exceed the height of four St. Paul's Cathedrals, piled one on the top of the other.

6. It often happens that the sinking of a shaft is incommoded by heavy springs of water, which it would be very expensive to have to pump from the bottom of the mine. It is, therefore, important, where it is possible, to dam them back, so as to prevent their pouring into the open space. This may be so successfully accomplished, that not a drop of water shall leak in, although the feeders or springs may have supplied above a thousand gallons a minute. The mode of effecting it, called "tubbing," is to choose a bed of firm rock, below the watery part, and from that foundation to build the sides of a kind of tub, as large as the shaft, which, instead of holding water inside it, shall keep it out. This is built up to some place above the watery portion of the ground, at which it can be securely connected again with water-tight rock, and thus the subterranean streams, being unable to penetrate, are obliged to take their course elsewhere.

The tubbing may be constructed either of wood or iron. In Belgium the former material is generally employed, and placed in a polygonal form: in England cast-iron is found to be less expensive, and is fitted in segmental portions, accurately cast to fit the circle. Whatever the material, a principal object of care is the filling of all the joints with wedges, first of soft and then of hard wood, driven in as long as a chisel can make room for them, and amounting sometimes to several thousand in the space of a few square feet.

7. WORKING AWAY OF THE MINERALS, OR EXPLOITATION.—When a sufficient portion of the mine has been opened out in the manner above described, the next operation is to extract the minerals of a deposit in a manner more economical than can be effected in the narrow openings of shafts and levels. In most lodes the valuable material is so unequally distributed that many portions have to be left untouched, and other parts may be profitable or not, according to the temporary price of the metal. In nearly all cases a large portion of the broken vein must be left underground as rubbish,

"deads," or "attle," and serves the useful purpose of filling up the excavations, and forming a support to the natural walls.

66

8. A lode may be attacked from any gallery downwards by "underhand stopeing." The ground is divided into a series of steps, on each of which a set of men may be employed, without interfering with others, and having one always in advance of the other. As the work proceeds in depth, timbers are stretched from wall to wall, and covered with planks, on which are piled the heaps of "attle." But as this method entails several disadvantages, it is generally replaced by the mode of working upwards from below, or overhand stopeing." The roof of a level is broken away upwards, and when the excavation is sufficiently high, a timber platform or "stull" is put in, upon which the men may stand to proceed with their work. The ground is similarly divided into steps, each one regularly in advance of the next, and the broken vein-stuff, or attle, forms, as the men ascend, a continually-increasing heap, upon which they can stand to attack the roof above them; or they may erect temporary stages to serve the same purpose.

Fig. 5 exhibits both these methods in progress in a mine of small extent, the underhand stopeing being seen below the 10-fathom level on the right-hand side.

9. It is a far more serious task to determine the arrangement which is most suitable for the working of a bed of coal. In this case the great variety of conditions, under which it may occur, renders it necessary to choose from among a great number of different plans of exploitation. The thickness of the bed, its quality and angle of inclination, as also the character of the strata immediately above and below it, technically called the "roof" and the "floor," must all be taken into account; and since a large district has often to be excavated from one starting point, it is needful for the manager or "viewer" to be far-seeing enough to calculate on having always a sufficient quantity of ground open to supply the required produce, and yet so to regulate the excavations that no part shall be injured by the exhaustion and abandonment of another.

Additional importance is lent to this subject by the fact, that the ventilation, upon which the lives of all the workmen depend, must be regulated by the plan which is adopted for the extraction of the coal.

10. To avoid entering too much on detail, we may consider that the numerous methods of working coal are divisible into two heads:-1st, that in which solid blocks or "pillars" of coal are left, as the excavations proceed, to support the overlying strata; and 2nd, that by which the entire bed is worked continuously Each of these we may examine separately.

away.

The first mode of working, by "bord and pillar,' or "post and stall," is extensively used in the north of England, and may be easily understood by aid of a plan or map which represents by a black shade that

portion of the coal which remains unwrought. The excavations of a single colliery are sometimes carried on over so large a space, that it resembles a goodsized town with an infinite number of streets and lanes laid out with great regularity; we must, therefore, for convenience, only include in our plan a small part of such workings, or consider it to be only in its com

mencement.

11. When the seam has been reached by the shafts U, D, and one or more principal roadways or levels have been carried to a sufficient distance from the shafts, "bords are driven out at regular intervals in the coal. These are chambers of from 10 to 15 feet in width, which, when they have extended to a certain length, are brought into communication by cross passages termed "thirls.” The rectangular" pillars" or pieces of coal thus shaped out, have been, where economy was not properly studied, intended solely for the support of the roof, and therefore left as small as was consistent with safety at the moment. In other cases they have been left only a little larger with a view to afterwards "robbing," or partially removing them. But ere long, under such circumstances, the pressure from above either crushes this solid coal so as to render it useless, or forces the softer floor to "creep" or heave up into the excavations, till, in many instances, a loss of more than half the entire coal has been sustained, an evil of the most serious nature when we regard the high importance of cheap coal to the community.

Of late years, especially in Durham and Northumberland, where coal-mining is conducted on a gigantic scale, the pillars have been formed of such dimensions (for example, 90 feet long by 60 feet wide) that they remain uninjured by the pressure: then after the coal has been 66 won by extending the labyrinth of passages over a considerable district, the colliers begin to attack the pillars farthest from the shaft, remove as much of them as possible, and with the falling roof continually following them, work their way back towards the

shafts.

A more modern practice is to open out only a

small district of pillars at a time, and to clear them away at once, as shown on the right-hand side of fig. 10, in the part marked G.

12. It is often with considerable risk that these latter operations are carried out, and a constant source of danger frequently exists in the "goaf" or "gob," the space from which the coal was extracted, now more or less filled with a confused mass of broken stone, but sufficiently open to act as a magazine for the fire-damp which oozes out from the coal or some of the adjacent beds. The goaf is represented at G, figs. 10

and 11.

The difficulties of excavating a bed or stratum by this method increase very much with the thickness of the deposit. Hence, in working the "thick coal" of Staffordshire, a seam averaging 30 feet of material which is removed in one excavation, the usual method is to limit the part operated on at one time to a parallelogram of small dimensions, the roof of which is upheld by several stout pillars of 8 or 10 yards square. These lofty chambers have somewhat the aspect of a blackened Egyptian rock-temple, and their funereal hue is but too suggestive of the numerous fatal accidents which occur, when large masses of the upper part of the seam suddenly fall without warning. A solid wall of coal is left surrounding the "side of work," pierced only by a single opening, which, as soon as the available coal has been extracted, is blocked up by a dam, for the purpose of excluding the atmospheric air, which would tend to excite spontaneous combustion in the small coal and refuse.

13. The second mode of working to which I have adverted above, and which is termed the "long wall," fig. 11, consists in removing the whole of the coal in a more or less continuous face, allowing the roof regularly to fall close behind the men as the work advances. This method may be conducted either by driving roadways in the coal to the limits of the field, and then cutting it away towards the shaft, or it may be commenced from the neighbourhood of the shaft outwards, in which case the roadways have to be maintained, often