pitching from 15° to 45°, where the top is brittle, the coal free, and the mine gaseous. Rooms or breasts are turned off the gangway in pairs, at intervals of about 60 yd. The breasts are about 8 yd. wide, and the pillar between, which is about 5 yd. wide, is drawn back as soon as the breasts reach the airway near the level above. In the middle of each large pillar between the several pairs of breasts, chutes about 4 yd. wide are driven from the gangway up to the airway above. These are provided with a traveling way on one side, giving the miners free access to the workings. Small headings are driven in the bottom bench of coal, at right angles to these chutes, and about 10 or 20 yd. apart. These headings are continued on either side of the chutes until they intersect the breasts. When the chute and headings are finished, the work of getting the coal in the panel is begun by going to the end of the uppermost heading and widening it out on the rise side until the airway above is reached and a working face oblique to the heading is formed. This face is then drawn back to the chute in the middle of the panel. After the working face in the uppermost section has been drawn back some 10 or 12 yd., work in the next section below is begun, and so on down to the gangway, working the various sections in the descending order. Both sides of the pillar are worked similarly and at the same time toward the chute.

Small cars, or buggies, are used to convey the coal from the working faces along the headings to the chute, where it is run down to the gangway below

and loaded into the regular mine cars. This system affords a great degree of safety to the workmen, because whenever any signs of a fall of roof or coal occur, the men can reach the heading in a very few seconds and be perfectly safe. A great deal of narrow work must be done before any great quantity of coal can be produced. The breasts are driven in pairs and at intervals, to get a fair quantity of coal while the narrow work is being done, and they are not an essential part of the system. It is claimed that the facility and cheapness with which the coal can be mined, handled, and cleaned in the mine more than counterbalance the extra expense for the narrow work.

The advantages of the panel system are: A more complete control of the ventilating current is possible, and the ventilation in any panel may be altered as circumstances may require; the powder smoke from each panel goes directly into the return air-current and does not go throughout the mine; an explosion or a fire occurring in one panel is usually confined to that panel; creeps or squeezes are of rare occurrence, and are confined to the panel in which they occur; the output of coal is better regulated and more reliable. The disadvantage of the system is the expense of entry driving, and the delayed extraction of the coal within the panel until the driving of the main and cross-headings has been completed.

MINING AND BLASTING COAL

SHOOTING OFF THE SOLID

Coal may be broken down at the working face by blasting from the solid; by blasting after having undercut or sheared the seam; and by a combination of the methods.

The term solid shooting, or shooting off the solid, is used to describe a method of working in which the coal is blasted from a solid face without previous

shearing or undercutting. It is practically the only method used in mining anthracite, and is also much used in bituminous mining. The chief labor in the production of coal by this method is the drilling of the holes for the powder and the loading of the coal into mine cars. The holes are drilled with a churn drill or with a rotary drill worked either by hand or by electric or compressed-air power.

The location of the holes, the depth to which they must be drilled, and their direction depend on the nature of the coal. If the coal is compact and without cleat, as is the case with anthracite, the holes are placed as they would be for a rock face worked under similar conditions. If the coal has a cleat, advantage must be taken of this to produce the maximum effect of the shot and to prevent the shot seaming out. The best method of blasting a particular coal can only be learned by experience.

Drill holes must be so placed that the explosion of the charges will increase the number of free faces (loose or open ends) exposed to the action of subsequent blasts and thus reduce the amount of powder otherwise necessary to bring down the coal.

FIG. 1

Fig. 1 shows a common method of placing the shots in shooting off the solid used both for rooms and wide entries, where the coal is 5 to 7 ft. thick and is strong and close-grained and without cleats or partings which need be considered in the blasting. When firing with squib or fuse, the holes are exploded singly or in the numbered order, except 1 and 2, which must be fired simultaneously. When firing with electric detonators, the shots are fired in pairs; first 1 and 2; then 3 and 4; and, lastly, 5 and 6. The mutual reinforcing action of two charges when fired together is very noticeable in the case of holes pitching toward one another as 1 and 2, which may be placed much farther apart and will break down much more coal when fired together than when fired singly. Shots 1 and 2 are sometimes called breaking, or busting, shots, as they break out the center and thus give loose ends for the shots 3 and 4, which should take out the greater part of the coal. The shots 5 and 6 are placed about 10 in. from the ribs and are intended mainly to straighten the ribs; they are often inclined toward the rib. A cut from 4 to 6 ft. deep across the face should be taken out by such a round of holes.

If any shot does not blow out the entire face from top to bottom, it is necessary to mine out the bottom or top coal that is left in order to square up the face in preparation for the next round of shots. Occasionally, a short hole,

out a piece abc; shot 2 is placed about 10 in. from the rib to straighten the rib; shot 3 takes out the greater part of the center coal, while 4 and 5 act similarly to 1 and 2. After the straight face has thus been broken, the location of the subsequent shots is largely a matter of judgment, as so much depends on the conditions. No definite rules can be given, except that in solid coal the direction of the hole should be parallel to a free face if possible, though even this general rule will be greatly modified by cleats, partings, etc.

Fig. 4 shows approximately the appearance of the face after the shots shown in Fig. 3 have been fired. If there was a cleavage to the coal, a shot placed about as shown might blow out the piece of coal within the dotted line and thus provide two free faces with respect to which side shots could then be placed. If there was no cleavage and the coal was hard and solid, the shot would be placed nearer the previous shot 4.

Precautions in Solid Shooting.-Where a single center shot, which is also known as the opening shot, such as is shown by the treble dotted line in Fig. 4, is employed to make an additional free face, its angle of grip and its length should bear such a relation to the strength of the coal that not to exceed 2 lb. of black powder (provided permissible powder is not used) will be required to bring the coal. This is commonly accomplished in coals where there are no marked cleats by limiting the angle between the straight face and the hole to 35°, in which case a 5-ft. hole, which is of the maximum allowable length, would have a line of least resistance 2 ft. 10 in.

long to work against. Where the cleats are favorable, the angle of grip may be as much as, but should never be greater than, 45°.

Gripping shots (see under Explosives) should not be permitted, as they are very apt to blow out, or to blow off the heel, leaving the toe in place, in which case a portion of the powder may burn in the air. On the other hand, holes should not be so pitched as to give them too thin a toe as the shot may blow out at the back leaving the heel standing.

Shots that are all dead (as would be 5 in Fig. 3 if fired alone) or are partly dead, should be prohibited. No shots should be drilled directly into the face and no shot should be drilled to such a depth that any portion of the charge is beyond the point where a perpendicular dropped from the drill hole will not cut a free face.

As a rule, shot holes should be parallel to a free face, as shown in Fig. 4, and should pitch a few degrees from the horizontal so as to cut across the bedding planes. It is advisable to incline the holes slightly in the direction of the length of the room, so that they may cut across the vertical cleavage planes. Shots should not be placed in soft streaks of coal, or shale, or mother coal, as they may blow out along these lines of weakness.

Shots should be well balanced with toe and heel of equal width, which should never be more than 5 ft. in any case and never greater than the thickness of the seam when less than 5 ft. A hole 5 ft. long, working against a toe and heel each 4 ft. wide, will give excellent results in seams 4 ft. thick and over.

Straight holes of uniform diameter give better results than crooked ones, and generally short holes are to be preferred to long ones. To ensure the holes being of a uniform size best adapted to the coal, the drills should be frequently tested.

In

Shots that cannot do their work when fired separately but depend for their successful action on the results accomplished by another shot or shots fired at the same time, and which are, consequently, known as dependent (or follow) shots, should not be permitted and are prohibited by law in some states. Fig. 1, while shots 1 and 2 are strictly dependent shots, since either one fired alone would blow out, yet with proper judgment they may be safely used if fired at the same instant with an electric battery. The holes 3 and 4 are in every sense dependent as they are dead unless a free face has been made by firing 1 and 2. Similarly, 5 and 6 are dependent on the successful firing of 3 and 4. If fuse or squibs are used, 3 and 4 may explode a few seconds before the others, resulting in blown-out shots, to be followed by the detonation of 1

and 2 simultaneously which will do their work, the final explosion of 5 and 6 resulting in blown-out shots. The reason for the failure to explode at the same instant is due to the impossibility of securing either squibs or fuse that will burn at exactly the same rate. In electric firing with a blasting machine, holes 1 and 2 may be fired first, and then, after the smoke and dust have cleared away, 3 and 4 and 5 and 6 in separate pairs. If delay-action detonators are used, all the holes may be fired in sequence in pairs by a single application of the current by using no-delay caps in 1 and 2, first-delay caps in 3 and 4, and seconddelay caps in 5 and 6. However, one of the strongest arguments against solid shooting is that it is so entirely impossible to place the second of a series of drill holes until the results accomplished by the detonation of the first hole have been studied. Good practice, then, demands, that the center shot or shots (as 1 and 2, Fig. 1) be fired first, and that the other shots be placed where needed after the face has been examined.

Objections to Solid Shooting.-The objections to solid shooting are twofold: It increases the percentage of slack coal produced and is dangerous to the men and mine, particularly where inexperienced or careless workers using black powder, are allowed to drill, charge, and fire their own shots, when, where, and how they please.

From an economic standpoint, the objection to solid shooting is that the usual employment of excessive charges of powder in poorly placed holes always leads to the production of an excessive amount of slack. While this may be an advantage where the coal is coked, for ordinary commercial use the coal must be lumpy and as free from slack as possible.

The same causes that tend to produce an excessive amount of fine coal, also tend to the production of blown-out shots, and these, in turn, have been the cause of many mine explosions. These dangers are largely reduced if permissible powders are used and particularly so if shot firers and electric blasting are employed. They are reduced to the minimum if, in addition, the coal is undercut before blasting.

If the duties of the shot firer are limited to firing the holes that have been previously drilled, charged, and otherwise prepared by the miner, the possibility of damage to property is not reduced, and the danger to life is merely transferred from the miner to the shot firer. Under these conditions the shot firer is killed in event of accident and not all the underground workers.

A distinct advance toward safety is made if the shot firers charge as well as blast the holes previously drilled by the miner, and are required to refuse to fire any and all holes that are improperly placed. In theory, the method is perfect, but leads to many accidents in practice, as the shot firers, through mistaken friendship for the miner, often fire shots their better judgment must condemn.

The highest degree of safety is attained by employing an inspector who is independent of the miners to oversee the placing and drilling of all shot holes. The inspector not only instructs the miner where to place the holes but determines their pitch, depth, and the amount of charge as well. Before the miner leaves his place, the inspector examines it, and compels the drilling of other holes in place of those drilled contrary to his previously issued instructions. The inspector commonly places a marker or flag (a piece of paper) in the mouth of each hole to be fired, the charging and blasting being left to the shot firers.

Notwithstanding all precautions taken by inspecting the holes as described, the fatality rate among shot firers is needlessly high by reason of the failure of many of them to blast the holes properly. Having a certain number of holes to charge and fire in order to complete their shift, the work is commonly done in from one-half to one-third the time that should be devoted to it; and this leads to carelessness in charging, in going back on delayed shots, and has led to serious mine explosions through blown-out shots igniting the dust thrown into suspension through rapid firing. To protect the shot firers against themselves has arisen the custom of firing all the shots at one time by means of a current of electricity applied from some point outside the mine, and after all the men have left the workings. This practice is not without danger to the mine, through its possible wrecking by a dust explosion caused by the detonation at a single instant of many hundred pounds of high explosives in an atmosphere charged with dust.

BLASTING AFTER UNDERCUTTING

The object of mining or undermining the seam of coal previous to blasting is to secure the advantages of an additional free face. The mining may be made in the bottom of the seam or in the fireclay underlying it (undercutting),