44 THOMAS COMMERFORD MARTIN 



tain extent the principle of the chain breast machine, consists 

 of an endless chain operated along a narrow frame or arm ex- 

 tending from the side of the machine and so adjusted that it 

 can be operated at any angle up to a right angle. A change 

 in the angle of the cutting frame will increase or diminish the 

 thrust on the track; in the same way the machine can be made 

 to follow any irregularity of the face of the coal. The cutting 

 frame can be extended over the right or left side, so that as 

 the machine moves the cutting can be accomplished in both 

 directions along the coal face. The height of the machine is 

 only 18 inches, and its weight is only about 3,000 pounds, so 

 that it can be operated in very thin veins. It has a wide 

 range of cutting speed, and can be made to undercut to any 

 practical depth. In going through narrow places and also 

 in moving or changing the cutting bits, the cutting frame can 

 be swung out behind the machine instead of from the side. 

 Like the ordinary chain machine it is operated by two men, 

 and is said to be as available for room and pillar work as for 

 long wall mining. 



Shearing machines constitute another class of apparatus 

 of this nature. They are built on the general plan of the chain 

 undercutting machine. The cutter frame is located in a posi- 

 tion normal to that of the undercutting, the shaft in the 

 armature of the motor being parallel to the center rail. 



The application of electric drills to mining work has been 

 the subject of considerable study and experimentation for 

 many years past, but not until recently have these drills been 

 successful. Drilling is one of the few classes of operation in 

 which a reciprocating motion is preferable if not necessary. 

 The air drill has a reciprocating motion; the electric drill, on 

 the other hand, necessarily has a rotary motion, the effort to 

 develop electric power practically by reciprocating motion 

 having been abandoned at a very early stage in the develop- 

 ment of the electric motor. Moreover, the air drill has shown 

 itself very hardy and capable of withstanding unlimited rough 

 usage; whereas the electric drill has usually proved to be a 

 delicate and sensitive piece of mechanism, easily deranged 

 and expensive to repair. On the other hand, the degree of 

 efficiency of the air drill is relatively low, the electric motor 



