MIXING. C A L Trigor, 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 cham ber, 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 pres sure 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 & Chau- dron in Belgium and Germany. For this purpose a horizon tal 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 GO 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 consider ably simplified the problem of sinking through water-bear ing 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 dis tricts, 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 re quired. 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. Ths 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. 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 tlie 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 Stafford shire 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 ad vantage. 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 un touched 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 brick work, 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 G 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