about 5 feet thick, which is being mined by the same company but from a different colliery. The presence of this upper bed made necessary either the leaving of enough support to hold the roof, or the mining of the coal in such a way that the roof would be lowered gradually to prevent the formation of cracks or squeezes that would break through to this upper bed.

Mining methods.—When the bed was first opened the company tried to mine it by the standard room-and-pillar method. Headings were driven 12 feet wide, and enough rock was lifted or shot down, preferably lifted, to make them 7 feet in the clear. The same procedure was followed for roadways in the rooms. As a result 4y2 feet of rock had to be handled for 2y2 feet of coal. This method of mining proved too expensive, not only because of the large amount of rock to be handled but also because of the large pillars that had to be left to support the upper workings.

The company then decided to attempt mining the bed by a longwall system, using conveyors to carry the coal from the working face to the heading and allowing the roof to settle slowly and gradually. As in the first plan adopted, the clearance between the heading of one pair of entries and the airway of another was 220 feet. Enough top or bottom was taken to make both entry and air course 7 feet high in the clear, but a spacing of only 220 feet between headings was not found profitable, so the distance was increased to 440 feet. This spacing permits a saving of approximately one-half of the narrow work and allows the large pillars to be worked on both advance and retreat. Furthermore, the bottom in the airway was left in place instead of being removed. A chain pillar 12 feet square was formed, with 20 feet between pillars. These openings were well bratticed.

Between the main heading and the first cut advancing and the last cut retreating, a pillar 60 feet thick was left, then preparations for the first cut were made by driving a doghole 45 feet long and 14 feet wide. As the airway is 12 feet wide and the chain pillar 12 feet thick, the doghole has to be driven in only 21 feet when driven from the airway but 45 feet when driven from the heading. The coal from the doghole is removed to the heading by hand.

As soon as the doghole is driven in 45 feet a shortwall mining machine is taken in and the coal undercut. The front end and take-up end of a conveyor are then placed in the doghole, 4 feet of the conveyor being allowed to project into the heading over the track. As the doghole is driven in, the conveyor is extended by pulling the take-up end 16 feet forward with the coal cutter and then inserting a 16-foot section of the conveyor. Each section of the conveyor consists of sixty-one to sixty-three 6-inch links. The These links travel in a steel trough, the top of which is 10 inches above the floor of the room.

As soon as the doghole has been driven in 222 feet (or a shorter or longer distance, depending on the character of the bed), the coal cutter makes a cut along the inside rib from the end of the doghole to the airway and then cuts back on the other rib to the end of the

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PlOUBB 21.—Longwall face in Orchard bed at Dodge mine of Baker
colliery. Circles represent props, black crosses cogs

doghole, as shown in Figure 21. The cutter thus makes two cuts without the conveyor being moved. The doghole is completed in three days of three shifts each.

Operating data.—As soon as the cutter has finished, the cut laborer loads out the coal. Each laborer is allotted a certain amount to load, this amount depending on what a man can do in a day's time. One man is usually required to load all the coal in a section of the conveyor—that is, he loads the coal for a 16-foot length of conveyor.

These links travel in a steel trough, the top of which is 10 inches above the floor of the room.

As soon as the doghole has been driven in 222 feet (or a shorter or longer distance, depending on the character of the bed), the coal cutter makes a cut along the inside rib from the end of the doghole to the airway and then cuts back on the other rib to the end of the

doghole, as shown in Figure 21. The cutter thus makes two cuts without the conveyor being moved. The doghole is completed in three days of three shifts each.

Operating data.—As soon as the cutter has finished, the cut laborer loads out the coal. Each laborer is allotted a certain amount to load, this amount depending on what a man can do in a day's time. One man is usually required to load all the coal in a section of the conveyor—that is, he loads the coal for a 16-foot length of conveyor.