Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/76

Rh U O A L [MINING. low bores, the boring is generally with wrought iron rods screwed together in lengths, armed with a cutting chisel and workin^ by percussion, the tool being lifted by hand and allowed to fall with its full weight upon the rock. The pounded material is removed at intervals, by substi tuting a shell pump or tube with valves at the bottom, whose action is similar to that of the foot valves of an ordinary lifting pump. The sludge brought to the sur face indicates the nature of the ground passed through. In very deep borings, however, the use of rigid rods and fixed tools is found to present two serious evils, namely, the excessive weight on the tool caused by the increased length of the rods, and the great length of time required to withdraw the tool and remove the detritus. The first of these difficulties has been overcome by the _use of the free falling cutters, where the tool, instead of being attached rigidly to&quot; the rod, moves in a guide-block in such a manner as to be lifted with the rods, falling freely when the top of the stroke is reached. The rods, when lowered, pick up the tool at the bottom of the hole in readiness for the next lift. By this means the momentum of the tool is kept constant whatever may be the weight of rods employed. The use of a wire rope winding on a drum, instead of rods for suspending the boring tool, allows the latter to be withdrawn and replaced with much greater rapidity than can be done with rods. This method has been very successfully adopted by Messrs Mather & Platt of Salford. But perhaps the best methods of expeditious boring are those (Fauvelle s) whereby the detritus is removed as it forms by con tinuously flushing out the hole with water, hollow rods being used down which the water flows while it rises through the annular space between the rod and the lining tube of the bore hole. This has the advantage of giving a clear surface for the tool to cut on, instead of its having to work through its own sludge, as is the case when the shell pump is only used at intervals. Of late years the value of boring for exploratory purposes has been much increased by the adoption of tubular or crown borers, which cut out an annu lar groove, leaving a core of unbroken rock in the centre, which is then brought out by a grapnel in a solid piece. One of. the m^st successful of these methods is that due to Leschot of Geneva, where a rotating cutter, armed with amorphous black diamond, the hardest known substance, is used, the detritus being continuously removed by water on Fauvelle s plan. The machinery adopted for this pur pose, as modified by Messrs Beaumont & Appleby. has been employed with great success to bore holes exceeding 2000 feet in depth. Methods of The working of coal may be conducted either by means working, 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 out crop 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 feet thick laid bare, over an area of several acres, by stripping off a superficial covering varying from 10 to 30 feet, in order to remove the whole of tbe 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 &quot;winning,&quot; the actual working or extraction of the coal being termed &quot;getting.&quot; In the accompanying figure, No. 4, A B is a cross cut-level, by which the seams of coal 1 and 2 are won, and C D a ver tical 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 &quot; level-free.&quot; 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 Fig. 4. 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 bo 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 coal-fields are in flat countries, and where the coal can only be reached by vertical sinking. 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. 4, which are driven across the stratification, or generally anywhere not in coal, are known as &quot; stone drifts.&quot; 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 sink in sc. The O O 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 countries, 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 excava tion with an impermeable casing of wood or iron, gene rally the latter, which is built up in segments forming rings, that 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 in 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 recep tion, tightly caulked with moss, and wedged into posi tion. Upon this the tubbing is built up in segments, usually from 10 to 12 being required for the entire cir cumference, the edges being made perfectly true. The thickness varies according to the pressure expected, but may be taken at from f to 1| inches. 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 gc.ieral use. The polygons adopted were of 20 or more sides approximating to a circular form. The second principal method of sinking through water bearing ground is that which was first adopted by M. Sinking shafts. Pnenma- sinking.