On the left of the diagram at g, is shown the method of laying out faces so that they are 90° with the line of pitch, so that the full influence of any roof pressure due to the pitch is exerted fully on the gob. In this case, the empty mine cars are taken into the lower road of the upper pair of cross-entries h, dropped through the room i, which connects the pair of cross-entries, then to the face g along which they are loaded, thence dropped through a gob roadway j to the upper road of the lower pair of cross-entries k. The gob roadway j has solid coal for one of its ribs, and is maintained by means of a substantial timber crib, built as the face progresses. On the left of the diagram, the faces advance directly up hill.

Longwall in Thick Seams.-A thick seam of coal is usually worked in several benches of moderate thickness. The longwall method is variously modified to suit the conditions, but the plan generally adopted, and which has given the best results in France, Bohemia, and other countries where such seams are worked, both with respect to the safety of the working and the percentage of coal obtained from the seam, is that of close packing, or completely filling with waste the space from which the coal is taken. Sufficient waste is not produced, ordinarily, in the working of the seam, and waste material is brought from the surface to fill in this space.

In the working of flat seams by this method, a longwall face is carried forwards in each bench of the coal, the face in each bench being kept from 80 to 100 yd. in advance of that in the bench next above. Fig. 3 shows the general plan and cross-section of the workings at the face of a thick flat seam, the seam being divided, as shown, into three benches. The first mining is done in the lowest bench, in which the parallel main roads ab and en are driven and connected by the cross-roads c. The roads o that lead to the face are protected by packs, and correspond to the temporary roads, or working places in the general longwall method. Above the cross-roads c and from 80 to 100 yd. back from their faces are the cross-roads c' in the second bench. Above the crossroads c' and from 80 to 100 yd. back from their faces are the cross-roads c" in the third bench. As the temporary roads o in the lower bench attain a length equal to the advance between the cross-roads c, they are cut off by a new cross-road in the lowest bench of the seam. At the same time, the corresponding temporary roads o in each of the benches above and which are in the

same line with the roads o in the lowest bench reach their limits. The roof of the cross-roads c in the lowest bench is then ripped or taken down and the road is packed with waste on which a new cross-road c' is laid in the bench above. In like manner, the roof of each cross-road c' in the second bench is taken down, and the old road is packed with waste on which the new cross-road is laid in the bench above, as before. The main roads on each side are treated in the same manner, the road being graded from the cross-road in the lower bench to the cross-road in the upper bench, as indicated in the cross-section by the dotted line across the gob and marked road. This inclined road advances with the work in each bench.

Longwall in Inclined Thick Seams.-In the working of inclined thick seams, when the inelination of the seam is moderate, the method just described may be used for the removal of the coal. In steeper inclinations, a slope road is driven in the lower bench of the coal on the floor of the seam, and gangways, or levels, are driven to the right and left in the seam from this slope. Crossdrifts are then driven from these gangways across the seam to the roof rock, at intervals varying from 16 to 20 yd. At the roof, they are holed across from one to the other, and the coal drawn back, on the retreating method, in horizontal slices from 5 to 6 ft. in thickness. The face is also broken into steps between the cross-drifts, and as each slice of coal is taken out, it is often necessary to fill in the space with waste.

Longwall in Contiguous Seams. In the application of longwall to contiguous seams, the methods of work at the face do not differ from those already described. Seams have been worked in England as one seam when separated by a slate parting 7 ft. in thickness. In this particular case, the lower seam was 7 ft., and the upper seam 2 ft., in thickness. A general method for work

ing contiguous seams is shown in Fig. 4, which is a cross-section of the working face. The lower seam a is worked first, the slate parting b being supported on three rows of props c, d, e set parallel to the face. By the withdrawal of the rear row of props e, the slate parting falls or is wedged down. The work of taking down the upper seam f then follows. The slate parting, as it falls, is broken up and leveled off, thus forming a convenient flat g at the top of the incline i. The roof above the incline is supported by pack walls j.

When contiguous seams are worked separately by the longwall method, either by driving cross-tunnels between the different seams, or by separate slopes or inclines, the longwall face in an overlying seam should generally be kept in advance of that in a lower one, as the working of an underlying seam by longwall will usually result in the crushing and crevicing of overlying seams to a certain extent.

DETAILS OF LONGWALL WORKING

Starting Longwall.-There are two methods of starting longwall workings. In the first, the work of extraction begins at the shaft itself, the coal being taken out all around and its place filled with solid packs, leaving only space for the roadways. In the second method, a pillar of solid coal is left to support the shaft, cut only by the roadways. The longwall work is then started uniformly all around this pillar. Great care is needed in building the first pack

walls around the shaft pillar, to see that they are solidly built and well rammed, in order to break the roof over the coal. The system will not work rightly, however, until the breast has been advanced some distance from the pillar, so as to secure the benefit from the weighting action of the roof upon the coal face. The mining will be more difficult in the start, and in some exceptional cases it may even be necessary to place some light shots; this, however, should be avoided, if possible.

Roadways. The general plan of laying out the roads is shown on pages 652, 655, and 656. The temporary roads connect the working places, or rooms, with the cross-roads, but as they are cut off from time to time by other cross-roads they are not protected by as substantial pack walls as the other roads. The length of the temporary roads may be different for each section of the mine, and must be determined in each case as the work progresses. A cross-road is started off the main road whenever the temporary road gives signs of closing. It will thus be seen that the distance between cross-roads, measured on the main entries, may not be a uniform distance for the different sections of the mine, and may even vary in the same section. Again, owing to a creep or crush closing some rooms, it may be necessary to turn a short stub road directly across the heads of such rooms or working places, the old roads in this case being gobbed tightly to counteract the effect of the squeeze. The distance between the temporary roads is decided mainly by the possibility of taking the cars along the face, which, in turn, depends on the clear width it is possible to keep open between the face of the coal and packs. When it can be done, roadways are laid along the face and protected by timbers. At the junction of the track along the face with the track running into the room, a turntable consisting of an iron plate that allows the car to be turned is sometimes used. This track may be made of oak mine rails spiked to cross-ties, or of light iron rails, held together by spreaders of "X1" strap iron.

The

If the mine car cannot be taken along the face and there is a hard, smooth bottom, the coal is often loaded on a sled, or buggy, which is dragged along the face to the head of the temporary road, where it is loaded into a car. distance between the temporary roads will then depend on the distance to which the coal can be thus conveniently carried, but is usually from 40 to 60 yd., though it has reached 100 yd. in exceptional cases.

When it is necessary to shovel the coal to the road head where it is loaded or to wheel it in barrows along the face, the roads are made from 15 to 20 yd apart, center to center.

In some mines a motor-driven conveyer laid along the face is used to carry the coal to one of the roads where it is loaded into mine cars. The conveyer

is usually of the trough type, is moved forwards bodily as the face is advanced, and works well where the face is reasonably straight.

Control of Roof Pressure. The removal of the coal and the slow advance of the working face is followed by a slow, but irresistible, downward movement of the cover or overburden, known as settlement. The immediate effect of this is to produce breaks in the roof strata over the area from which the coal has been taken, more or less parallel to the working face, and, consequently at right angles to the line of advance. The effect of removing the coal is to divide the overburden into two portions:

1. An underweight of broken material from 10 to 40 ft. thick which may be likened to heavy falls of the rocks immediately overlying the seam in roomand-pillar work. The weight of this broken material is small, compared with the overweight, and it may be temporarily supported by the timbers and by the face of the coal and the packs. By the withdrawal of the timber next the packs, the weight is thrown, or settles, forwards on the coal and breaks it. The amount of the underweight thrown on the coal face is controlled, as far as possible, by the posts set parallel to the face. The amount of timber, number of rows, and the distances apart of the rows and of the timbers in each row depend on the conditions at the face. For given conditions of roof and floor, more weight is thrown on the face of the coal by decreasing the amount of timber, while increasing the timber decreases the weight on the coal. With a hard roof and floor, the posts should be set on some soft material, or be provided with a thick soft cap that will yield and allow the post to take the weight gradually, or the post is sometimes tapered at the end for the same purpose. 2. An overweight due to the weight of the rocks from the top of that portion of them causing the underweight to the surface, and which is practically equal to the weight of the entire overburden. It is irresistible and its effect is to compress the pack walls and gob into spaces between the walls until the resistance offered to the weight is practically equal to that of the coal that has

been removed. The amount of settlement due to this overweight is regulated mainly by the proportion of pack walls to the entire area from which the coal has been removed and by the manner of building these pack wails.

The aim in longwall work is to so control the roof pressure that it may be just sufficient to break the coal from the face and yet not crush it. When this pressure can be controlled, it is done by means of the number and size of the pack walls, by rows of props placed parallel to the face, and by varying the open space left along the face between the coal and the pack walls. Experience under the given conditions alone determines in just what way and to what extent this control may be secured. The pack walls provide for a gradual and uniform settlement of the roof over the entire area from which the coal has been removed. They relieve the timbers at the face of a great part of the weight they would otherwise have to carry and permit the weight on the coal face to be regulated largely by means of timbers so that the coal may be properly broken and not crushed.

Excessive weight on the face of the coal is shown by increased hardness of the under clay and the increased difficulty of undercutting in it, and also by the crushing and nipping of the coal. This excessive weight may be due to too small packs and too little timbering, or to the attempt to carry too wide an area between the packs and the face. The remedy is to increase the amount of timber and the size or number of packs, or to decrease the distance between the face and the packs.

Too small a weight on the face of the coal is evidenced by the slow breaking of the coal. This may be due to too large a proportion of packs, to too many timbers, or to too narrow a space between the face of the coal and the packs. The remedy is to decrease the proportion of packs or the amount of timber, or to increase the open width at the face.

Building Pack Walls and Stowing.-Pack walls should be built large enough at first and kept well up to the face, to prevent the weight coming upon the timber and also to permit the roof to settle rapidly when the timber is taken out of the face. Often the roof will not stand this second movement without breaking, and possibly closing in the entire face. The face should therefore be kept in shape, and just as soon as there is room for a prop or chock, it should be put in immediately, and the pack walls likewise should be extended after each cut or web is loaded out.

As a general thing, the pack walls in the gob are not so wide as the roadside ones, particularly when the seam produces enough waste material to stow the marches, cundies, or gobs, between these pack walls. Usually about 50% of the cubic contents of the solid seam taken out will stow the spaces between the pack walls in thick pitching seams, where the entire gob must be completely filled or nearly so. No waste material, except such as will hasten spontaneous combustion, should be taken out of the mine to the surface.

Timbering a Longwall Face.-The method of timbering the working face depends on the nature of the roof, floor, coal, etc. The action of the roof on the coal face is regulated almost entirely by timber; consequently, when the coal is of such a nature as to require little weight to make it mine easily, the roof must be timbered with rows of chocks and, if necessary, a few props.

The ends of all stone packs nearest the face of the coal should be in line, and the ends of these Timbers set at

pack walls should form a line parallel to the face of the coal. equal distances and in line along a longwall face are much more efficient in supporting the roof than irregularly set timbers. The accompanying figure shows the proper way of locating the pack walls and the face timber.

EXPLOSIVES AND BLASTING

CLASSIFICATION OF EXPLOSIVES

The characteristics of a good blasting explosive are: (1) Sufficient stability or difficulty of detonating by mechanical shock, and strength; (2) convenience in form and safety in handling; (3) absence of injurious effects upon the user. Explosives are of two general classes: low explosives, or those discharged by fire, and high explosives, or those that require a detonator.

Low Explosives.-Ordinary gunpowder and black, or blasting, powder are low explosives. Blasting powder consists of 73 parts of Chili saltpeter (nitrate of soda, NaNO3), 16 parts of charcoal, and 11 parts of sulphur. Blasting powder is graded according to the size of the grains passing through and over various sizes of round holes in sieves. In low explosives, the force of shattering effect is produced by the rapid combination of the oxygen of the saltpeter with the carbon of the charcoal; the sulphur is added merely to make the mixture more easily ignitable.

SIZES OF GRAINS OF BLACK BLASTING POWDER

High Explosives. High explosives, which are very commonly called dynamites regardless of their composition, usually consist of a base or absorbent known as dope, which may or may not be an aid to the explosive, and an explosive proper. If the active agent is a liquid, as it is in the case of true dynamites, the dope is added to hold or absorb the explosive; if the active agent is a solid, the base is added to reduce the temperature of the gaseous products of combustion or to shorten the length of the flame produced by their burning, and, in general, to render their use safer. The high explosives generally derive their names from those of their active principle.

Nitroglycerin is prepared by slowly running glycerin into a mixture of concentrated nitric and sulphuric acids, the bath being stirred and kept cool during the process of mixing. It is a dense, oily liquid, white when pure but of a yellowish cast as found on the market. It is very poisonous, not only when taken through the mouth but also through the inhalation of its fumes, producing violent headaches which, however, tend to diminish in intensity after repeated exposures. Nitroglycerin may freeze after some exposure to a temperature of 52° F. It explodes when confined at 360° F. It takes fire at 306° F. and, if unconfined, usually burns harmlessly unless in large quantities, so that a part of it, before coming in contact with the air, becomes heated to the exploding point.

Picric acid, which forms the basis of many explosives, is made by treating carbolic acid (derived from coal tar) with concentrated nitric acid.

Ammonium nitrate is a salt similar to sodium and potassium nitrate and is largely used in the manufacture of a class of explosives known as ammonium (or ammonia) nitrate powders, ammonia dynamite, etc. It has been used in the United States for nearly 30 yr. in the manufacture of some dynamites, taking the place of sodium nitrate over which it has the advantage that upon exploding it goes completely into gases. It is also largely used here and abroad in the making of the so-called permissible, or permitted, powder for use in gaseous and dusty coal mines, for the reason that upon exploding it forms large amounts of water, and this lowers the temperature of all the products of combustion.