employ about 7000 hands. There are various other works, including the building and repairing yards of the Clyde Navigation Trust. Municipal buildings, including baths, are being erected at a cost of £40,000. Population Population (1891), 9998; (1901), 18,654.

Coahuila, a state of Mexico, bounded on the N. by the United States, on the E. by the state of Nuevo León, on the S. by those of San Luis Potosí and Zacatecas, on the W. and S.W. by Durango, and on the N.W. and W. by Sonora, has an area of 62,376 square miles. In 1879 the population was 130,026, and in 1895 it was 241,026. The flora comprises over sixty varieties of trees of the cold and temperate zones, and fifty belonging to the hot lands. Agriculture is the principal industry, cotton, maize, wheat, beans, sugar-cane, linseed, and about thirty species of leguminous plants being the chief products. Cattle-raising is also extensively followed. The mines are being rapidly developed, especially in the Sierra Mojada, Sierra del Carmen, and in the valley of Santa Rosa. The state, which is divided into five districts and thirty-three municipalities, is one of the most prosperous commercial regions of the Republic, due principally to its excellent railway system. The capital, Saltillo (population, 26,801), is 615 miles from Mexico City by rail. It has good public buildings, a State college, public library, &c., and is noted for its manufacture of shawls (sarapes), cotton cloth, knit goods, and flour. Amongst other towns are Parras (8326), Monclova, Ciudad Porfirio Díaz, Viesca, Matamoros.

Coal. During the period that has elapsed since the publication of the article contained in the ninth edition of this Encyclopædia, the development in the methods of winning and working coal has been very considerable, especially in the direction of increasing the output from individual centres of production concurrently with a general diminution in the length of the working day. This has been attended, at any rate in the older coal-fields, with a rapid increase in the depth of workings and greatly increased cost and difficulty in opening new mines, with the result that the comparatively rough methods and appliances of earlier times have given way to more economical methods of working underground, and machinery of more refined construction. The main winding engines, especially, are now constructed upon the most improved types, in order to save fuel in working. Similar improvements have been introduced in the methods of underground haulage, and horses on underground lines have been largely replaced by mechanical traction, the substitution of electric for steam driven motors, and the use of electric locomotives being specially noticeable. In the main operation of getting or removing the coal, machine-cutting has to some extent taken the place of hand labour, although the progress in this direction has been more marked in America than in Europe, and even there the proportion of the output obtained with the use of such mechanical aids is only small when compared with that due to hand labour. This, however, is rapidly changing, owing to the increased flexibility in working of electric motors, which will in the near future probably take the first place in this as in other branches of coal-mining. To enter, however, into the details of these and other changes would involve discussion of mechanical and other technical matters beyond the scope of the present work, which is to be regarded as supplementary to the article in the ninth edition, the additional matter being noticed somewhat in the same order as that previously adopted.

During the past few years the question of the origin and mode of formation of coal has received considerable attention from geologists, but it cannot be said that any

coal.

authoritative new solution of the problem has been propounded. Speaking generally, the tendency is towards a modification of the view of which Logan, De Origin and la Beche, Dawson, and Newberry may be taken composias the principal exponents, that coal seams are tion of essentially the remains of forests upon the sites of their original growth, a detrital origin being supposed for a part, if not the whole, of the carbonaceous material, which may have been derived from adjacent higherThis view, lying land by the action of river currents. known as the delta hypothesis, has found considerable favour in France, especially from study of the coal-field of Saint Etienne, where the seams vary very irregularly in thickness and character, in a way which seems to be incompatible with the hypothesis of a tranquil accumulation in situ. As regards the changes involved in the actual transformation of plant structures into lignite and coal, one of the most important series of researches is due to M. B. Renault (Bulletin de la Société de l'Industrie Minérale, 3 Ser., vol. xiii. p. 865), who, starting from the study of peat, finds that the chief agents in the transformation of cellulose into peaty substances are saproAs the former are phytic fungi and bacterial ferments.

only active in the presence of air, while the latter are anaerobic, the greater or less activity of either agent is conditioned by variation in the water-level of the bog. The destructive agency of bacteria seems to be limited by the production of ulmic acid of the composition, carbon 65.31, hydrogen 3.85 per cent., 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 ratio of carbon to oxygen and hydrogen in the residual product increases in the following manner :—

The constituents of lignite are generally similar to those of peat, with the addition of some animal (infusorial) remains, the degraded vegetable tissues forming a paste originally plastic, which has converted the more resisting parts of the plants into a compact mass. From the figures given above it will be seen that oxygen is less rapidly eliminated than hydrogen, the change to lignite being similar to that obtaining in the case of peat, but further advanced.

Bituminous shale and Boghead or Torbane Hill coal are considered by Renault to be mainly alterationproducts of masses of gelatinous fresh-water algae, which by an almost complete elimination of oxygen have been transformed into substances approximating to the formulæ CH, and C,H,, where C: H=7'98 and C:O+N=46.3. In cannel coals the prevailing vegetable constituents are spores of cryptogamic plants, algae being rare and in many cases absent. The detection of bacilli in coal is a difficult matter, owing to its opacity; but by making very thin sections and employing high magnification, 1000 to 1200 diameters, Renault has been enabled to detect numerous forms in the woody parts included in coal. One of these, named Micrococcus carbo, in many respects resembles the living Cladothryx found in the wood of trees buried in peat-bogs in process of formation. Clearer evidence has been obtained from wood partially mineralized by silica or carbonate of lime included in the coal.

The transformation of woody fibre into coal is attended with considerable contraction, which may be from 11 to 13 of the original volume, but this is unequally distributed; it is mainly in the direction of the thickness, so that minute objects seen on the flat may keep nearly their

original dimensions. The evidence for this is to be seen in the large number of microphotographs in the memoir just referred to, which represents the result of twenty-one years' work. Approximately the change of wood into coal, and the proportion of the valuable product eliminated, may be represented as follows:

4(C&H100%)=C,H2O+7CH, +8CO2+3H2O.

Cellulose.

Coal. Methane. Carbon Water.
dioxide.

The solid product CHO corresponds to an average bituminous coal of the composition, carbon 83.1, hydrogen 4.6, oxygen 12.3 per cent., and represents about 20 per cent. of the original weight of cellulose and 45 per cent. of its heating power. Another aid to the study of the structure of coal has been found by H. M. Couriot (Annales de la Société Géologique de Belgique, vol. xxiii. p. 105) in Röntgen photography, the carbonaceous or combustible portion being readily permeable by X-rays, while the mineral matter is comparatively opaque. In this way photographs showing the arrangement of the ash in parallel thin bands may be obtained from a piece of coal about an inch thick when exposed perpendicularly to the planes of bedding; included masses of iron pyrites appear as dark spots. The suggestion has also been made to use this agent as a means of determining the proportion of incombustible matter in coal, the sample being ground to powder and enclosed in a wooden box of a long, tapered wedge form, with a fluorescent screen along one side, which becomes sensitive to the rays passing through a greater or less thickness of the coal wedge in proportion to its freedom from mineral matter. According to Kotte (Stahl u. Eisen, vol. xx. p. 392), however, this method is unreliable for quantitative purposes, since for equal contents of ash the permeability varies with the nature of the mineral matter, a small proportion of iron being more effective in increasing the opacity than much larger amounts of the ordinary ash constituents, silica, alumina, &c. Thus for equal total amounts of ash, that containing the least amount of iron, 0.07 per cent., gave the clearest photograph, and that with 54 per cent. the worst.

The existence of coal-fields below the secondary strata in the south-east of England, along the line joining the South Wales and Westphalian basins, as Development of inferred by the late Mr Godwin Austen, has of coal- late years been verified by the discovery of fields. carboniferous strata with workable coal seams below the chalk and other secondary formations in the vicinity of Dover, where in 1890 a borehole reached coal at a depth of 1180 feet from the surface, and in a further depth of 1042 feet several seams were subsequently proved of various thicknesses up to 4 feet. In another boring at Ropersole, between Dover and Canterbury, 1774 feet deep, the last 197 feet are in carboniferous strata with two coal seams. In the Dover sinking a bed of oolitic brown iron ore, resembling that of Cleveland and Luxemburg, has been discovered at a depth of 600 feet. Most extensive developments have also been made on the eastern side of the great Midland coal-field, and numerous pits have been sunk through the magnesian limestone and other overlying strata along the whole length of the basin, from Yorkshire to Nottinghamshire. At South Carr, near Gainsborough, in Lincolnshire, the regular succession of the seams in the coal-field has been proved under 1700 feet of New Red Sandstone rocks down to the Barnsley hard coal at 3186 feet, the greatest depth at which coal has as yet been proved in the United Kingdom. It has been suggested that the coal brought up by the trawlers on the fishing bank known as the Coal Pit in the North Sea, 65 miles east of the mouth of the Humber, may be derived from the eastern outcrop of the seams in this

basin, which, if this be the case, would be the largest coal-field in Europe.

In the Rhenish Westphalian coal basin, the most important one in Continental Europe, great activity prevails both in exploration and opening of new mines. This is in many respects similar in structure to that of South Wales, a large number of seams, none very thick, being distributed through a great thickness of strata. These are folded transversely into three principal troughs whose axes have a general north-westerly strike, but with the important difference that the carboniferous strata are exposed for only a short distance along the southern margin, the greater part being covered by secondary and tertiary rocks, which increase in thickness rapidly to the The newer sinkings have therefore to pass through constantly increasing depths of water-bearing measures, with the result that special methods of overcoming such difficulties have been brought to a high degree of perfection in this region. According to Schultz (Mittheilungen über den Niederrheinisch-Westfälischen Steinkohlen-Bergbau, 1901, p. 28) the proved area in 1900 was 1157 square miles, estimated to contain in workable seams :—

north.

substituted for hand labour. Twelve or more holes are bored and charged, and after the removal of the machines and their support to a safe height, are fired either simultaneously or in groups, the boring frame being replaced after the removal of the broken stuff. Much time can also be saved when sinking and walling are carried on simultaneously by the method used in several deep sinkings in South Wales by Professor W. Galloway, where the bricklayers work upon a suspended platform with hinged flaps which completely fill the hollow of the shaft when in use, but can be rapidly shifted by the engine at the surface as the top of the wall rises. One or more central holes are provided with wire-rope guides, to allow of the passage of the bucket bringing up the débris broken by the sinkers. In sinking through soft or waterbearing strata within moderate depths, excavation by hand is practised, the ground being secured with segmented cast-iron tubbing and pumps or water tubs used to keep the bottom dry when the inflow does not exceed 6 or 8 tons of water per minute. Beyond this, the watercost becomes so great that the Kind and Chaudron system of boring, which has of late years been considerably improved in detail, particularly by the addition of methods for continuously removing the boring detritus, is usually to be preferred. 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 feet, the bottom rings in a shaft 14 feet in diameter are about 4 inches 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 and 5 feet, 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 of late years 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 feet 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 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 a recent

more.

application of this method at Vicq, near Anzin, two shafts of 12 and 16-4 feet diameter, in a covering of cretaceous strata, were frozen to a depth of 300 feet in fifty days, the actual sinking and lining operations requiring ninety days 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. In some cases cement concrete has been usefully employed in lining shafts instead of brickwork, a layer about 10 inches thick being much stronger than an equal thickness of brickwork. This is especially applicable to the repair of old shafts, and also to the lining of the excavations for underground pumping engines. Some excellent examples of this method were shown at Paris in 1901 by the Cockerill Company of Seraing. The cement used, as well as the ballast in the concrete, was produced from blast-furnace slags.

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

The subject of the limiting depth of working has been very fully studied in Belgium by Professor Stassart of Mons ("Les Conditions d'exploitation à grande profondeur en Belgique," Bulletin de la Société de l'Industrie Minérale, 3 Ser., vol. xiv.), who finds that no special difficulty has been met with in workings above 1100 metres deep from increased temperature or atmospheric pressure. The extreme temperatures in the working faces at 1150 metres were 79 degrees and 86 degrees F., and the maximum in the end of a drift, 100 degrees; and these were quite bearable on account of the energetic ventilation maintained, and the dryness of the air. The yield per man on the working faces was 4.5 tons, and for the whole of the working force underground, 0.846 tons, which is not less than that realized in shallower mines. From the experience of such workings it is considered that 1500 metres would be a possible workable depth, the rock temperature being 132 degrees, and those of the intake and return galleries, 92 degrees and 108 degrees respectively. Under such conditions work would be practically impossible except with very energetic ventilation and dry air. It would be scarcely possible to circulate more than 120,000 to 130,000 cubic feet per minute under such conditions, and the number of working places would thus be restricted, and consequently the output reduced to about 500 tons per shift of ten hours, which could be raised by a single engine at the surface without requiring any very different appliances from those in current use.

Except in modifications of details, no great alterations in the methods of working away coal seams are to be noted, the pillar-and-stall system of removal by two Methods of stages and the long-wall or continuous method

working.

being representative of all the systems in use. In Europe the tendency is toward the substitution of the latter method wherever possible, but in America pillar-and-stall work in some form is most prevalent. 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 per cent. 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 America culm and waste are washed into the workings by water, giving a compact mass when the water has drained away.

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, the forest areas of Northern Europe and Russia and other countries being laid under contribution, in addition to native woods, to furnish the ever-increasing quantities of pit wood required. 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 by Mr Firth. 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. 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.

Coalcutting

The substitution of machinery for hand labour has been comparatively slow as compared with the changes in other directions, and is by no means general in Europe, although in America the progress has been conmachines. siderable, especially since the systematic introduction of electric power underground. Of the earlier types, those with a swinging pick, imitating the action of a miner in undercutting, represented by the machines of Firth & Donisthorpe and Jones & Levick, have been superseded by those of the circular or chain saw types, to which have been added others with percussive and rotatory drill cutters. In the North of England and Midland districts the circular-saw type, cutting in a horizontal plane at or near the ground-level, is largely used, one of the best known being the Diamond coalcutter of Mr W. Garforth, which is similar in construction to Winstanly & Barker's machine (vol. vi. Fig. 14, p. 68), but cuts to a depth of 5 or 6 feet. The Baird type of chain-saw machine, working round a fixed overhanging frame, is still used in Scotland, and a modified form adapted for electric driving has been lately proposed by Mr E. K. Scott (Proc. Inst. Civ. Eng. vol. cxliv. p. 247). In the United States percussive and chain-saw machines are used almost exclusively. The former of these are

represented by the Harrison, Sullivan, & 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 square feet per hour. The chain machines represented by the Jeffrey, Link-Belt, and Morgan-Gardner coal-cutters 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 feet 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 (31 to 4 feet), while its depth may be varied up to the maximum travel (8 feet) 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 square feet 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. The saving effected by the substitution of machines for hand coalcutting in English Midland collieries varies from about 9-75d. to 21d. per ton, about two-thirds of this being due to the smaller fall of slack as compared with that produced in hand driving. In America the saving is less apparent, owing to the increased wages demanded by the drivers and assistants, and the principal advantage is in the increased rate of production, which is from 6 to 7 tons daily per man underground, instead of 3 to 4 tons with hand work. In 1898, 25.3 per cent. of the output of Pennsylvania (15 out of 59 million tons) was obtained by machine cutting, which was exclusively confined to the bituminous coal district; the coal in the anthracite districts is too hard and the seams too much disturbed to allow the cutting to be done except by hand.

Ventilation and lighting.

In new mines the ventilation is now generally effected by an exhausting fan, the old system of ventilating furnaces being almost obsolete. The large slowgoing fan of the Guibal type still maintains its character for efficiency, although the tendency is towards using smaller and more rapidly-driven machines; and the heavy casings and chimneys in brickwork are generally giving way to lighter structures in sheet-iron. Fans with curved instead of flat blades, and with spiral diffusers resembling turbines, are now largely used, that of M. Rateau being specially popular with Continental colliery engineers, on account of its high mechanical efficiency. The use of small auxiliary blowing ventilators underground, for carrying air into workings away from the main circuits, which was largely advocated a few years since, has lost its popularity, but a useful substitute has

1

been found in the induced draught produced by jets of | compressed air or high-pressure water blowing into ejectors. With a jet of inch area, a pipe discharging 1 gallon of water per minute at 165 b pressure per square inch, a circulation of 850 cubic feet 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 ib 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. Electric driving from central-power stations has been found to be well suited for this particular use.

The numerous forms of safety-lamps employed in fiery mines have received several additions in late years, and old forms have been improved and modified to meet the requirements of safety in air-currents travelling at a high velocity. Prominent among the new forms is the Hepplewhite-Gray lamp, which has a conical glass surrounding the light, with a gauze chimney, protected by an outer metal cylinder; the air supply to the flame is carried downwards through three tubes forming the standards of the cage. This, in addition to giving a good light overhead owing to the shape of the glass, is peculiarly sensitive to gas, and therefore valuable in testing for fire-damp. Other approved lamps are the Deflector and those of Marsaut & Mueseler when specially bonneted to resist extra high-speed currents. The illuminant now generally used in Great Britain is a mixture of rape oil with half its volume of petroleum, which is more suitable than vegetable or animal oil alone. In Germany Wolf's lamp, burning benzoline or petroleum spirit upon an asbestos wick, is very popular, as giving a much better light than oil. Special care is, however, required in filling, so that no free liquid may be left in the holder; the spirit must be entirely absorbed by a filling of sponge, and any superfluous quantity poured off. Portable electric lamps, supplied by accumulators or dry batteries, have been introduced into coal-mines; but owing to the weight and cost, their use is as yet very restricted. For the use of exploring parties after explosions, where irrespirable gases are encountered and compressed air or oxygen must be carried, they are especially valuable, as light is obtained without any demand on the air supply. Fire-damp, when present in the air, lengthens the flame of an ordinary safety-lamp, but the effect is not apparent with less than about 2 per cent. of gas; and for more delicate testing, special lamps with non-luminous flames are adopted. In Pieler's lamp, which is of the ordinary Davy form, alcohol is burned on a silk wick, and a screen is provided so that the flame can be hidden. When exposed in air containing per cent., a cap of 1 inch is formed, which increases to 2 inches with per cent., and with 1 per cent. the lamp is filled with a deep blue glow. Another and more useful method is that of Dr. F. Clowes, who uses a hydrogen flame 0.4 inch long, obtained by attaching a cylinder containing compressed hydrogen to an ordinary safety-lamp; the gas is turned into the oil flame, which is for the time extinguished, and relighted when the observation is finished. As little as 0-2 per cent. of gas can be detected by this method.

The danger arising from the presence of coal dust in the air of dry mines, with or without the addition of firedamp, has, since it was first pointed out by Coal dust. Professor W. Galloway, been made the subject of special inquiries in the principal European countries interested in coal mining; and although certain points are still debatable, the fact is generally admitted as

one

calling for special precautions. The conclusions arrived at by the Royal Commission of 1891, which may be taken as generally representative of the views of British colliery engineers, are as follows:

1. The danger of explosion when gas exists in very small quantities is greatly increased by the presence of coal dust.

2. A gas explosion in a fiery mine may be intensified or indefinitely propagated by the dust raised by the explosion itself. 3. Coal dust alone, without any gas, may cause a dangerous explosion if ignited by a blown-out shot; but such cases are likely to be exceptional.

4. The inflammability of coal dust varies with different coals, but none can be said to be entirely free from risk.

5. There is no probability of a dangerous explosion being produced by the ignition of coal dust by a naked light or ordinary Hlame.

Danger arising from coal dust is best guarded against by systematically sprinkling or watering the main roads leading from the working faces to the shaft, where the dust falling from the trams in transit is liable to accumulate. This may be done by water-carts or hose and jet, but preferably by finely divided water and compressed air distributed from a network of pipes carried through the workings. This is now generally done, and in some countries is compulsory, when the rocks are deficient in natural moisture. According to Behrens, the quantity of water required to keep down the dust in a mine raising 850 tons of coal in a single shift was 28.8 tons, apart from that required by the jets and motors. The distributing network extended to more than 30 miles of pipes, varying from 3 inches to 1 inch in diameter.

explosives.

A

In all British coal-mines, when gas in dangerous quantities has appeared within three months, and in all places that are dry and dusty, blasting is prohibited, except with permitted explosives, whose Safety composition and properties have been examined at the testing station at the Royal Arsenal, Woolwich. list of those sanctioned is published by the Home Office. They are mostly distinguished by special trade names, and are mainly of two classes-those containing ammonium nitrate and nitro-benzole or nitro-naphthalene, and those. containing nitro-glycerine and nitro-cellulose, which are essentially weak dynamites. The safety property attributed to them is due to the depression of the temperature of the flame or products of explosion to a point below that necessary to ignite fire-damp or coal dust in air from a blown-out shot. New explosives that are found to be satisfactory when tested are added to the list from time to time, the composition being stated in all

cases.

Pumping.

The most noticeable feature in the arrangements for draining modern collieries is the general abandonment of surface engines, with heavy wooden or iron rods in the shaft, in favour of high-speed engines placed underground, and supplied with power either by steam sent down from the surface, or in a less direct manner by water circulating under high pressure or by electric transmission. Compressed air may also be used, but is mostly restricted to small installations, on account of its low mechanical efficiency. The earlier underground steam-pumps were very wasteful machines, on account of the low steam pressures available and the loss by condensation in the steam conduit pipes, but with improvements in construction and the adoption of multiple expansion in several cylinders with high initial steam pressure, the fuel consumption has been reduced nearly to the level of that of good surface or marine engines. Several engines of this class of considerable size have been erected in the deep Westphalian pits, e.g., one of 1900 to 2000 H.P., lifting a maximum quantity of 17 tons of water per minute 1300 feet high, with an expenditure of