The principle of proving a mineral field by boring is illustrated by figure 3, which represents a line direct from the dip to the rise of the field, the inclination of the strata being one in eight. No. 1 bore is commenced at the dip, and reaches a seam of coal A, at 40 fathoms; at this depth it is considered proper to remove nearer to the outcrop, so that lower strata may be bored into at a less depth, and a second bore is commenced. To find the position of No. 2, so as to form a continuous section, it is necessary to reckon the inclination of the strata, which is 1 in 8: and as bore No. 1 was 40 fathoms in depth, we multiply the depth by the rate of inclination, 40 × 8 = 320 fathoms, which gives the point at which the coal seam A should reach the surface. But there is generally a certain depth of alluvial cover which requires to be deducted, and which we call 3 fathoms, then (40 - 3 = 37) × 8 = 296 fathoms ; or say 286 fathoms is the distance that the second bore should be placed to the rise of the first, so as to have for certain the seam of coal A in clear connection with the seam of coal B. In bore No. 3, where the seam B, according to the same system of arrangement, should have been found at or near the surface, another seam C is proved at a considerable depth, differing in character and thickness from either of the preceding. This derangement being carefully noted, another bore to the outcrop on the same principle is put down for the purpose of proving the seam C; the nature of the strata at first is found to agree with the latter part of that bored through in No. 3, but immediately on crossing the dislocation seen in the figure it is changed, and the deeper seam D is found.

The evidence therefore of these bores (3 and 4) indicates some material derangement, which is then proved by other bores, either towards the dip or the outcrop, according to the judgment of the borer, so as to ascertain the best position for sinking pits.

The methods of boring are similar to those adopted for deep wells, or in other departments of mining. For shallow bores, the boring is generally with wrought iron rods screwed together in lengths, armed with a cutting chisel, and working 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 substituting 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 surface 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 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 continuously 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 annular 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 most 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 purpose, as modified by Messrs Beaumont & Appleby, has been employed with great success to bore holes exceeding 2000 feet in depth.

The working of coal may be conducted either by means 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 outcrop 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 the 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 “winning,” the actual working or extraction of the coal being termed “getting.” 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 vertical 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 “level-free.” 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 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 “stone drifts.” 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 sinking. The actual coal measure strata, consisting mainly of shales and clays, are generally imperγious 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 waterbearing 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 excavation with an impermeable casing of wood or iron, generally 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 reception, tightly caulked with moss, and wedged into position. 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 ¾ 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 general use. The polygons adopted were of 20 or more sides approximating to a circular form.

The second principal method of sinking through waterbearing ground is that which was first adopted by Μ.

Triger, in France, and has also been used by civil engineers in putting down deep foundations for bridge piers, namely, by compressed air. The shaft is lined with a cylinder of "wrought iron, within which a tubular chamber, provided with doors above and below, known as an air-lock, is fitted by a telescopic joint, which is tightly packed so as to close the top of the shaft air-tight. Air is then forced into the inclosed space by means of a compressing engine, until the pressure is sufficient to oppose the flow of water into the excavation, and to drive out any that may collect i n the bottom of the shaft through a pipe which is carried through the air-sluice to the surface. The miners work in the bottom in the same manner as divers in an - ordinary diving-bell. Access to the surface is obtained through the double doors of the air sluice, the pressure being reduced to that of the external atmosphere when it is desired to open the upper door, and increased to that of the working space below when it is intended to communicate with the sinkers, or to raise the stuff broken in the bottom. This method has been adopted in various sinkings on the Continent. At Bracquenie, near Mons, the miners worked in an atmosphere up to 45 ft pressure on the square inch, without experiencing any great difficulty, but they were found to be more susceptible to pulmonary disorder upon changes of weather than those who worked under the ordinary conditions of pressure.

The third method of sinking through water-bearing strata is that of boring, adopted by Messrs Kind & Chaudron in Belgium and Germany. For this purpose a horizontal bar armed with vertical cutting chisels is used, which cuts out the whole section of the shaft simultaneously. In the first instance, a smaller cutting frame is used, boring a hole from 3 to 5 feet in diameter, which is kept some 50 or 60 feet in advance, so as to receive the detritus, which is removed by a shell pump of large size. The large trepan or cutter weighs about 16 tons, and cuts a hole of from 9 to 15 feet in diameter. The water-tight lining may be either a wrought iron tube, which is. pressed down by jack screws as the bore hole advances, or cast-iron tubbing put together in short complete rings, in contradistinction to the old plan of building them up of segments. The tubbing, which is considerably let« in diameter than the bore hole, is suspended by rods from the surface until a bed suitable for a foundation is reached, upon which a sliding length of tube, known as the moss box, bearing a shoulder, which is filled with dried moss, is placed. The whole weight of the tubbing is made to bear on the moss, which squeezes outwards, forming a completely water-tight joint. The interval between the back of the tubbing and the sides of the bore hole is then filled up with concrete, which on setting fixes the tubbing firmly in position.

The introduction of these special methods has considerably simplified the problem of sinking through water-bearing strata. Some of the earlier sinkings of this kind, when pumps had to be depended on for keeping down the water, were conducted at great cost, as, for instance, at South Hetton, and more recently Ryhope, near Sunderland, through the magnesian limestone of Durham.

The size and form of colliery shafts varies in different districts, but the tendency is now generally to make them round, and from 12 to 15 feet in diameter. In the Midland counties, from 7 to 9 feet is a very common size, but larger dimensions are adopted where a large production is required. At Bagillt, on the Dee, a shaft of 22 feet in diameter was commenced a few years ago, but was reduced in diameter a short distance down. Since the accident at Hartley colliery, caused by the breaking of the pumping engine beam, which fell into the shaft and blocked it up, whereby the whole company of men in the mine were starved to death—it has been made compulsory upon mine owners to have two pits for each working, in place of the single one divided by walls or brattices which was formerly thought sufficient. The use of two independent connections—whether separate pits or sections of the same pit, between the surface and the workings—is necessary for the service of the ventilation,—fresh air from the surface being carried down one, known as the “downcast,” while the foul or return air of the mine rises through the other or “upcast” pit back to the surface. Where the mine is heavily watered, it is often necessary to establish a special engine pit, with pumps permanently fixed, or a division of one of the pits may be devoted to this purpose. The use of direct-acting high-pressure pumping engines placed at the bottom of the shaft has become common during the last ten years. They have the advantage of doing away with the heavy reciprocating rod from the engine at the surface, and may be worked either by steam pipes carried down the pit, or, what is now more common, by boilers underground, which supply also steam for the underground hauling engines. Where the water does not accumulate very rapidly'it is a very common practice to allow it to" collect in a pit or sump below the working bottom of the shaft, and to draw it off in a water tub or bucket by the main engine, when the latter is not employed in raising coal.

The laying out of a colliery, after the coal has been won, by sinkings or levels, may be accomplished in various ways, according to the nature of the coal, its thickness and dip, and the extent of ground to be worked. In the South Staffordshire and other Midland coal-fields, where only shallow pits are required, and the coals are thick, a pair of pits may be sunk for a very few acres, while in the North of England, on the other hand, where sinking is expensive, an area of some thousands of acres may be commanded from the same number of pits. In the latter case, which represents the most approved practice, the sinking is usually placed about the centre of the ground, so that the workings may radiate in every direction from the pit bottom, with the view of employing the greatest number of hands to advantage. Where a large area cannot be commanded, it is best to sink to the lowest point of the field for the convenience of drawing the coal and water which become level-free in regard to the pit. Where properties are much divided, it is always necessary to maintain a thick barrier of unwrought coal between the boundary of the mine and the neighbouring workings, especially if the latter are to the dip. If a prominent line of fault crosses the area, it may usually be a convenient division of the field into sections or districts. The first process in laying out the workings consists in driving a gallery on the level along the course of the coal seam, which is known as a “dip head level,” and a lower parallel one, in which the water collects, known as a “lodgment level.” Galleries driven at right angles to these are known as “dip” or “rise headings,” according to their position above or below the pit bottom. In Staffordshire the main levels are also known as “gate roads.” To secure the perpendicularity of the shaft, it is necessary to leave a large mass or pillar of the seam untouched around the pit bottom. This pillar is known in Scotland as the “pit bottom stoop.” The junction of the levels with the pit is known as the “pit eye;” it is usually of an enlarged section, and lined with masonry or brickwork, so as to afford room for handling the waggons or trams of coal brought from the working faces. In this portion of the pit are generally placed the furnaces for ventilation, and the boilers required for working steam-engines underground, as well as the stables and lamp cabin.

Figs. 5 and 6 represent the pit bottom arrangements at Cambois colliery in Northumberland, which are of an extremely commodious character. There are four large Cornish boilers, supplying steam to the engines drawing coals from the workings, as well as to a direct-acting pumping engine, the flame and smoke being discharged by drifts into the upcast pit. For the purpose of handling large pieces of machinery and boilers, the level at the bottom is increased to a chamber 18 feet high, and roofed with rolled iron girders of a double T section. To protect the fillers working at the bottom, strong diagonal guard timbers are placed at S in order to deflect any materials falling down the shaft, and prevent them falling into the workings. This is an unusually large example, but is taken from a pit in the highest state of development, and making a very large daily outturn.

The removal of the coal after the roads have been driven may be effected in many different ways, according to the custom of the district, These may, however all be considered as modifications of two systems viz., pillar work and long-wall work, In the former, which is also known as “port and stall” or “bord and pillar” in the north of England, “pillar and stall” in South Wales, and “stoop and room” in Scotland, the field is divided into strips by numerous openings driven parallel to the main rise headings, called “bords” or “bord gates,” which are again divided by cutting through them at intervals, so as to leave a series of pillars arranged chequer-wise over the entire area. These pillars are left for the support of the roof as the workings advance, so as to keep the mine open and free from waste. Fig. 1, Plate III. represents the oldest form of this class of working as practised in Scotland, from which it will be seen that if the size of the pillar is equal to the width of the stall or excavation, about ¾ of the whole seam will be removed, the remainder being left in the pillars. A portion of this may be got by the process known as robbing the pillars, but the coal so obtained...