The excess of hydrogen in a coal, above the amount necessary Physical pro= perties. to combine with its oxygen to form water, is known as " disposable " hydrogen, and is a measure of the fitness of the coal coal. for use in gas-making. This excess is greatest in what is Gas known as cannel coal, the Lancashire kennel or candle coal, so named from the bright light it gives out when burning. This, although of very small value as fuel, commands a specially high price for gas-making. Cannel is more compact and duller than ordinary coal, and can be wrought in the lathe and polished.
oxygen and hygroscopic water are much higher than in true coals. The property of caking or yielding a coherent coke is usually absent, and the ash is often very high. The specific gravity is low when not brought up by an excessive amount of earthy matter. Sometimes it is almost pasty, and crumbles to powder when dried, so as to be susceptible of use as a pigment, forming the colour known as Cologne earth, which resembles umber or sepia. In Nassau and Bavaria woody structure is very common, and it is [[Table I]].-Elementary Composition of Coal (the figures denote the amounts per cent). These properties are most highly developed in the substance known as jet, which is a variety of cannel found in the lower oolitic strata of Yorkshire, and is almost entirely used for ornamental purposes, the whole quantity produced near Whitby, together with a further supply from Spain, being manufactured into articles of jewellery at that town.
When coal is heated to redness out of contact with the air, the more volatile constituents, water, hydrogen, oxygen, and nitrogen are in great part expelled, a portion of the carbon being also volatilized in the form of hydro carbons and carbonic oxide,-the greater part, however, remaining behind, together with all the mineral matter or ash, in the form of coke, or, as it is also called, " fixed carbon." The proportion of this residue is greatest in the more anthracitic or drier coals, but a more valuable product is yielded by those richer in hydrogen. Very important distinctions-those of caking or non-caking-are founded on the behaviour of coals when subjected to the process of coking. The former class undergo an incipient fusion or softening when heated, so that the fragments coalesce and yield a compact coke, while the latter (also called free-burning) preserve their form, producing a coke which is only serviceable when made from large pieces of coal, the smaller pieces being incoherent and of no value. The caking property is best developed in coals low in oxygen with 25 to 30% of volatile matters. As a matter of experience, it is found that caking coals lose that property when exposed to the action of the air for a lengthened period, or by heating to about 300° C., and that the dust or slack of non-caking coal may, in some instances, be converted into a coherent coke by exposing it suddenly to a very high temperature, or compressing it strongly before charging it into the oven.
Lignite or brown coal includes all varieties which are intermediate in properties between wood and coals of the older. formations. A coal of this kind is generally to be Lignite distinguished by its brown colour, either in mass or in the blacker varieties in the streak. The proportion of carbon is comparatively low, usually not exceeding 70%, while the from this circumstance that the term lignite is derived. The best varieties are black and pitchy in lustre, or even bright and scarcely to be distinguished from true coals. These kinds are most common in Eastern Europe. Lignites, as a rule, are generally found in strata of a newer geological age, but there are many instances of perfect coals being found in such strata.
By the term " ash " is understood the mineral matter remaining unconsumed after the complete combustion of the carbonaceous portion of a coal. According to Couriot (Annales de la societe geologique de Belgique, vol. xxiii. p. 105) the stratified character of the ash may be rendered apparent in an X-ray photograph of a piece of coal about an inch thick, when it appears in thin parallel bands, the combustible portion remaining transparent. It may also be rendered visible if a smooth block of free-burning coal is allowed to burn away quickly in an open fire, when the ash remains in thin grey or yellow bands on the surface of the block. The composition of the ashes of different coals is subject to considerable variation, as will be seen by Table II.
The composition of the ash of true coal approximates to that of a fire-clay, allowance being made for lime, which may be present either as carbonate or sulphate, and for sulphuric acid. Sulphur is derived mainly from iron Sulphur p p }' in coal.
pyrites, which yields sulphates by combustion. An indication of the character of the ash of a coal is afforded by its colour, white ash coals being generally freer from sulphur than those containing iron pyrites, which yield a red ash. There are, however, several striking exceptions, as for instance in the anthracite from Peru, given in Table I., which contains more than io% of sulphur, and yields but a very small percentage of a white ash. In this coal, as well as in the lignite of Tasmania, known as white coal or Tasmanite, the sulphur occurs in organic combination, but is so firmly held that it can only be very partially expelled, even by exposure to a very high and continued heating out of contact with the air. An anthracite occurring in connexion with the old volcanic rocks of Arthur's Seat,Edinburgh, which contains a large amount of sulphur in proportion to the Caking coals. Ash of coal. [[Table Ii]].-Composition of the Ashes of Coals. ash, has been found to behave in a similar manner. Under ordinary conditions, from s to 4 of the whole amount of sulphur in a coal is volatilized during combustion, the remaining 4 to being found in the ash.
The amount of water present in freshly raised coals varies very considerably. It is generally largest in lignites, which may sometimes contain 30% or even more, while in the coals of the coal measures it does not usually exceed from 5 to io%. The loss of weight by exposure to the atmosphere from drying may be from z to I of the total amount of water contained.
Coal is the result of the transformation of woody fibre and other vegetable matter by the elimination of oxygen and hydrogen in proportionally larger quantity than carbon, so that the percentage of the latter element is increased in the manner shown in Table III., given by J. Percy, the mineral matter being also changed by the removal of silica and alkalis and the substitution of substances analogous in composition to fire-clay. The causes and methods of these changes are, however, not very exactly defined. Accord Table 111.-Composition of Fuels (assuming Carbon = Ioo).
ing to the elaborate researches of B. Renault (Bulletin de la Societe de l'Industrie minerale, 3 ser. vol. xiii. p. 865), the agents of the transformation of cellulose into peaty substances are saprophytic fungi and bacterial ferments. As the former are only active in the air while the latter are anaerobic, the activity of either agent is conditioned by variation in the water level of the bog. The ultimate term of bacterial activity seems to be the production of ulmic acid, containing carbon 65.31 and hydrogen 3.85%, which is a powerful antiseptic. By the progressive elimination of oxygen and hydrogen, partly as water and partly as carbon dioxide and marsh gas, the ratios of carbon to oxygen and hydrogen in the rendered product increase in the following manner: The resulting product is a brown pasty or gelatinous substance which binds the more resisting parts of the plants into a compact mass. The same observer considers Boghead coal, kerosene shale and similar substances used for the production of mineral oils to be mainly alteration products of gelatinous fresh water algae, which by a nearly complete elimination of oxygen have been changed to substances approximating in composition to C 2 H 3 and C 3 H 5, where C: H = 7.98 and C: O ±N = 46.3. In cannel coals the prevailing constituents are the spores of cryptogamic plants, algae being rare or in many cases absent. By making very thin sections and employing high magnification (1000-1200 diameters), Renault has been enabled to detect numerous forms of bacilli in the woody parts preserved in coal, one of which, Micrococcus carbo, bears a strong resemblance to the living Cladothrix found in trees buried in peat bogs. Clearer evidence of their occurrence has, however, been found in fragments of wood fossilized by silica or carbonate of lime which are sometimes met with in coal seams.
The subsequent change of peaty substance into coal is probably due to geological causes, i.e. chemical and physical processes similar to those that have converted ordinary sediments into rock masses. Such changes seem, however, to have been very rapidly accomplished, as pebbles of completely formed coal are commonly found in the sandstones and coarser sedimentary strata alternating with the coal seams in many coalfields.
The variation in the composition of coal seams in different parts of the same basin is a difficult matter to explain. It has been variously attributed to metamorphism, consequent upon igneous intrusion, earth movements and other kinds of geothermic action, greater or less loss of volatile constituents during the period of coaly transformation, conditioned by differences of permeability in the enclosing rocks, which is greater for sandstones than for argillaceous strata, and other causes; but none of these appears to be applicable over more than limited areas. According to L. Lemiere, who has very fully reviewed the relation of composition to origin in coal seams (Bulletin de la Societe de l'Industrie minerale, 4 ser. vol. iv. pp. 851 and 1299, vol. v. p. 273), differences in composition are mainly original, the denser and more anthracitic varieties representing plant substance which has been more completely macerated and deprived of its putrescible constituents before submergence, or of which the deposition had taken place in shallow water, more readily accessible to atmospheric oxidizing influences than the deeper areas where conditions favourable to the elaboration of compounds richer in hydrogen prevailed.
The conditions favourable to the production of coal seem therefore to have been-forest growth in swampy ground about the mouths of rivers, and rapid oscillation of level, the coal produced during subsidence being covered up by the sediment brought down by the river forming beds of sand or clay, which, on re-elevation, formed the soil for fresh growths, the alternation being occasionally broken by the deposit of purely marine beds. We might therefore expect to find coal wherever strata of estuarine origin are developed in great mass. This is actually the case; the Carboniferous, Cretaceous and Jurassic systems (qq.v.) contain coal-bearing strata though in unequal degrees,- the first being known as the Coal Measures proper, while the others are of small economic value in Great Britain, though more productive in workable coals on the continent of Europe. The Coal Measures which form part of the Palaeozoic or oldest of the three great geological divisions are mainly confined to the countries north of the equator. Mesozoic coals are more abundant in the southern hemisphere, while Tertiary coals seem to be tolerably uniformly distributed irrespective of latitude.
The nature of the Coal Measures will be best understood by v1.19 considering in detail the areas within which they occur in Britain, together with the rocks with which they are most intimately associated. The commencement of the Carboniferous period is marked by a mass of limestones known as the Carboniferous or Sequences Mountain Limestone,which contains a large assemblage of carbon- of marine fossils, and has a maximum thickness in iferous S.W. England and Wales of about 2000 ft. The strata. upper portion of this group consists of shales and sandstones, known as the Yoredale Rocks, which are highly developed in the moorland region between Lancashire and the north side of Yorkshire. These are also called the Upper Limestone Shale, a similar group being found in places below the limestone, and called the Lower Limestone Shale, or, in the north of England, the Tuedian group. Going northward the beds of limestone diminish in thickness, with a proportional increase in the intercalated sandstones and shales, until in Scotland they are entirely subordinate to a mass of coal-bearing strata, which forms the most productive members of the Scotch coalfields. The next member of the series is a mass of coarse sandstones, with some slates and a few thin coals, known as the Millstone Grit, which is about equally developed in England and in Scotland. In the southern coalfields it is usually known by the miners' name of " Farewell rock," from its marking the lower limit of possible coal working. The Coal Measures, forming the third great member of the Carboniferous series, consist of alternations of shales and sandstones, with beds of coal and nodular ironstones, which together make up a thickness of many thousands of feet - from 12,000 to 14,000 ft. when at the maximum of development. They are divisible into three parts, the Lower Coal Measures, the middle or Pennant, a mass of sandstone containing some coals, and the Upper Coal Measures, also containing workable coal. The latter member is marked by a thin limestone band near the top, containing Spirorbis carbonarius, a small marine univalve.
The uppermost portion of the Coal Measures consists of red sandstone so closely resembling that of the Permian group, which are next in geological sequence, that it is often difficult to decide upon the true line of demarcation between the two formations. These are not, however, always found together, the Coal Measures being often covered by strata belonging to the Trias or Upper New Red Sandstone series.
The areas containing productive coal measures are usually known as coalfields or basins, within which coal occurs in more or less regular beds, also called seams or veins, which can often be followed over a considerable length of country without change of character, although, like all stratified rocks, their continuity may be interrupted by faults or dislocations, also known as slips, hitches, heaves or troubles.
The thickness of coal seams varies in Great Britain from a mere film to 35 or 40 ft.; but in the south of France and in India masses of coal are known up to 200 ft. in thickness. These very thick seams are, however, rarely constant in character for any great distance, being found commonly to degenerate into carbonaceous shales, or to split up into thinner beds by the intercalation of shale bands or partings. One of the most striking examples of this is afforded by the thick or ten-yard seam of South Staffordshire, which is from 30 to 45 ft. thick in one connected mass in the neighbourhood of Dudley, but splits up into eight seams, which, with the intermediate shales and sandstones, are of a total thickness of 400 ft. in the northern part of the coalfield in Cannock Chase. Seams of a medium thickness of 3 to 7 ft. are usually the most regular and continuous in character. Cannel coals are generally variable in quality, being liable to change into shales or black-band ironstones within very short horizontal limits. In some instances the coal seams may be changed as a whole, as for instance in South Wales, where the coking coals of the eastern side of the basin pass through the state of dry steam coal in the centre, and become anthracite in the western side. (H. B.) The most important European coalfields are in Great Britain, Belgium and Germany. In Great Britain there is the South Welsh field, extending westward from the march of Monmouth shire to Kidwelly, and northward to Merthyr Tydfil. A midland group of coalfields extends from south Lancashire to the West Riding of Yorkshire, the two greatest industrial districts Geo= in the country, southward to Warwickshire and graphical Staffordshire, and from Nottinghamshire on the east to distribu- Flintshire on the west. In the north of England are tion of co the rich field of Northumberland and Durham, and field als. a lesser field on the coast of Cumberland (White haven, &c.). Smaller isolated fields are those of the Forest of Dean (Gloucestershire) and the field on either side of the Avon above Bristol. Coal has also been found in Kent, in the neighbourhood of Dover. In Scotland coal is worked at various points (principally in the west) in the Clyde-Forth lowlands.. In Belgium the chief coal-basins are those of Hainaut and Liege. Coal has also been found in an extension northward from this field towards Antwerp, while westward the same field extends into north-eastern France. Coal is widely distributed in Germany. The principal field is that of the lower Rhine and Westphalia, which centres in the industrial region of the basin of the Ruhr, a right-bank tributary of the Rhine. In the other chief industrial region of Germany, in Saxony, Zwickau and Lugau, are important mining centres. In German Silesia there is a third rich field, which extends into Austria (Austrian Silesia and Galicia), for which country it forms the chief home source of supply (apart from lignite). Part of the same field also lies within Russian territory (Poland) near the point where the frontiers of the three powers meet. Both in Germany and in Austria-Hungary the production of lignite is large - in the first-named especially in the districts about Halle and Cologne; in the second in northwestern Bohemia, Styria and Carniola. In France the principal coalfield is that in the north-east, already mentioned; another of importance is the central (Le Creusot, &c.) and a third, the southern, about the lower course of the Rhone. Coal is pretty widely distributed in Spain, and occurs in several districts in the Balkan peninsula. In Russia, besides the Polish field, there is an important one south of Moscow, and another in the lower valley of the Donetz, north of the Sea of Azov. The European region poorest in coal (proportionately to area) is Scandinavia, where there is only one field of economic value - a small one in the extreme south of Sweden.
In Asia the Chinese coalfields are of peculiar interest. They are widely distributed throughout China Proper, but those of the province of Shansi appear to be the richest. Proportionately to their vast extent they have been little worked. In a modified degree the same is true of the Indian fields; large supplies are unworked, but in several districts, especially about Raniganj and elsewhere in Bengal, workings are fully developed. Similarly in Siberia and Japan there are extensive supplies unworked or only partially exploited. Those in the neighbourhood of Semipalatinsk may be instanced in the first case and those in the island of Yezo in the second. In Japan, however, several smaller fields (e.g. in the island of Kiushiu) are more fully developed. Coal is worked to some extent in Sumatra, British North Borneo, and the Philippine Islands.
In the United States of America the Appalachian mountain system, from Pennsylvania southward, roughly marks the line of the chief coal-producing region. This group of fields is followed in importance by the " Eastern Interior " group in Indiana, Illinois and Kentucky, and the " Western Interior " group in Iowa, Missouri and Kansas. In Arkansas, Oklahoma and Texas, and along the line of the Rocky Mountains, extensive fields occur, producing lignite and bituminous coal. The lastnamed fields are continued northward in Canada (Crow's Nest Pass field, Vancouver Island, &c.). There is also a group of coalfields on the Atlantic seaboard of the Dominion, principally in Nova Scotia. Coal is known at several points in Alaska, and there are rich but little worked deposits in Mexico.
In the southern countries coal-production is insignificant compared with that in the northern hemisphere. In South America coal is known in Venezuela, Colombia, Peru, northern Chile, Brazil (chiefly in the south), and Argentina (Parana, the extreme south of Patagonia, and Tierra del Fuego), but in no country are the workings extensive. Africa is apparently the continent poorest in coal, though valuable workings have been developed at various points in British South Africa, e.g. at Kronstad, &c., in Cape Colony, at Vereeniging, Boksburg and elsewhere in the Transvaal, in Natal and in Swaziland. Australia possesses fields of great value, principally in the south-east (New South Wales and Victoria), and in New Zealand considerable quantities of coal and lignite are raised, chiefly in South Island.
The following table, based on figures given in the Journal of the Iron and Steel Institute, vol. 72, will give an idea of the coal production of the world: TABLE IV.
The questions, what is the total amount of available coal in the coalfields of Great Britain and Ireland, and how long it may be expected to last, have frequently been discussed since the early part of the 19th century, and particular attention was directed to them after the publication of Stanley Jevons's book on The Coal Question in 1865. In 1866 a royal commission was appointed to inquire into the subject, and in its report, issued in 1871, estimated that the TABLE V.
coal resources of the country, in seams of i ft. thick and upwards situated within 4000 ft. of the surface, amounted to 90,207,285,398 tons. A second commission, which was appointed in 1901 and issued its final report in 1905, taking 4000 f t. as the limit of practicable depth in working and i ft. as the minimum workable thickness, and after making all necessary deductions, estimated the available quantity of coal in the proved coalfields of the United Kingdom as 100,914,668,167 tons. Although in the years 1870-1903 the amount raised was 5,694,928,507 tons, this later estimate was higher by 10,707,382,769 tons than that of the previous commission, the excess being accounted for partly by the difference in the areas regarded as productive by the two commissions, and partly by new discoveries and more accurate knowledge of the coal seams. In addition it was estimated that in the proved coalfields at depths greater than 4000 ft. there were 5,239,433,980 tons, and that in concealed and unproved fields, at depths less than 4000 ft. there were 39,4 8 3, 8 44, 000 tons, together with 854,608,307 tons in that part of the Cumberland coalfield beyond 5 m. and within 12 m. of high-water mark, and 383,024,000 tons in the South Wales coalfield under the sea in St Bride's Bay and part of Carmarthen Bay.
In Table V. below column I. shows the quantity of coal still remaining unworked in the different coalfields at depths not exceeding 4000 f t. and in seams not less than i ft. thick, as estimated by seven district commissioners; column II. the total estimated reductions on account of loss in working due to faults and other natural causes in seams and of coal required to be left for barriers, support of surface buildings, &c.; and column III. the estimated net available amount remaining unworked.
As regards the duration of British coal resources, the commissioners reported (1905): " This question turns chiefly upon the maintenance or the variation of the annual output. The calculations of the last Coal Commission as to the future exports and of Mr Jevons as to the future annual consumption make us hesitate to prophesy how long our coal resources are likely to last. The present annual output is in round numbers 230 million tons, and the calculated available resources in the proved coalfields are in round numbers 100,000 million tons, exclusive of the 40,000 million tons in the unproved coalfields, which we have thought best to regard only as probable or speculative. For the last thirty years the average increase in the output has been 22% per annum, and that in the exports (including bunkers) 42% per annum. ' It is the general opinion of the District Commissioners that owing to physical considerations it is highly probable that the present rate of increase of the putput of coal can long continue - indeed, they think that some districts have already attained their maximum output, but that on the other hand the developments in the newer coalfields will possibly increase the total output for some years.
In view of this opinion and of the exhaustion of the shallower collieries we look forward to a time, not far distant, when the rate of increase of output will be slower, to be followed by a period of stationary output, and then a gradual decline." According to a calculation made by P. Frech in 1900, on the basis of the then rate of production, the coalfields of central France, central Bohemia, the kingdom of Saxony, the Prussian province of Saxony and the north of England, would be exhausted in 100 to 200 years, the other British coalfields, the Waldenburg-Schatzlar and that of the north of France in 250 years, those of Saarbriicken, Belgium, Aachen and Westphalia in 600 to Boo years, and those of Upper Silesia in more than 1000 years. (0. J. R. H.; H.' M. R.) ' Coal-Mining. The opening and laying out, or, as it is generally called, "winning," of new collieries is rarely Prelimin- undertaken without a ary trial preliminary examination of coal= of the character of the workings. strata by means of borings, either for the purpose of determining the Coal resources of Great Britain. number and nature of the coal seams in new ground, or the position of the particular seam or seams which it is proposed to work in extensions of known coalfields.
The principle of proving a mineral field by boring is illustrated by fig. i, which represents a line direct from the dip to the rise of the field, the inclination of the strata being one in eight. No. i bore is commenced at the dip, and reaches a seam of coal A, at 40 fathoms; at this depth it is considered proper to remove nearer to the outcrop so that lower strata may be bored into at a less depth, and a second bore is commenced. To find the position of No. 2, so as to form a continuous section, it is necessary to reckon the inclination of the strata, which is i in 8; and as FIG. i. - Proving by Boreholes.
bore No. i was 40 fathoms in depth, we multiply the depth by the rate of inclination, 40 X 8 = 320 fathoms, which gives the point at which the coal seam A should reach the surface. But there is generally a certain depth of alluvial cover which requires to be deducted, and which we call a fathoms, then (40 - 3 =37)X 8 = 296 fathoms; or say 286 fathoms is the distance that the second bore should be placed to the rise of the first, so as to have, for certain, the seam of coal A in clear connexion with the seam of coal B. In bore No. 3, where the seam B, according to the same system of arrangement, should have been found at or near the surface, another seam C is proved at a considerable depth, differing in character and thickness from either of the preceding. This derangement being carefully noted, another bore to the outcrop on the same principle is put down for the purpose of proving the seam C; the nature of the strata at first is found to agree with the latter part of that bored through in No. 3, but immediately on crossing the dislocation seen in the figure it is changed and the deeper seam D is found.
The evidence therefore of these bores (3 and 4) indicates some material derangement, which is then proved by other bores, either towards the dip or the outcrop, according to the judgment of the borer, so as to ascertain the best position for sinking pits. (For the methods of boring see Boring.) The working of coal may be conducted either by means of levels or galleries driven from the outcrop in a valley, or by shafts or pits sunk from the surface. In the early days of coal-mining, open working, or quarrying from the outcrop of the seams, was practised to a considerable extent; but there are now few if any places in England where this can be done. In 1873 there could be seen, in the thick coal seams of Bengal, near Raniganj, a seam about 50 ft. thick laid bare, over an area of several acres, by stripping off a superficial covering varying from 10 to 30 ft., in order to remove the whole of the coal without loss by pillars. Such a case, however, is quite exceptional. The operations by which the coal is reached and laid out for removal are known as " winning," the actual working or extraction of the coal being termed " getting." In fig. 2 A B is a cross cut level, by which the seams of coal 1 and 2 are won, and C D a vertical shaft by which the seams 1, 2 and 3 are won. When the field is won by the former method, the coal lying above the level is said to be "level-free." The mode of winning by level is of less general application than that by shafts, as the capacity for production is less, owing to the smaller size of roadways by which the coal must be brought to the surface, levels of large section being expensive and difficult to keep open when the mine has been for some time at work. Shafts, on the other hand, may be made of almost any capacity, owing to the high speed in drawing which is attainable with proper mechanism, and allow of the use of more perfect arrangements at the surface than can usually be adopted at the mouth of a level on a hill-side. A more cogent reason, however, is to be found in the fact that the principal coalfields are in flat countries, where the coal can only be reached by vertical sinking.
The methods adopted in driving levels for collieries are generally similar to those adopted in other mines. The ground is secured by timbering, or more usually by arching in masonry or brick-work. Levels like that in fig. 2, which are driven across the stratification, or generally anywhere not in coal, are known as " stone drifts." The sinking of colliery shafts, however, differs considerably from that of other mines, owing to their generally large size, and the difficulties nkingof g g y g ? afts. that are often encountered from water during the sinking. The actual coal measure strata, consisting mainly of shales and clays, are generally impervious to water, but when strata of a permeable character are sunk through, such as the magnesian limestone of the north of England, the Permian sandstones of the central counties, or the chalk and greensand in the north of France and Westphalia, special methods are required in order to pass the water-bearing beds, and to protect the shaft and workings from the influx of water subsequently. Of these methods one of the chief is the plan of tubbing, or lining the excavation with an impermeable casing of wood or iron, generally the latter, built up in segments forming rings, which are piled upon each other throughout the whole depth of the water-bearing strata. This method necessitates the use of very considerable pumping power during the sinking, as the water has to be kept down in order to allow the sinkers to reach a water - tight stratum upon which the foundation of the tubbing FIG. 2. - Shaft and Level.
can be placed. This consists of a heavy cast iron ring, known as a wedging crib, or curb, also fitted together in segments, which is lodged in a square-edged groove cut for its reception, tightly caulked with moss, and wedged into position. Upon this the tubbing is built up in segments, of which usually from 10 to 12 are required for the entire circumference, the edges being made perfectly true. The thickness varies according to the pressure expected, but may be taken at from 4 to i 2 in. The inner face is smooth, but the back is strengthened with angle brackets at the corners. A small hole is left in the centre of each segment, which is kept open during the fitting to prevent undue pressure upon any one, but is stopped as soon as the circle is completed. In the north of France and Belgium wooden tubbings, built of polygonal rings, were at one time in general use. The polygons adopted were of 20 or more sides approximating to a circular form.
The second principal method of sinking through water-bearing ground is by compressed air. The shaft is lined with a cylinder of wrought iron, within which a tubular chamber, provided with doors above and below, known as an P g air-lock, is fitted by a telescopic joint, which is tightly sinkin packed so as to close the top of the shaft air-tight. Air is then forced into the inclosed space by means of a compressing engine, until the pressure is sufficient to oppose the flow of water into the excavation, and to drive out any that may collect in the bottom of the shaft through a pipe which is carried through the air-sluice to the surface. The miners work in the bottom in the same manner as divers in an ordinary diving-bell. Access to the surface is obtained through the double doors of the air-sluice, 2 N. 3 the actual sinking and lining operations requiring ninety days more. The freezing machines were kept at work for 200 days, and 2191 tons of coal were consumed in supplying steam for the compressors and circulating pumps.
The introduction of these special methods has considerably simplified the problem of sinking through water-bearing strata. Some of the earlier sinkings of this kind, when pumps had to be depended on for keeping down the water, were conducted at great cost, as, for instance, at South Hetton, and more recently Ryhope, near Sunderland, through the magnesian limestone of Durham.
The size and form of colliery shafts vary in different districts. In the United States and Scotland rectangular pits secured by timber framings are still common, but the tendency the pressure being reduced to that of the external atmosphere when it is desired to open the upper door, and increased to that of the working space below when it is intended to communicate with the sinkers, or to raise the stuff broken in the bottom. This method has been adopted in various sinkings on the continent of Europe.
The third method of sinking through water-bearing strata is that of boring, adopted by Messrs Kind & Chaudron in Belgium and Germany. For this purpose a horizontal bar Shaft armed with vertical cutting chisels is used, which cuts boring. g out the whole section of the shaft simultaneously. In the first instance, a smaller cutting frame is used, boring a hole from 3 to 5 ft. in diameter, which is kept some 50 or 60 ft. in advance, so as to receive the detritus, which is removed by a shell pump of large size. The large trepan or cutter weighs about 16 tons, and cuts a hole of from 9 to 15 ft. in diameter. The water-tight lining may be either a wrought iron tube, which is pressed down by jack screws as the borehole advances, or cast iron tubbing put together in short complete rings, in contradistinction to the old plan of building them up of segments. The tubbing, which is considerably less in diameter than the borehole, is suspended by rods from the surface until a bed suitable for a foundation is reached, upon which a sliding length of tube, known as the moss box, bearing a shoulder, which is filled with dried moss, is placed. The whole weight of the tubbing is made to bear on the moss, which squeezes outwards, forming a completely water-tight joint. The interval between the back of the tubbing and the sides of the borehole is then filled up with concrete, which on setting fixes the tubbing firmly in position. With increase in depth, however, the thickness and weight of the cast iron tubbing in a large shaft become almost unmanageable; in one instance, at a depth of 1215 f t., the bottom rings in a shaft 142 ft. in diameter are about 4 in. thick, which is about the limit for sound castings. It has therefore been proposed, for greater depths, to put four columns of tubbings of smaller diameters, 82 and 52 ft., in the shaft, and fill up the remainder of the boring with concrete, so that with thinner and lighter castings a greater depth may be reached. This, however, has not as yet been tried. Another extremely useful method of sinking through water-bearing ground, introduced by Messrs A. & H. T. Poetsch in 1883, and originally applied to shafts passing through quicksands above brown coal seams, has been applied with advantage in opening new pits through the secondary and tertiary strata above the coal measures in the north of France and Belgium, some of the most successful examples being those at Lens, Anzin and Vicq, in the north of France basin. In this system the soft ground or fissured water-bearing rock is rendered temporarily solid by freezing the contained water within a surface a few feet larger in diameter than the size of the finished shaft, so that the ground may be broken either by hand tools or blasting in the same manner as hard rock. The miners are protected by the frozen wall, which may be 4 or 5 ft. thick. The freezing is effected by circulating brine (calcium chloride solution) cooled to 5° F. through a series of vertical pipes closed at the bottom, contained in boreholes arranged at equal distances apart around the space to be frozen, and carried down to a short distance below the bottom of the ground to be secured. The chilled brine enters through a central tube of small diameter, passes to the bottom of the outer one and rises through the latter to the surface, each system of tubes being connected above by a ring main with the circulating pumps. The brine is cooled in a tank filled with spiral pipes, in which anhydrous ammonia, previously liquefied by compression, is vaporized in vacuo at the atmospheric temperature by the sensible heat of the returncurrent of brine, whose temperature has been slightly raised in its passage through the circulating tubes. When hard ground is reached, a seat is formed for the cast iron tubbing, which is built up in the usual way and concreted at the back, a small quantity of caustic soda being sometimes used in mixing the concrete to prevent freezing. In an application of this method at Vicq, two shafts of 12 and 16.4 ft. diameter, in a covering of cretaceous strata, were frozen to a depth of 300 ft. in fifty days, is now generally to make them round, 20 ft. being about g y ? g shafts. the largest diameter employed. In the Midland counties, from 7 to 9 ft. is a very common size, but larger dimensions are adopted where a large production is required. Since the accident at Hartley colliery in 1862, caused by the breaking of the pumping-engine beam, which fell into the shaft and blocked it up, whereby the whole of the men then at work in the mine were starved to death, it has been made compulsory upon mine-owners in the United Kingdom to have two pits for each working, in place of the single one divided by walls or brattices which was formerly thought sufficient. The use of two independent connexions - whether separate pits or sections of the same pit, between the surface and the workings - is necessary for the service of the ventilation, fresh air from the surface being carried down one, known as the " downcast," while the foul or return air of the mine rises through the other or " upcast " pit back to the surface. In a heavily-watered mine it is often necessary to establish a special engine-pit, with pumps permanently fixed, or a division of one of the pits may be devoted to this purpose. The pumps, placed close to the point where the water accumulates, may be worked by an engine on the surface by means of heavy reciprocating rods which pass down the shaft, or by underground motors driven by steam, compressed air or electricity.
Where the water does not accumulate very rapidly it is a common practice to allow it to collect in a pit or sump below the working bottom of the shaft, and to draw it off in a water tub or" hoppet "by the main engine, when the latter is not employed in raising coal.
The laying out of a colliery, after the coal has been won, by sinkings or levels, may be accomplished in various ways, according to the nature of the coal, its thickness and dip, and the extent of ground to be worked. In the South Staffordshire and other Midland coalfields, where only shallow pits are required, and the coals are thick, a pair of pits may be sunk for a very few acres, while in the North of England, on the other hand, where sinking is expensive, an area of some thousands of acres may be commanded from the same number of pits. In the latter case, which represents the most approved practice, the sinking is usually placed about the centre of the ground, so that the workings may radiate in every direction from the pit bottom, with the view of employing the greatest number of hands to advantage. Where a large area cannot be commanded, it is best to sink to the lowest point of the field for the convenience of drawing the coal and water which become level-freein regard to the pit. Where properties are much divided, it is always necessary to maintain a thick barrier of unwrought coal between the boundary of the mine and the neighbouring workings, especially if the latter are to the dip. If a prominent line of fault crosses the area it may usually be a convenient division of the fields into sections or districts. The first process in laying out the workings consists in driving a gallery on the level along the course of the coal seam, which is known as a " dip head level," and a lower parallel one, in which the water collects, known as a " lodgment level." Galleries driven at right angles to these are known as a " dip " or " rise headings," according to their position above or below the pit bottom. In Staffordshire the main levels are also known as Laying gate roads." To secure the perpendicularity of the shaft, it is necessary to leave a large mass or pillar of the seam untouched around the pit bottom. This pillar is known in Scotland as the " pit bottom stoop." The junction of the levels with the pit is known as the " pit eye "; it is usually of an enlarged section, and lined with masonry or brick-work, so as to afford room for handling the wagons or trams of coal brought from the working faces. In this portion of the pit are generally placed the furnaces for ventilation, and the boilers required for working steam engines underground, as well as the stables and lamp cabin.
The removal of the coal after the roads have been driven may be effected in many different ways, according to the custom of the district. These may, however, all be considered as modifications of two systems, viz. pillar work and long-wall work. In the former, which is also known as " post and stall" or "bord and pillar " in the north of England, " pillar and stall " in South Wales, and " stoop and room " in Scotland, the field is divided into strips by numerous openings driven parallel to the main rise headings, called " bords " or " bord gates," which are again divided by cutting through them at intervals, so as to leave a series of pillars arranged chequer-wise over the entire area. These pillars are left for the support of the roof as the workings advance, so as to keep the mine open and free from waste. In the oldest form of this class of working, where the size of the pillar is equal to the width of the stall or excavation, about 4 of the whole seam will be removed, the remainder being left in the pillars. A portion of this may be got by the process known as robbing the pillars, but the coal so obtained is liable to be very much crushed from the pressure of the superincumbent strata. This crushing may take place either from above or below, producing what are known as " creeps " or " sits." A coal seam with a soft pavement and a hard roof is the most subject to a " creep." The first indication is a dull hollow sound heard when treading on the pavement or floor, probably occasioned r FIG. 3. - " Creeps " in Coal-Mines.
by some of the individual layers parting from each other as shown at a fig. 3; the succeeding stages of creep are shown at b, c, d, f, and g, in the same figure; the last being the final stage, when the coal begins to sustain the pressure from the overlying strata, in common with the disturbed pavement.
FIG. 4. - " Sits " in Mines.
" Sits " are the reverse of creeps; in the one case the pavement is forced up, and in the other the roof is forced or falls down, for want of proper support or tenacity in itself. This accident generally arises from an improper size of pillars; some roofs, however, are so difficult to support that sits take place where the half of the coal is left in pillars. Fig. 4 will convey a general idea of the appearance of sits, - k, m, n showing different stages.
The modern method of pillar working is shown in fig. 5. In the Northumberland steam coal district, where it is carried out in the most perfect manner, the bords are 5 to 6 yds. in width, while the pillars are 22 yds. broad and 30 yds. long, which are subsequently got out on coming back. In the same figure is also shown the method of working whole coal and pillars at the same time, a barrier of two or three ranges of pillars or a rib of solid coal being left between the working in the solid and those in the pillars. The space from which the entire quantity of coal G//////,/ %/ /?O /%/ FIG. 5. - Pillar Working.
has been removed is known in different districts as the " goaf," " gob," or " waste." Fig. 6 represents the Lancashire system of pillar working. The area is laid out by two pairs of level drifts, parallel to each other, about 150 yds. apart, which are carried to the boundary. About Ioo yds. back from the boundary a communication is made between these levels, from which other levels are driven forward, dividing the coal into ribs of about 25 or 30 yds. wide, which are then cut back by taking off the coal in slices from FIG. 6. - Lancashire method of working Coal.
the level towards the rise in breadths of about 6 yds. By this method the whole of the coal is got backwards, the main roads being kept in solid coal; the intermediate levels not being driven till they are wanted, a greater amount of support is given, and the pillars are less crushed than is usual in pillar working.
In the South Wales system of working, cross headings are driven from the main roads obliquely across the rise to get a sufficiently easy gradient for horse roads, and from these the stalls are opened out with a narrow entrance, in order to leave support on either side of the road, but afterwards widening to as great a breadth as the seam will allow, leaving pillars of a minimum thickness. The character of such workings is very irregular in plan, and as the ventilation is attended with considerable difficulty, it is now becoming generally superseded by more improved methods.
The second great principle of working is that known as longwall or long-work, in which the coal is taken away either in broad faces from roads about 40 or 50 yds. apart and parallel to each other, or along curved faces between roads radiating from the pit bottom - the essential feature in both cases being the removal of the whole of the coal at once, without first sub-dividing it into pillars, to be taken away at a FIG. 7. - Long-wall method of working Coal in Derbyshire.
second working. The roof is temporarily supported by wooden props or pack walling of stone, for a sufficient breadth along the face to protect the workmen, and allow them to work together behind. The general character of a long-wall working is shown in fig. 7, which represents an area of about 500 acres of the bottom hard steam coal at Shipley in Derbyshire. The principal road extends from the shafts southward; and on both sides of it the coal has been removed from the light-shaded area by cutting it back perpendicularly towards the boundaries, along faces about 50 yds. in length, those nearest to the shaft being kept in advance of those farther away, producing a step-shaped outline to the face of the whole coal. It will be seen that by this method the whole of the seam, with the exception of the pillars left to protect the main roadways, is removed. The roads for drawing the coal from the working faces to the shaft are kept open by walling through the waste or goaf produced by the fall of the unsupported roof. The straight roads are the air-ways for carrying pure air from the down-cast shaft to the working faces, while the return air passes along the faces and back to the up-cast by the curved road. The above is the method of working long-wall forward, i.e. taking the coal in advance from the pit towards the boundary, with roads kept open through the gob. Another method consists in driving towards the boundary, and taking the coal backward towards the shafts, or working homeward, allowing the waste to close up without roads having to be kept open through it. This is of course preferable, but is only applicable where the owner of the mine can afford to expend the capital required to reach the limit of the field in excess of that necessary when the raising of coal proceeds pari passu with the extension of the main roads. Fig. 6 is sub stantially a modification of this kind of long-wall work. Fig. 8 represents a method of working practised in the South Yorkshire district, known as bords and banks. The field is divided by levels and headings into rectangular banks, while from the main levels bords or wickets about 30 yds. wide, separated from each other by banks of about the same width, are carried forward in long-wall work, as shown on the left side of the figure, the waste being carefully packed behind so as to secure the ventilation. When these have been worked up to the extremity, as shown on the right side, the intermediate bank is removed by working backward towards the level. This system, therefore, combines both methods of longwall working, but it is not generally applicable, owing to the difficulty of ventilation, due to the great length of air-way that has to be kept open around the waste on each bank.
The relative advantages of the different methods may be generally stated as follows. Long-wall work is best suited for thin coals, and those having a good roof, i.e. one that gives way gradually and fills up the excavation made by removing the coal without scaling off suddenly and falling into the working faces, when practically the whole of the coal maybe removed. Against these advantages must be placed the difficulties attending the maintenance of roads through the goaves, and in some cases the large proportion of slack to round or large coal obtained. Pillar working, in the whole coal, is generally reputed to give a more advantageous proportion of round coal to slack, the latter being more abundantly produced on the removal of the pillars, but as these form only a small portion of the whole seam, the general yield is more advantageous than in the former method. The ventilation of pillar working is often attended with difficulty, and the coal is longer exposed to the influence of the air, a point of importance in some coals, which deteriorate in quality when exposed to a hot damp atmosphere. The great increase in the size of the pillars in the best modern collieries worked upon this principle has, however, done much to approximate the two systems to an equality in other respects.
Where the whole of the coal is removed at once there is less chance of surface damage, when the mines are deep, than with pillar workings. A notable instance of this was afforded at Newstead, Notts, where the ruined front of Newstead Abbey was lowered several feet without any injury to the structure.
The working of very thick seams presents certain special peculiarities, owing to the difficulties of supporting the roof in the excavated portions, and supplying fresh air to the workings. The most typical example of this kind of working in England is afforded by the thick coal of South Staffordshire, which consists of a series of closely associated coal seams, varying from 8 to 12 or 13, divided FIG. 8. - Bords and Banks.
from each other by their partings, but making together one great bed of from 25 to 40 ft. or more in thickness. The partings together do not amount to more than 2 or 3 ft. The method of working which has been long in use is represented in fig. 9. The main level or gate road is driven in the benches coal, or lower part of the seam, while a smaller drift for ventilation, called an air heading, is carried above it in one of the upper beds called the slipper coal. From the gate road a heading called a bolt-hole is opened, and extended into a large rectangular chamber, known as a " side of work," large pillars being left at regular intervals, besides smaller ones or cogs. The order in which the coal is cut is shown in the dotted and numbered squares in the figure. The coal is first cut to the top of the slipper coal from below, after which the upper portion is either broken down by wedging or falls of itself. The working of these upper portions is exceedingly ti ??
? ,? 'rr iii/ r ?? ?/j ? 1 s? ?// //% %??%??i.. ???? ??'y///I%. r??
t%/ / Reference ' -u ,E??%/%?'/,??/FA // %/////A %/// dangerous, owing to the great height of the excavations, and fatal accidents from falls of roof are in consequence more common in South Staffordshire than in any other coalfield in this country. The air from the down-cast shaft enters from the gate road, and passes to the up-cast through the air heading above. About onehalf of the total coal (or less) is obtained in the first working; the roof is then allowed to fall, and when the gob is sufficiently consolidated, fresh roads are driven through it to obtain the ribs and pillars left behind by a second or even, in some cases, a third , _!?/i _ ? ?m?i?r?rrri ?rir?? ??rm?? ? i??a? ?ii?? u a i o i a i i of u , ? .?: ?iiiarrr FIG. 9. - South Staffordshire method of working Thick Coal.
working. The loss of coal by this method is very considerable, besides great risk to life and danger from fire. It has, therefore, been to some extent superseded by the long-wall method, the upper half being taken at the first working, and removed as completely as possible, working backwards from the boundaries to the shaft. The lower half is then taken in the same manner, after the fallen roof has become sufficiently consolidated to allow the mine to be re-opened.
In the working of thick seams inclined at a high angle, such as those in the south of France, and in the lignite mines of Styria and Bohemia, the method of working in horizontal slices, about i 2 or r 5 ft. thick, and filling up the excavation with broken rock and earth from the surface, is now generally adopted in preference to the systems formerly used. At Monceaux les Mines, in France, a seam 40 ft. thick, and dipping at an angle of 20°, is worked in the following manner. A level is driven in a sandstone forming the floor, along the course of the coal, into which communications are made by cross cuts at intervals of 16 yds., which are driven across to the roof, dividing up the area to be worked into panels. These are worked backwards, the coal being taken to a height of 20 ft., the opening being packed up with stone sent down from the surface. As each stage is worked out, the floor level is connected with that next below it by means of an incline, which facilitates the introduction of the packing material. Stuff containing a considerable amount of clay is found to be the best suited for the purpose of filling, as it consolidates readily under pressure.
In France and Germany the method of filling the space left by the removal of the coal with waste rock, quarried underground or sent down from the surface, which was originally used in connexion with the working of thick inclined seams by the method of horizontal slices, is now largely extended to long-wall workings on thin seams, and in Westphalia is made compulsory where workings extend below surface buildings, and safety pillars of unwrought coal are found to be insufficient. With careful packing it is estimated that the surface subsidence will not exceed 40% of the thickness of the seam removed, and will usually be considerably less. The material for filling may be the waste from earlier workings stored in the spoil banks at the surface; where there are blast furnaces in the neighbourhood, granulated slag mixed with earth affords excellent packing. In thick seams packing adds about 5d. per ton to the cost of the coal, but in thinner seams the advantage is on the other side.
In some anthracite collieries in America the small coal or culm and other waste are washed into the exhausted workings by water which gives a compact mass filling the excavation when the water has drained away. A modification of this method, which originated in Silesia, is now becoming of importance in many European coalfields. In this the filling material, preferably sand, is sent down from the surface through a vertical steel pipe mixed with sufficient water to allow it to flow freely through distributing pipes in the levels commanding the excavations to be filled; these are closed at the bottom by screens of boards sufficiently close to retain the packing material while allowing the water to pass by the lower level to the pumping-engine which returns it to the surface.
The actual cutting of the coal is chiefly performed by manual labour, the tool employed being a sharp-pointed double-armed pick, which is nearly straight, except when required for use in hard rock, when the arms are made with an « „ inclination or anchored. The terms pike, pick, mandril and slitter are applied to the collier's pick in different districts, the men being known as pikemen or hewers. In driving levels it is necessary to cut grooves vertically parallel to the walls, a process known as shearing; but the most important operation is that known as holing or kirving, which consists in cutting a notch or groove in the floor of the seam to a depth of about 3 ft., measured back from the face, so as to leave the overhanging part unsupported, which then either falls of its own accord within a few hours, or is brought down either by driving wedges along the top, or by blasting. The process of holing in coal is one of the severest kinds of human labour. It has to be performed in a constrained position, and the miner lying on his side has to cut to a much greater height, in order to get room to carry the groove in to a sufficient depth, than is required to bring the coal down, 0 y - ??- giving rise to a great 0 1 `(waste in slack as corno pared with machine o %j ,G?O A?F/ 1 r work. This is sometimes obviated by holing in the beds below the coal, or in any portion of a seam of inferior quality that may not be worth working. This loss .is proportionately greater in thin than in thick seams, the same quantity being cut to waste in either case. The method of cutting coal on the long-wall system is seen in fig. io, representing the working at the Shipley colliery. The coal is 40 in. thick, with a seam of fire-clay and a roof of black shale; about 6 in. of the upper part, known as the roof coal, not being worth working, is left behind. A groove of triangular section of 30 in. base and 9 in. high is cut along the face, inclined timber props being placed at intervals to support the overhanging portion until the required length is cut. These are then removed, and the coal is allowed to fall, wedges or blasting being employed when necessary. The roof of the excavation is supported as the coal is removed, by packing up the waste material, and by a double row of props, 2 ft. from each other, placed temporarily along the face. These are placed 5 ft. apart, the props of the back row alternating with those in front.
