THE MINERAL DEPTHS 



one in which the sawing is done by an endless chain, 

 the links of which are provided with a cutting blade. 

 These have been very generally replacing the 

 pressed-air or pick type of machine, and their 

 larity accounts largely for the enormous increase in 

 the use of coal-mining machinery during the past 

 decade. Thus in 1898 there were 2,622 coal-mining 

 machines in use in the United States. Four years 

 later this number had more than doubled, the increase 

 being due largely to the adoption of chain machines. 



Like electric locomotives, and for similar reasons, the 

 coal-cutting machines are low, broad, flat machines, 

 from eighteen to twenty-eight inches high. They 

 rest upon a flat shoeboard that can be moved easily 

 along the face of the coal. An ordinary machine 

 weighs in the neighborhood of a ton, and requires 

 two men to operate. The apparatus is described 

 briefly as follows: 



"On an outside frame, consisting of two steel channel 

 bars and two angle irons riveted to steel cross ties, 

 rests a sliding frame consisting of a heavy channel 

 or centre rail, to which is bolted the cutter head. The 

 cutter head is made entirely of two milled steel plates, 

 which bolt together, forming the front guide for the 

 cutter chain. This chain, which is made of solid cast 

 steel links connected by drop forge straps, is carried 

 around idlers or sprockets placed at each end of the 

 cutter head and along the chain guides at the side to 

 the rear of the machine, where it engages with and re- 

 ceives its power from a third sprocket, under the 

 motor. The electric motor, which is of ironclad 



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