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 COAL 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 debris 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 14J 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<j 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. 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 Yicq, 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

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application of this method at Yicq, near Anzin, two shafts of 12 and IGA 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 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. 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. With the increased activity of working characteristic of modern coal mining, the depth of the mines has rapidly increased, and at the present time the level of coal4000 feet, formerly assumed as the possible limit working for working, has been nearly attained. The at great following list gives the depths reached in the depths. deepest collieries in Europe in 1900, from which it will be seen that the larger number, as well as the deepest, are in Belgium:— Metres. Feet. Saint Henriette, Cie des Produits, Flenu, Belgium 1150 3773 Yiviers Gilly ,, 1143 3750 Marcinelle, No. 11, Charleroi . . ,, 1075 3527 Marchienne, No. 2 ,, . . ,, 1065 3494 Agrappe, Mons ,, 1060 3478 Pendleton dip workings . . . Lancashire 1059 3474 Sacre Madame, Charleroi. . . Belgium 1055 3461 Ashton Moss dip workings. . . Lancashire 1024 3360 Ronchamp, No. 11 pit .... France 1015 3330 Yiernoy, Anderlues .... Belgium 1006 3301 Astley Pit, Dukinfield, dip workings. Cheshire 960 3150 Saint AndiA, Poirier, Charleroi. . Belgium 950 3117 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 a grande profondeur en Belgique,” Bulletin de la Societe de VIndustrie Minerals, 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, O’SdG 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 difierent appliances from those in current use.