the coal when shot tends to be thrown toward -room roadways, thus lessening to some extent the amount of coal that a miner has to handle in these very thin beds. A permissible explosive is used to shoot the coal, and in a 60-foot room six to eight holes are necessary to loosen the coal. Ordinarily one 25-pound box of explosives lasts one miner two weeks.

One machine can easily undercut two 60-foot places, and some machine runners average as high as 200 feet of face cutting a day when they do not have to move too far between cuts.

In driving the headings the rock is gobbed on the chain-pillar side of the gangway, and the crosscuts are driven to the full height of the roadway.

The mine cars in the colliery have the following inside dimensions: Depth, 1 foot 6 inches; width, 4 feet iy2 inches; and length, 9 feet 5 inches. Their capacity is therefore 63 cubic feet. Their over-all length is 11 feet. Three 8-ton cable-and-reel electric locomotives are used to handle the coal from this bed. In 1925 the company had five mining machines, four of which were worked and one was held in reserve. Two more machines of the same type had been ordered.

During November, 1923, there were 50 working places and one man to a place. The 50 men employed mined 4,716 tons of coal in 24 working-days, a daily rate of 3.9 tons of coal per miner. The company has approximately the following employees in this district, exclusive of company men used from time to time: 1 assistant foreman, 50 miners, 4 machine runners, 4 machine helpers, 3 motormen, 3 brakemen, and 2 trackmen, a total of 67. This system of mining has allowed coal to be produced at a profit.

SEMIIONGWALL MINING OF A THIN BED, WITH UNDEBCTJTTEBS AND CONVEYOKS AT THE FACE

One problem in the mining of thin coal beds is that of haulage from the face to the gangway. Brushing the roof to allow a mine car to enter the working place is expensive, therefore it is very desirable to move the coal from the face without having to resort to brushing. For a number of years at various places in the anthracite region drag scrapers have been employed to remove the coal from the room or from the face of the longwall. A few other methods of handling coal in working places have already been described; the use of traveling conveyors is still another method.

Bed mined.—A conveyor system has been in service for a number of years at the Dodge mine of the Baker colliery, Glen Alden Coal Co., Scranton, Pa. At this colliery the Orchard bed is 16 to 34 inches thick, it generally is flat, and the cover over the bed is 80 to 150

about 5 feet thick, which is being rained 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 4i^ 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 1 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 pillaf 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