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COAL

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 working 0t stages and the long-wall or continuous method 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 wToods, 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. 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, CoaI ’ 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 51 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 ip breadth (3J 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 2Id. 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. 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 lighting. 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 fiat blades, and with spiral diffusers resembling turbines, are now largely used, that of M. Bateau 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