TABLE III.--_Composition of Fuels (assum_Encyclopaedia Britannica, 11th Edition, "Clervaux" to "Cockade" Volume 6, Slice 5

+------------------------------+-------+---------+---------+-----------+

+------------------------------+-------+---------+---------+-----------+

| Wood | 100 | 12.18 | 83.07 | 1.80 |

| Peat | 100 | 9.85 | 55.67 | 2.89 |

| Lignite | 100 | 8.37 | 42.42 | 3.07 |

| Thick Coal, S. Staffordshire | 100 | 6.12 | 21.23 | 3.47 |

| Hartley Steam Coal | 100 | 5.91 | 18.32 | 3.62 |

| South Wales Steam Coal | 100 | 4.75 | 5.28 | 4.09 |

| American Anthracite | 100 | 2.84 | 1.74 | 2.63 |

+------------------------------+-------+---------+---------+-----------+

Origin of Coal.

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. According 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:--

C : H C : O

Cellulose 7.2 0.9

Peat 9.8 1.8

Lignite, imperfect 12.2 2.4

" perfect 12.6 3.6

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 C2H3 and C3H5, 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.

Sequences of carboniferous strata.

The nature of the Coal Measures will be best understood by 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 Mountain Limestone, which

contains a large assemblage of marine fossils, and has a maximum

thickness in S.W. England and Wales of about 2000 ft. The 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.)

Geographical distribution of coalfields.

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 Monmouthshire 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 in the country, southward to Warwickshire and Staffordshire,

and from Nottinghamshire on the east to Flintshire on the west. In the

north of England are the rich field of Northumberland and Durham, and a

lesser field on the coast of Cumberland (Whitehaven, &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 north-western 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 last-named

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.

Europe:-- Tons.

United Kingdom 1905 236,128,936

Germany, coal " 121,298,167

" lignite " 52,498,507

France " 35,869,497

Belgium " 21,775,280

Austria, coal " 12,585,263

" lignite " 22,692,076

Hungary, coal 1904 1,031,501

" lignite " 5,447,283

Spain 1905 3,202,911

Russia 1904 19,318,000

Holland " 466,997

Bosnia, lignite 1905 540,237

Rumania " 1903 110,000

Servia 1904 183,204

Italy, coal and lignite 1905 412,916

Sweden " 322,384

Greece, lignite 1904 466,997

Asia:--

India 1905 8,417,739

Japan 1903 10,088,845

Sumatra 1904 207,280

Africa:--

Transvaal 1904 2,409,033

Natal 1905 1,129,407

Cape Colony 1904 154,272

America:--

United States 1905 350,821,000

Canada 1904 7,509,860

Mexico " 700,000

Peru 1905 72,665

Australasia:--

New South Wales 1905 6,632,138

Queensland " 529,326

Victoria " 153,135

Western Australia " 127,364

Tasmania " 51,993

New Zealand " 1,585,756

Coal resources of Great Britain.

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 coal resources of the

country, in seams of 1 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

ft. as the limit of practicable depth in working and 1 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 39,483,844,000 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 ft.

and in seams not less than 1 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.

Table V.

+--------------------------+----------------+---------------+----------------+

| Coalfield. | I. | II. | III. |

+--------------------------+----------------+---------------+----------------+

| and Monmouthshire | 33,443,000,339 | 6,972,003,760 | 26,470,996,579 |

| Forest of Dean | 305,928,137 | 47,394,690 | 258,533,447 |

| North Stafford | 5,267,833,074 | 89,782,727 | 4,368,050,347 |

| South Stafford | 1,953,627,435 | 538,179,363 | 1,415,448,072 |

| Warwickshire | 1,448,804,556 | 321,822,653 | 1,126,981,903 |

| Leicestershire | 2,467,583,205 | 642,124,654 | 1,825,458,551 |

| Shropshire | 369,174,620 | 48,180,921 | 320,993,699 |

| Lancashire | 5,349,554,437 | 1,111,046,710 | 4,238,507,727 |

| Cheshire | 358,998,172 | 87,165,901 | 291,832,271 |

| North Wales | 2,513,026,200 | 776,558,371 | 1,736,467,829 |

| Northumberland | 7,040,348,127 | 1,530,722,486 | 5,509,625,641 |

| Cumberland | 2,188,938,830 | 661,230,025 | 1,527,708,805 |

| Durham | 6,607,700,522 | 1,336,584,176 | 5,271,116,346 |

| Scotland | 21,259,767,661 | 5,579,311,305 | 15,681,456,356 |

+--------------------------+----------------+---------------+----------------+

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 2-1/2% per annum, and

that in the exports (including bunkers) 4-1/2% 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 output 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 Saarbrucken, Belgium, Aachen and

Westphalia in 600 to 800 years, and those of Upper Silesia in more than

1000 years. (O. J. R. H.; H. M. R.)

_Coal-Mining._

Preliminary trial of coalworkings.

The opening and laying out, or, as it is generally called, "winning," of

new collieries is rarely undertaken without a preliminary examination of

the character of the strata by means of borings, either for the purpose

of determining the 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.

[Illustration: FIG. 1.--Proving by Boreholes.]

The principle of proving a mineral field by boring is illustrated by

fig. 1, which represents a line direct from the dip to the rise of the

field, the inclination of the strata being one in eight. No. 1 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 1 in 8; and as bore No. 1 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 3 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.)

Methods of working.

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.

Sinking of shafts.

Tubbing.

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 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 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 3/4 to 1-1/2 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.

[Illustration: FIG. 2.--Shaft and Level.]

Pneumatic sinking.

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 air-lock, is fitted by a telescopic joint, which is tightly

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, 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.

Shaft boring.

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 armed with vertical cutting chisels is

used, which cuts 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 ft., the bottom rings in a shaft 14-1/2 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, 8-1/2 and 5-1/2 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 deg. 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 return-current 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, 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.

Size of shafts.

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 is now generally to make them round,

20 ft. being about 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.

Laying out workings.

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-free in

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 "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.

Method of working coal.

Pillar working.

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 3/4 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."

[Illustration: FIG 3.--"Creeps" in Coal-Mines.]

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 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.

[Illustration: 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.

[Illustration: FIG. 5.--Pillar Working.]

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 has been removed is known in different districts as the

"goaf," "gob," or "waste."

[Illustration: FIG. 6.--Lancashire method of working Coal.]

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 100 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 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.

[Illustration: FIG. 7.--Long-wall method of working Coal in Derbyshire.]

Long-wall working.

South Yorkshire method.

The second great principle of working is that known as long-wall 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

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 substantially 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 long-wall 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

may be 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.

[Illustration: FIG. 8.--Bords and Banks.]

Working thick seams.

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 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 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 one-half 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 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.

[Illustration: FIG. 9.--South Staffordshire method of working Thick

Coal.]

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 12 or 15 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 deg., 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.

[Illustration: FIG. 10.--Long-wall working-face--Plan and Section.]

Methods of cutting coal.

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, giving rise to a great waste in slack as compared with machine

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. 10, 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. The props used are preferably of small oak or English

larch, but large quantities of fir props, cut to the right length, are

also imported from the north of Europe. As the work proceeds onwards,

the props are withdrawn and replaced in advance, except those that may

be crushed by the pressure or buried by sudden falls of the roof.

In Yorkshire hollow square pillars, formed by piling up short blocks of

wood or chocks, are often used instead of props formed of a single stem.

In securing the roof and sides of coal workings, malleable iron and

steel are now used to some extent instead of timber, although the

consumption of the latter material is extremely large. As a substitute

for timber props at the face, pieces of steel joists, with the web cut

out for a short distance on either end, with the flanges turned back to

give a square bearing surface, have been introduced. In large levels

only the cap pieces for the roof are made of steel joists, but in

smaller ones complete arches made of pieces of rails fish-jointed at the

crown are used. In another system introduced by the Mannesmann Tube

Company the prop is made up of weldless steel tubes sliding

telescopically one within the other, which are fixed at the right height

by a screw clamp capable of carrying a load of 15 to 16 tons. These can

be most advantageously used on thick seams 6 to 10 ft. or upwards. For

shaft linings steel rings of H or channel section supported by

intermediate struts are also used, and cross-bearers or buntons of steel

joists and rail guides are now generally substituted for wood.

When the coal has been under-cut for a sufficient length, the struts are

withdrawn, and the overhanging mass is allowed to fall during the time

that the workmen are out of the pit, or it may be brought down by

driving wedges, or if it be of a compact character a blast in a borehole

near the roof may be required. Sometimes, but rarely, it happens that it

is necessary to cut vertical grooves in the face to determine the limit

of the fall, such limits being usually dependent upon the cleet or

divisional planes in the coal, especially when the work is carried

perpendicular to them or on the end.

Coal-cutting machines.

The substitution of machinery for hand labour in cutting coal has long

been a favourite problem with inventors, the earliest plan being that of

Michael Meinzies, in 1761, who proposed to work a heavy pick underground

by power transmitted from an engine at the surface, through the agencies

of spear-rods and chains passing over pulleys; but none of the methods

suggested proved to be practically successful until the general

introduction of compressed air into mines furnished a convenient motive

power, susceptible of being carried to considerable distances without

any great loss of pressure. This agent has been applied in various ways,

in machines which either imitate the action of the collier by cutting

with a pick or make a groove by rotating cutters attached to an endless

chain or a revolving disk or wheel. The most successful of the first

class, or pick machines, that of William Firth of Sheffield, consists

essentially of a horizontal pick with two cutting arms placed one

slightly in advance of the other, which is swung backwards and forwards

by a pair of bell crank levers actuated by a horizontal cylinder engine

mounted on a railway truck. The weight is about 15 cwt. At a working

speed of 60 yds. per shift of 6 hours, the work done corresponds to that

of twelve average men. The width of the groove cut is from 2 to 3 in. at

the face, diminishing to 1-1/2 in. at the back, the proportion of waste

being very considerably diminished as compared with the system of holing

by hand. The use of this machine has allowed a thin seam of cannel, from

10 to 14 in. in thickness, to be worked at a profit, which had formerly

been abandoned as too hard to be worked by hand-labour. Pick machines

have also been introduced by Jones and Levick, Bidder, and other

inventors, but their use is now mostly abandoned in favour of those

working continuously.

In the Gartsherrie machine of Messrs Baird, the earliest of the flexible

chain cutter type, the chain of cutters works round a fixed frame or jib

projecting at right angles from the engine carriage, an arrangement

which makes it necessary to cut from the end of the block of coal to

the full depth, instead of holing into it from the face. The forward

feed is given by a chain winding upon a drum, which hauls upon a pulley

fixed to a prop about 30 yds. in advance. This is one of the most

compact forms of machine, the smaller size being only 20 in. high. With

an air pressure of from 35 to 40 lb. per sq. in., a length of from 300

to 350 ft. of coal is holed, 2 ft. 9 in. deep, in the shift of from 8 to

10 hours. The chain machine has been largely developed in America in the

Jeffrey, Link Bell, and Morgan Gardner coal cutters. These are similar

in principle to the Baird machine, the cutting agent being a flat link

chain carrying a double set of chisel points, which are drawn across the

coal face at the rate of about 5 ft. per second; but, unlike the older

machines, in which the cutting is done in a fixed plane, the chain with

its motor is made movable, and is fed forward by a rack-and-pinion

motion as the cutting advances, so that the cut is limited in breadth

(3-1/2 to 4 ft.), while its depth may be varied up to the maximum travel (8

ft.) of the cutting frame. The carrying frame, while the work is going

on, is fixed in position by jack-screws bearing against the roof of the

seam, which, when the cut is completed, are withdrawn, and the machine

shifted laterally through a distance equal to the breadth of the cut and

fixed in position again. The whole operation requires from 8 to 10

minutes, giving a cutting speed of 120 to 150 sq. ft. per hour. These

machines weigh from 20 to 22 cwt., and are mostly driven by electric

motors of 25 up to 35 h.p. as a maximum. By reason of their intermittent

action they are only suited for use in driving galleries or in

pillar-and-stall workings.

[Illustration: FIG. 11.--Winstanly & Barker's Coal-cutting

Machine--Plan.]

A simple form of the saw or spur wheel coal-cutting machine is that of

Messrs Winstanly & Barker (fig. 11), which is driven by a pair of

oscillating engines placed on a frame running on rails in the usual way.

The crank shaft carries a pinion which gears into a toothed wheel of a

coarse pitch, carrying cutters at the ends of the teeth. This wheel is

mounted on a carrier which, being movable about its centre by a screw

gearing worked by hand, gives a radial sweep to the cutting edges. When

at work it is slowly turned until the carrier is at right angles to the

frame, when the cut has attained the full depth. The forward motion is

given by a chain winding upon a crab placed in front, by which it is

hauled slowly forward. With 25 lb pressure it will hole 3 ft. deep, at

the rate of 30 yds. per hour, the cut being only 2-3/4 in. high, but it

will only work on one side of the carriage. This type has been greatly

improved and now is the most popular machine in Great Britain,

especially in long-wall workings. W. E. Garforth's Diamond coal cutter,

one of the best known, undercuts from 5-1/2 to 6 ft. In some instances

electric motors have been substituted for compressed-air engines in such

machines.

Another class of percussive coal-cutters of American origin is

represented by the Harrison, Sullivan and Ingersoll-Sergeant machines,

which are essentially large rock-drills without turning gear for the

cutting tool, and mounted upon a pair of wheels placed so as to allow

the tool to work on a forward slope. When in use the machine is placed

upon a wooden platform inclining towards the face, upon which the miner

lies and controls the direction of the blow by a pair of handles at the

back of the machine, which is kept stationary by wedging the wheels

against a stop on the platform. These machines, which are driven by

compressed air, are very handy in use, as the height and direction of

the cut may be readily varied; but the work is rather severe to the

driver on account of the recoil shock of the piston, and an assistant is

necessary to clear out the small coal from the cut, which limits the

rate of cutting to about 125 sq. ft. per hour.

Coal-wedging machines.

Another kind of application of machinery to coal mining is that of

Messrs Bidder & Jones, which is intended to replace the use of blasting

for bringing down the coal. It consists of a small hydraulic press,

which forces a set of expanding bits or wedges into a bore-hole

previously bored by a long screw augur or drill, worked by hand, the

action of the press being continued until a sufficient strain is

obtained to bring down the coal. The arrangement is, in fact, a

modification of the plug and feather system used in stone quarrying for

obtaining large blocks, but with the substitution of the powerful

rending force of the hydraulic press for hand-power in driving up the

wedges. This apparatus has been used at Harecastle in North

Staffordshire, and found to work well, but with the disadvantage of

bringing down the coal in unmanageably large masses. A method of wedging

down coal sufficiently perfected to be of general application would add

greatly to the security of colliers.

Underground conveyance.

The removal of the coal broken at the working face to the pit bottom may

in small mines be effected by hand labour, but more generally it is done

by horse or mechanical traction, upon railways, the "trams" or "tubs,"

as the pit wagons are called, being where possible brought up to the

face. In steeply inclined seams passes or shoots leading to the main

level below are sometimes used, and in Belgium iron plates are sometimes

laid in the excavated ground to form a slide for the coal down to the

loading place. In some instances travelling belts or creepers have been

adopted, which deliver the coal with a reduced amount of breakage, but

this application is not common. The capacity of the trams varies with

the size of the workings and the shaft. From 5 to 7 cwt. are common

sizes, but in South Wales they are larger, carrying up to one ton or

more. The rails used are of flat bottomed or bridge section varying in

weight from 15 to 25 lb to the yd.; they are laid upon cross sleepers in

a temporary manner, so that they can be easily shifted along the working

faces, but are carefully secured along main roads intended to carry

traffic continuously for some time. The arrangement of the roads at the

face is shown in the plan, fig. 10. In the main roads to the pit when

the distance is not considerable horse traction may be used, a train of

6 to 15 vehicles being drawn by one horse, but more generally the

hauling or, as it is called in the north of England, the leading of the

trains of tubs is effected by mechanical traction.

In a large colliery where the shafts are situated near the centre of the

field, and the workings extend on all sides, both to the dip and rise,

the drawing roads for the coal may be of three different kinds--(1)

levels driven at right angles to the dip, suitable for horse roads, (2)

rise ways, known as jinny roads, jig-brows, or up-brows, which, when of

sufficient slope, may be used as self-acting planes, i.e. the loaded

waggons may be made to pull back the empty ones to the working faces,

and (3) dip or down-brows, requiring engine power. A road may be used as

a self-acting or gravitating incline when the gradient is 1 in 30 or

steeper, in which case the train is lowered by a rope passing over a

pulley or brake drum at the upper end, the return empty train being

attached to the opposite end of the rope and hauled up by the descending

load. The arrangements for this purpose vary, of course, with the amount

of work to be done with one fixing of the machinery; where it is likely

to be used for a considerable time, the drum and brake are solidly

constructed, and the ropes of steel or iron wire carefully guided over

friction rollers, placed at intervals between the rails to prevent them

from chafing and wearing out on the ground. Where the load has to be

hauled up a rising gradient, underground engines, driven by steam or

compressed air or electric motors, are used. In some cases steam

generated in boilers at the surface is carried in pipes to the engines

below, but there is less loss of power when compressed air is sent down

in the same way. Underground boilers placed near the up-cast pit so that

the smoke and gases help the ventilating furnace have been largely used

but are now less favourably regarded than formerly. Water-pressure

engines, driven by a column of water equal to the depth of the pit, have

also been employed for hauling. These can, however, only be used

advantageously where there are fixed pumps, the fall of water generating

the power resulting in a load to be removed by the expenditure of an

equivalent amount of power in the pumping engine above that necessary

for keeping down the mine water.

The principal methods in which power can be applied to underground

traction are as follows:--

1. Tail rope system.

2. Endless chain system.

3. Endless rope system on the ground.

4. Endless rope system overhead.

The three last may be considered as modifications of the same principle.

In the first, which is that generally used in Northumberland and Durham,

a single line of rails is used, the loaded tubs being drawn "out bye,"

i.e. towards the shaft, and the empty ones returned "in bye," or towards

the working faces, by reversing the engine; while in the other systems,

double lines, with the rope travelling continuously in the same

direction, are the rule. On the tail rope plan the engine has two drums

worked by spur gearing, which can be connected with, or cast loose from,

the driving shaft at pleasure. The main rope, which draws out the loaded

tubs, coils upon one drum, and passes near the floor over guide sheaves

placed about 20 ft. apart. The tail rope, which is of lighter section

than the main one, is coiled on the second drum, passes over similar

guide sheaves placed near the roof or side of the gallery round a pulley

at the bottom of the plane, and is fixed to the end of the train or set

of tubs. When the load is being drawn out, the engine pulls directly on

the main rope, coiling it on to its own drum, while the tail drum runs

loose paying out its rope, a slight brake pressure being used to prevent

its running out too fast. When the set arrives out bye, the main rope

will be wound up, and the tail rope pass out from the drum to the end

and back, i.e. twice the length of the way; the set is returned in bye,

by reversing the engine, casting loose the main, and coupling up the

tail drum, so that the tail rope is wound up and the main rope paid out.

This method, which is the oldest, is best adapted for ways that are

nearly level, or when many branches are intended to be worked from one

engine, and can be carried round curves of small radius without

deranging the trains; but as it is intermittent in action, considerable

engine-power is required in order to get up the required speed, which is

from 8 to 10 m. per hour. From 8 to 10 tubs are usually drawn in a set,

the ways being often from 2000 to 3000 yds. long. In dip workings the

tail rope is often made to work a pump connected with the bottom pulley,

which forces the water back to the cistern of the main pumping engine in

the pit.

For the endless chain system, which is much used in the Wigan district,

a double line of way is necessary, one line for full and the other for

empty tubs. The chain passes over a pulley driven by the engine, placed

at such a height as to allow it to rest upon the tops of the tubs, and

round a similar pulley at the far end of the plane. The forward edge of

the tub carries a projecting pin or horn, with a notch into which the

chain falls which drags the tub forward. The road at the outer end is

made of a less slope than the chain, so that on arrival the tub is

lowered, clears the pin, and so becomes detached from the chain. The

tubs are placed on at intervals of about 20 yds., the chain moving

continuously at a speed of from 2-1/2 to 4 m. per hour. This system

presents the greatest advantages in point of economy of driving power,

especially where the gradients are variable, but is expensive in first

cost, and is not well suited for curves, and branch roads cannot be

worked continuously, as a fresh set of pulleys worked by bevel gearing

is required for each branch.

The endless rope system may be used with either a single or double line

of way, but the latter is more generally advantageous. The rope, which

is guided upon sheaves between the rails, is taken twice round the head

pulley. It is also customary to use a stretching pulley to keep the rope

strained when the pull of the load diminishes. This is done by passing a

loop at the upper end round a pulley mounted in a travelling frame, to

which is attached a weight of about 15 cwt. hanging by a chain. This

weight pulls directly against the rope; so if the latter slacks, the

weight pulls out the pulley frame and tightens it up again. The tubs are

usually formed into sets of from 2 to 12, the front one being coupled up

by a short length of chain to a clamping hook formed of two jaws moulded

to the curve of the rope which are attached by the "run rider," as the

driver accompanying the train is called. This system in many respects

resembles the tail rope, but has the advantage of working with one-third

less length of rope for the same length of way.

The endless rope system overhead is substantially similar to the endless

chain. The wagons are attached at intervals by short lengths of chain

lapped twice round the rope and hooked into one of the links, or in some

cases the chains are hooked into hempen loops on the main rope. In mines

that are worked from the outcrop by adits or day levels traction by

locomotives driven by steam, compressed air or electricity is used to

some extent. The most numerous applications are in America.

Ventilation.

One of the most important branches of colliery work is the management of

the ventilation, involving as it does the supply of fresh air to the men

working in the pit, as well as the removal of inflammable gases that may

be given off by the coal. This is effected by carrying through the

workings a large volume of air which is kept continually moving in the

same direction, descending from the surface by one or more pits known as

intake or downcast pits, and leaving the mine by a return or upcast pit.

Such a circulation of air can only be effected by mechanical means when

the workings are of any extent, the methods actually adopted being--(1)

The rarefaction of the air in the upcast pit by a furnace placed at the

bottom; and (2) Exhaustion by machinery at the surface. The former plan,

being the older, has been most largely used, but is becoming replaced by

some form of machine.

The usual form of ventilating furnace is a plain fire grate placed under

an arch, and communicating with the upcast shaft by an inclined drift.

It is separated from the coal by a narrow passage walled and arched in

brickwork on both-sides. The size of the grate varies with the

requirements of the ventilation, but from 6 to 10 ft. broad and from 6

to 8 ft. long are usual dimensions. The fire should be kept as thin and

bright as possible, to reduce the amount of smoke in the upcast. When

the mine is free from gas, the furnace may be worked by the return air,

but it is better to take fresh air directly from the downcast by a

scale, or split, from the main current. The return air from fiery

workings is never allowed to approach the furnace, but is carried into

the upcast by a special channel, called a dumb drift, some distance

above the furnace drift, so as not to come in contact with the products

of combustion until they have been cooled below the igniting point of

fire-damp. Where the upcast pit is used for drawing coal, it is usual to

discharge the smoke and gases through a short lateral drift near the

surface into a tall chimney, so as to keep the pit-top as clear as

possible for working. Otherwise the chimney is built directly over the

mouth of the pit.

Mechanical ventilation may be effected either by direct exhaustion or

centrifugal displacement of the air to be removed. In the first method

reciprocating bells, or piston machines, or rotary machines of varying

capacity like gas-works exhausters, are employed. They were formerly

used on a very large scale in Belgium and South Wales, but the great

weight of the moving parts makes it impossible to drive them at the high

speed called for by modern requirements, so that centrifugal fans are

now generally adopted instead. An early and very successful machine of

this class, the Guibal fan, is represented in fig. 12. The fan has eight

arms, framed together of wrought iron bars, with diagonal struts, so as

to obtain rigidity with comparative lightness, carrying flat

close-boarded blades at their extremities. It revolves with the smallest

possible clearance in a chamber of masonry, one of the side walls being

perforated by a large round hole, through which the air from the mine is

admitted to the centre of the fan. The lower quadrant of the casing is

enlarged spirally, so as to leave a narrow rectangular opening at the

bottom, through which the air is discharged into a chimney of gradually

increasing section carried to a height of about 25 ft. The size of the

discharge aperture can be varied by means of a flexible wooden shutter

sliding in a groove in a cast iron plate, curved to the slope of the

casing. By the use of the spiral guide casing and the chimney the

velocity of the effluent air is gradually reduced up to the point of

final discharge into the atmosphere, whereby a greater useful effect is

realized than is the case when the air streams freely from the

circumference with a velocity equal to that of the rotating fan. The

power is applied by steam acting directly on a crank at one end of the

axle, and the diameter of the fan may be 40 ft. or more.

[Illustration: FIG. 12.--Guibal Fan.]

The Waddle fan, represented in fig. 13, is an example of another class

of centrifugal ventilator, in which a close casing is not used, the air

exhausted being discharged from the circumference directly into the

atmosphere. It consists of a hollow sheet iron drum formed by two

conoidal tubes, united together by numerous guide blades, dividing it up

into a series of rectangular tubes of diminishing section, attached to a

horizontal axle by cast iron bosses and wrought iron arms. The tubes at

their smallest part are connected to a cast iron ring, 10 ft. in

diameter, but at their outer circumference they are only 2 ft. apart.

The extreme diameter is 25 ft.

[Illustration: FIG. 13.--Waddle Fan.]

By the adoption of more refined methods of construction, especially in

the shape of the intake and discharge passages for the air and the forms

of the fan blades, the efficiency of the ventilating fan has been

greatly increased so that the dimensions can be much reduced and a

higher rate of speed adopted. Notable examples are found in the Rateau,

Ser and Capell fans, and where an electric generating station is

available electric motors can be advantageously used instead of steam.

Distribution of air underground.

The quantity of air required for a large colliery depends upon the

number of men employed, as for actual respiration from 100 to 200 cub.

ft. per minute should be allowed. In fiery mines, however, a very much

larger amount must be provided in order to dilute the gas to the point

of safety. Even with the best arrangements a dangerous increase in the

amount of gas is not infrequent from the sudden release of stored-up

masses in the coal, which, overpowering the ventilation, produce

magazines of explosive material ready for ignition when brought in

contact with the flame of a lamp or the blast of a shot. The management

of such places, therefore, requires the most constant vigilance on the

part of the workmen, especially in the examination of the working places

that have been standing empty during the night, in which gas may have

accumulated, to see that they are properly cleared before the new shift

commences.

The actual conveyance or coursing of the air from the intake to the

working faces is effected by splitting or dividing the current at

different points in its course, so as to carry it as directly as

possible to the places where it is required. In laying out the mine it

is customary to drive the levels or roads in pairs, communication being

made between them at intervals by cutting through the intermediate

pillar; the air then passes along one and returns by the other. As the

roads advance other pillars are driven through in the same manner, the

passages first made being closed by stoppings of broken rock, or built

up with brick and mortar walls, or both. When it is desired to preserve

a way from one road or similar class of working to another, double doors

placed at sufficient intervals apart to take in one or more trams

between them when closed are used, forming a kind of lock or sluice.

These are made to shut air-tight against their frames, so as to prevent

the air from taking a short cut back to the upcast, while preserving

free access between the different districts without following the whole

round of the air-ways. The ventilation of ends is effected by means of

brattices or temporary partitions of thin boards placed midway in the

drift, and extending to within a few feet of the face. The air passes

along one side of the brattice, courses round the free end, and returns

on the other side. In many cases a light but air-proof cloth, specially

made for the purpose, is used instead of wood for brattices, as being

more handy and more easily removed. In large mines where the air-ways

are numerous and complicated, it often happens that currents travelling

in opposite directions are brought together at one point. In these cases

it is necessary to cross them. The return air is usually made to pass

over the intake by a curved drift carried some distance above in the

solid measures, both ways being arched in brickwork, or even in some

cases lined with sheet iron so as to ensure a separation not likely to

be destroyed in case of an explosion (see figs. 5 and 8). The use of

small auxiliary blowing ventilators underground, for carrying air into

workings away from the main circuits, which was largely advocated at one

time, has lost its popularity, but a useful substitute has been found in

the induced draught produced by jets of compressed air or high-pressure

water blowing into ejectors. With a jet of 1/200 in. area, a pipe

discharging 1-2/3 gallon of water per minute at 165 lb pressure per sq.

in., a circulation of 850 cub. ft. of air per minute was produced at the

end of a level, or about five times that obtained from an equal volume

of air at 60 lb pressure. The increased resistance, due to the large

extension of workings from single pairs of shafts, the ventilating

currents having often to travel several miles to the upcast, has led to

great increase in the size and power of ventilating fans, and engines

from 250 to 500 H.P. are not uncommonly used for such purposes.

Lighting.

The lighting of underground workings in collieries is closely connected

with the subject of ventilation. In many of the smaller pits in the

Midland districts of England, and generally in South Staffordshire, the

coals are sufficiently free from gas, or rather the gases are not liable

to become explosive when mixed with air, to allow the use of naked

lights, candles being generally used. Oil lamps are employed in many of

the Scotch collieries, and are almost universally used in Belgium and

other European countries. The buildings near the pit bottom, such as the

stables and lamp cabin, and even the main roads for some distance, are

often in large collieries lighted with gas brought from the surface, or

in some cases the gas given off by the coal is used for the same

purpose. Where the gases are fiery, the use of protected lights or

safety lamps (q.v.) becomes a necessity.

Composition of gas evolved by coal.

The nature of the gases evolved by coal when freshly exposed to the

atmosphere has been investigated by several chemists, more particularly

by Lyon Playfair and Ernst von Meyer. The latter observer found the

gases given off by coal from the district of Newcastle and Durham to

contain carbonic acid, marsh gas or light carburetted hydrogen (the

fire-damp of the miner), oxygen and nitrogen. A later investigation, by

J. W. Thomas, of the gases dissolved or occluded in coals from South

Wales basin shows them to vary considerably with the class of coal. The

results given below, which are selected from a much larger series

published in the _Journal of the Chemical Society_, were obtained by

heating samples of the different coals in vacuo for several hours at the

temperature of boiling water:--

+------------+------------------+---------+----------------------------------+

| Quality. | Colliery. | per ton +---------+-------+------+---------|

| | | ft. | Acid. |Oxygen.| gas.| |

+------------+------------------+---------+---------+-------+------+---------+

| Bituminous | Cwm Clydach | 19.72 | 5.44 | 1.05 | 63.76| 29.75 |

| " | Lantwit | 14.34 | 9.43 | 2.25 | 31.95| 56.34 |

| Steam | Navigation | 89.62 | 13.21 | 0.49 | 81.64| 4.66 |

| Anthracite | Bonville's Court | 198.95 | 2.62 | .. | 93.13| 4.25 |

+------------+------------------+---------+---------+-------+------+---------+

In one instance about 1% of hydride of ethyl was found in the gas from a

blower in a pit in the Rhondda district, which was collected in a tube

and brought to the surface to be used in lighting the engine-room and

pit-bank. The gases from the bituminous house coals of South Wales are

comparatively free from marsh gas, as compared with those from the steam

coal and anthracite pits. The latter class of coal contains the largest

proportion of this dangerous gas, but holds it more tenaciously than do

the steam coals, thus rendering the workings comparatively safer. It was

found that, of the entire volume of occluded gas in an anthracite, only

one-third could be expelled at the temperature of boiling water, and

that the whole quantity, amounting to 650 cub. ft. per ton, was only to

be driven out by a heat of 300 deg. C. Steam coals being softer and more

porous give off enormous volumes of gas from the working face in most of

the deep pits, many of which have been the scene of disastrous

explosions.

The gases evolved from the sudden outbursts or blowers in coal, which

are often given off at a considerable tension, are the most dangerous

enemy that the collier has to contend with. They consist almost entirely

of marsh gas, with only a small quantity of carbonic acid, usually under

1%, and from 1 to 4% of nitrogen.

Fire-damp when mixed with from four to twelve times its volume of

atmospheric air is explosive; but when the proportion is above or below

these limits it burns quietly with a pale blue flame.

Coal dust.

Encyclopaedia Britannica, 11th Edition, "Clervaux" to "Cockade" Volume 6, Slice 5 · 第 33 / 41 题

TABLE III.--_Composition of Fuels (assuming Carbon = 100)._