depth of 860 feet, buildings were considerably damaged at a distance. of 660 feet from the limit of the excavation, in spite of the fact that the seam dipped at a gradient of 1 in 10 from the houses.

The drainage of old workings, or the flooding of a mine, may set up fresh movements a long time after the original ones have ceased.

Reduction of Subsidence.-Careful and efficient stowing is the only remedy. One of the most interesting experiments of modern mining has been tried with success near Shenandoah City, Pennsylvania, at the Kohinoor Colliery. It consists in the filling in of the breasts or stalls with a mixture of small waste coal from the culm heaps, and water. At first, the method was introduced for the purpose of preventing the subsidence of a large house in Shenandoah City, beneath which a chamber 700 feet long, 300 feet wide, and 60 feet high, was found to exist. An 8-inch bore hole was drilled from the surface into the cavity, and down it was poured a thick mixture of small coal and water. The former was obtained from a neighbouring waste heap, and was carried to the top of the bore-hole by an ordinary push plate conveyor, working in a semi-circular trough. At the top of the bore-hole a stream of

water met the small coal and carried it into the excavation. Only a small quantity of water is required, and this readily drains away, leaving the small coal packed into heaps underground. Not only has this reduced the area of ground required on the surface for waste heaps, but experience has proved that the packing thus introduced sets so solidly, that after a short interval roads may be driven through it, and the coal previously left in ribs can be mined, thus increasing the amount won.

This practice introduced to prevent subsidence, is now carried on as a method of packing to increase the yield from a given area. Up to the time of the author's visit, thirteen bore-holes, 8 inches diameter, varying from 308 to 398 feet in depth, had been put down at variouspoints during the six years the system has been in operation. The annual cost is about £2,000, half the expense being borne by the colliery company and half by the lessor. The bore-holes are located on the apex of anticlinal ridges, and each one will fill an area of about 4 acres. The sludge takes a natural angle of about 5°. The present length of conveyors is about 1600 feet, in five sections. The engine supplying power is direct-acting, and has a cylinder 18 inches diameter by 36 inches stroke, while the pump delivering water is 9

inches diameter by 38 inches stroke. Two locomotive boilers, carrying 90 lbs. pressure, supply steam for the entire plant.

Arrangement of Labour.-Before describing the methods of mining, some reference should be made to the two systems under which the labour is carried out. In one, the miner not only gets the

coal, but carries out odd work, such as packing, repairs to roadways, &c., while, in the other, a skilled class of colliers are employed simply as hewers at the face, all dead work being performed by separate staffs of men. In the latter system the labour is subdivided, each class of men carrying out special duties.

If the hewers are employed solely at the face, a larger tonnage is produced per man, and hence less extent of workings is required, with a corresponding reduction in the cost of maintaining a smaller area under timber. The face moves faster, the weight has less time to break up the coal, the roof is always "green" (or fresh), and there is consequently less liability to accident.* Except under special circumstances, coal is invariably mined cheapest where the face travels fastest; the exception is a seam having a very strong roof and floor, as here it is possible to move too fast.

The division of labour does not actually produce more coal with fewer men, for other colliers have to be employed to perform the work the hewers originally did. The old out-put is, however, produced from a smaller extent of workings; and, on an average, about one-half of colliery cost accounts are capable of reduction in proportion as the output is increased, and the area from which it is produced is reduced. So far as maintaining roads is concerned, the chief point is to see that the gob is carefully packed, and that all props are removed, so that the roof can settle and not break down.

The above observations do not apply so strongly except in such places where, from the nature of the roof and the seam itself, the amount of repairs is large, and keeps the miner away from the actual coal-getting for a considerable part of his time. Timber drawing is certainly best performed by a separate staff of men, who should, preferably, be set the work by contract.

Bord and Pillar Working.†-After driving out the main roads, the first operation is to divide the coal into a series of rectangular blocks (Fig. 210) by means of drivages, called "bords" and "walls," the line of the latter being generally spoken of as "headways course.' The bords are driven from 4 to 5 yards wide, and always in a direction at right angles to the cleat of the coal, or, as it is generally termed, "on the face"-that is to say, the working faces of the bords are parallel with the lines of cleavage of the coal. Headways course is at right angles to these, or, in other words, parallel with the cleat, so that the working face in the walls is at right angles to the cleavage planes, and as the cleat runs approximately north and south in the North of England coalfield, headways course is generally taken to mean north and south, and bordways east and west. As a rule, walls are driven about 2 yards wide, but sometimes both bords and walls are driven 5 yards wide, and the roof allowed to fall.

The first procedure is to drive out the main roads.

*

So. Wales Inst., xv., 114.

At large

+Also known as "post and stall," "pillar and stall," and in Scotland as "stoop and room.'

collieries there are usually four proceeding at once, two intakes and two returns. Before these roads have gone far, bords and walls can be commenced on either side of them, leaving, however, sufficient coal on both sides to prevent any risk of creep. At one time, the pillars which were cut off by the bords and walls were made only just large enough to keep the roof up, and were left. The walls were as little as 12 to 15 yards apart, and the bords turned out of them commenced at 3 yards wide and were gradually widened out, until at the centre of the pillar only a thin piece of coal remained. As the bords approached. the other wall they were narrowed down again. As much as 35 per cent. of the coal was lost. This has been quite abandoned, and the pillars are now made very large, quite a common size being two, and often three, chains square. They should be made of such a size as to prevent the risk of creep when the floor is soft, or of the cracking and fissuring of the coal, known as "thrust," which happens when both floor and roof are hard. There does not, however,. appear to be any common system regulating the dimensions of pillars, as nearly every conceivable size and shape can be found in practice, the procedure at each colliery depending on the individual opinion of the manager. Mr. Atkinson, in a report to the New South Wales Government, quotes numerous instances in seams varying from 2 feet 2 inches to 8 feet thick and at a depth varying from 210 feet to 1800 feet, where amounts of from 59 to 95 per cent. of the coal is left in pillars after the bords and walls have been driven. In the best modern practice, never more than 30 to 35 per cent. of the coal is removed in the "whole" workings. If more than 40 per cent. is taken out there is great risk of "creep," while, if 50 per cent. is removed, the latter catastrophe seems inevitable.

It was customary some time ago to work these pillars years after they had been formed, but it is now more common to commence to remove them after a very short interval, before the roof has had time to settle down, and while the driving of bords still proceeds only a short distance away. In this manner the percentage of large coal has been materially increased, and the pillars are scarcely crushed at all, this being especially noticeable where both the coal and roof are tender. The work is concentrated, ventilation is easier, and the dead charges. are reduced, for the roof has rarely fallen in the bords or walls, and, consequently, no cost is incurred for ridding or cleansing them. Indeed, "ridding" out the fallen roof is seldom resorted to where the breakdown is serious, because no coal is obtained by the operation, and, consequently, the whole of the cost is dead loss. Where the roof has fallen, a narrow place is generally turned away skirting the débris.

Another practice which was introduced by the celebrated viewer Buddle, about the beginning of the century, is the method of dividing the colliery up into what are known as " panels," or "districts" (Fig. 210), these consisting of an area of from 30 to 40 acres, surrounded on all sides by a rib of coal, called a "barrier," these barriers being holed through at points where roads are necessary. This system of panels is particularly advantageous with a tender roof and soft floor; only a small area of the seam is opened at once, the roof does not weight so badly, nor require so much timber, and more round coal is produced. In addition, the risk of creep is, to a certain extent, prevented, or

it may be confined to the panel in which it arises. The risk of explosion is decreased, as each district has its own current of air, and should anything happen in one, there is a smaller probability of it extending to the others.

The preliminary work of driving the bords and walls is called "working in the whole," the removal of the pillars, which follows

afterwards, "working in the broken."

In the latter it is of the

utmost importance that a proper line of operations, usually a diagonal one, should be adhered to, as if portions of a pillar lag behind, or

lig. 210.

become surrounded by broken workings, the coal is very much crushed, and quantities are very often lost. The tonnage price paid for getting the coal in the whole is always larger than that paid in the broken. The working of the "broken," immediately following the "whole," seems desirable in every case, except where the seam makes a lot of gas, and has a hard roof that does not fall readily, because under these conditions large open spaces are formed and filled with gas, and this gas may be suddenly driven out into the airways when the fall of roof does happen.

The removal of the pillars is carried out by a series of drivages, technically called "jenkins" and "skirtings;" the former is a place driven in a pillar in a bord ways direction, while the latter is a similar place driven headways way, although, as a rule, any place driven alongside the fallen roof is called a skirting. At Eppleton Colliery the pillars, which are 44 yards by 33 yards, are worked by driving a fast skirting out of the waggon-way, the length of four pillars, as shown in Fig. 211, leaving

6 feet of coal against the fallen roof in the head ways. A jenkin is then carried up the pillar alongside the old bords, and then lifts or "juds" are driven right across, these being 5 yards wide. As soon as one of these reaches the fallen roof on the west side of the pillar, a second is commenced out of the jenkin. Several pillars are attacked at the same time, the lifts in each lying back in step fashion, as shown. The roof is kept up in the juds by a series of chocks or cogs, formed of timber 22 inches long by

Fig. 212.

4 inches square, placed 4 feet apart, and, say, 6 feet from the coal side. The space between the loose side and the chocks is secured by ordinary props and laggings, these, except three rows at the face, being drawn every night and the roof allowed to fall behind.

With pillars 44 yards square at Murton Colliery in a seam 4 feet thick and 490 yards deep, lifts of equal width are driven simultaneously from the bord and the wall until they meet, when second lifts are commenced by the side of the first (Fig. 212).