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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 in 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 b 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 less 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

The use

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 indepen-
dent connections-whether separate pits or sections of the
same pit, between the surface and the workings-is neces-
sary 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.
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, Laying out by sinkings or levels, may be accomplished in various ways, workings. 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 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 steamengines underground, as well as the stables and lamp cabin.

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Figs. 5 and 6 represent the pit bottom arrangements at Cambois colliery in Northumberland, which are of an ex tremely 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 pump- | ing 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.

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 is liable to be very much crushed from the pressure of the superincumbent strata. This crushing may take place either from above or below, producing what are known as "creeps" or "sits."

A coal seam with a soft pavement and a hard roof is the most subject to a "creep." The first indication is a dull hollow sound heard when treading on the pavement or floor, probably occasioned by some of the individual layers parting from each other as shown at a fig. 7;

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FIG. 7.-"Creeps" in Coal-Mines.

the succeeding stages of creep are shown at b, c, d, f, and the coal begins to sustain the pressure from the overlying g, in the same figure; the last being the final stage, when strata, in common with the disturbed pavement.

"Sits" are the reverse of creeps; in the one case the pavement is forced up, and in the other the roof is forced or falls down, for want of proper support or tenacity in itself. This accident generally arises from an improper size of pillars; some roofs, however, are so difficult to support that sits take place where the half of the coal is left in pillars.

Fig. 8 will convey a general idea of the appearance of sits,-k, m, n showing different stages.

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Pillar

FIG. 6.-Pit bottom arrangements, Cambois Colliery 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 working. 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 of the

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FIG. 8.-"Sits" in Mines.

The modern method of pillar working is shown in Plate IV. In the Northumberland steam-coal district, where it is carried out in the most perfect manner, the boards are 5 to 6 yards in width, while the pillars are 22 yards broad and 30 yards long, which are subsequently got out on coming back. In the same figure is also shown the method of working whole coal and pillars at the same time, a barrier of two or three ranges of pillars or a rib of solid coal being left between the working in the solid and those in the pillars. The space from which the entire quantity of coal has been removed is known in different districts as the "goaf," "gob," or "waste."

Fig. 9 represents the Lancashire system of pillarworking. The area is laid out by two pairs of level drifts, parallel to each other, about 150 yards apart, which are carried to the boundary. About 100 yards back from the boundary a communication is made between these levels, from which other levels are driven forward, dividing the coal into ribs of about 25 or 30 yards wide, which are then cut back by taking off the coal in slices from the level towards the rise in breadths of about six yards. By this method the whole of the coal is got backwards, the main roads being kept in solid coal; the intermediate levels not being driven till they are wanted, a greater amount of sup

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