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10| to 11 tons of steam at 180 H> pressure per hour, tire loss by condensation in the steam supply pipe, which is rather more than a quarter of a mile long, being only 12 cwt. per hour. The engines are of the four-cylinder triple expansion class in twin tandem form. A still larger example of the same kind, now under construction for the Harpen Collieries, is intended for a maximum duty of 25 tons of water raised 1640 feet per minute, representing about 3500 H.P. Probably the heaviest existing colliery pumping plant is that at the Miike Colliery in Japan, which has a capacity of 40‘3 tons per minute lifted 900 feet. This is done by four Davy compound engines placed side by side at the surface, each working two lines of rods in the shaft, which is of the unusual size of 40 by 12 feet. In the indirect system of hydraulic transmission a steam-engine at the surface pumps water continuously into a system of pipes against a resistance of 2500 to 3000 Eb per square inch, and the pressure so obtained drives a pump underground through a pipe carried down the shaft, the water returning by another pipe to the surface engine in a continuous circuit. This system, originally used as a method of transmitting small power underground by Messrs West & Darlington in Cornwall, and since developed to a high degree by Mr R. Moore, Mr H. Davey, Messrs Kaselowsky & Prdtt, and other hydraulic engineers, has many advantages, particularly in the small dimension of the engines, which require much smaller walled chambers in the mine than steam-engines of similar power; but great care is required to keep the driving water perfectly clean, to prevent wear on the valves, as on account of the high pressure employed a very small leakage gives rise to a large waste of power. Engines of this class are made in a great variety of forms, both reciprocating with long stroke, and rotating with short stroke high speed pumps. The largest class in use forces 7 tons per minute 2000 to 2300 feet high. Pumping by electric transmission, although of comparatively recent introduction, has taken up a leading position in deep mining, and probably will be almost exclusively used at no very distant date. At first it was mainly applied to small special purposes, such us raising feeders of water from isolated workings to a main pumping engine, but now large self-contained installations, with generating dynamos from 800 to 1000 H.P., are in use. In the largest plant of this class, intended for a maximum duty of 15 tons per minute lifted 1260 feet, three compound engines of 750 H.P. are provided, each driving its pump by a separate generator and motor by an alternating current of 2000 volts. Under ordinary conditions the water charge in coal mining is considerable, and in old mines with a large extent of open workings it becomes very burdensome, the weight of water to be lifted being often many times that of the coal produced. In the Westphalian coal-field in 1899, 169'5 million tons of wTater were lifted for an output of 55 million tons of coal, or rather more than 3 tons to 1 ton of coal. In the older and partly worked-out districts the proportion rises to as much as 8 to 1. In South Staffordshire, where the ground is honeycombed with old thick coal workings, from 24 to 28| tons of water per ton of coal raised were pumped in 1898. The principal improvements in the hoisting arrangements of modern colliery plants have been in the direction of larger engine power, and arrangements for loadWitiding ;ng anq discharging the cages with the smallest mines.eeP l°ss °f time. Where deep cages, carrying from four to six tiers of tubs superposed, are employed, Fowler’s hydraulic arrangement, using auxiliary cages upon hydraulic lifts at the surface and pit bottom {Ency. Brit. vol. vi. p. 76), is one of the best devices for
the latter purpose. It has been lately extended by Tomson to six- and eight-decked cages. In another arrangement by Darphin the cage is received at the surface by hydraulic keeps, which support it at its highest level and progressively lower it to bring each deck in succession down to the landing place, when the loaded tubs are run out and replaced by empty ones without moving the main engine. The time occupied in landing and changing the load of twelve tubs is from 20 to 25 seconds. Many improvements have been made in the construction of winding engines of late years, in order to reduce the consumption of steam, by the adoption of variable expansion gear, and the substitution of compound for single engines. The two-cylinder compound engine has been used in some cases, but a preferable form is the twin tandem compound adopted by Mr W. Galloway at Llanbradach in 1894, and since introduced into many of the deep pits in France and Germany. The steam is exhausted into a central condenser, where a permanent vacuum is maintained, as the intermittent nature of the work prevents the use of a separate condenser. In very deep mines the moment of the load at different points of the lift varies considerably when a cylindrical drum is used, and becomes negative during the last part of the lift, owing to the excess weight of the rope on the descending side. In order to keep the work of the engines more nearly constant, various systems of counterbalancing by auxiliary chains, &c., have been adopted, as previously described. In addition to these, Koepe’s method of winding has of late years obtained some favour. In this system the drum is replaced by a disc with a grooved rim for the rope, which passes from the top of one cage over the guide pulley, round the disc, and back over the second guide to the second cage, and a tail rope, passing round a pulley at the bottom of the shaft, connects the bottoms of the cages, so that the dead weight of cage, tubs, and rope is completely counterbalanced at all positions of the cages, and the work of the engine is confined to the useful weight of coal raised. Motion is communicated to the rope by frictional contact with the drum, which is covered through about one-half of the circumference. This system was used for some time at Bestwood, in Nottinghamshire, and is still employed at Sneyd, in North Staffordshire. In Belgium it was tried in a pit 940 metres deep, but has since been replaced by flat hempen ropes, and is now restricted to shallower workings at 250 metres. In Westphalia it is applied in about thirty different pits to a maximum depth of 761 metres. The system of counterbalancing by the use of spiral drums is limited by their excessive size and weight wdien both ropes wind on the same drum. This has to some extent been met by Tomson’s modification, where a separate conical drum is used for each rope. They are mounted upon parallel axes, but turned in opposite directions, the base of one towards the point of the other. In this wray the weight and breadth of the drum is notably diminished, but the engine is complicated by the addition of a rocking beam and additional coupling rod for working the second drum. An engine of this kind has been in use since 1896 at Preussen Colliery, Westphalia, and has also been adopted for the 1015-metre pits at Ronchamp, in France. As regards the form and material of winding ropes, while round ropes of steel wire of high tensile strength and tapered in thickness for great depths are generally preferred in England and Germany, opinion in Belgium, and to some extent in the north of France, favours the use of flat ropes made of “ aloes ” or Manilla hemp, really plantain fibre. Those in deep pits are of considerable size and weight, e.g., tapering from 15‘6 inches to 9 inches in breadth and 2 inches to 1^ inch in thickness, and weighing 14'3
