46 THOMAS COMMERFORD MARTIN 



being transmitted from the motor to tl)e drill by means of a 

 telescopic shaft and machine cut bevel gears. The telescopic 

 shaft consists of a steel tube in which slide two solid shafts, 

 each of which is fitted with a universal joint, and is fastened 

 to the motor and to the drill machine by means of an auto- 

 matic coupling. 



A number of electric percussion drills, striking a percus- 

 sive or hammer blow, are in use. They are of various sizes. 

 In some of them the power is transmitted from the motor to 

 the drill by a flexible shaft, which may be several feet in 

 length. In one form, equipped with a self contained motor, 

 the hammer proper is operated by a pair of eccentrics on 

 a shaft connected through simple gearing to the armature 

 of the motor. The raising of the hammer is effected dur- 

 ing a three quarter revolution of the eccentric shaft, and the 

 blow is struck during the remaining quarter. A very power- 

 ful hammer blow is thus secured, at the rate of about 400 to 

 500 l:»lows per minute. 



The electric method of shot firing in mines has reached 

 considerable development of late years, and is now in general 

 use. There are two methods of electric blasting or shot firing 

 — known as the high tension and the low tension — in ordinary 

 use at the present time. In the high tension method the 

 explosion is caused by a spark which is made to jump between 

 two points inside the detonator. The current for this spark 

 is created by what is known as a magneto machine — an ar- 

 mature revolving rapidly in front of a set of permanent mag- 

 nets, the whole mechanism being inclosed in a small box from 

 which the handle attached to the armature extends. This 

 box is portable and can be set down anywhere, and the wires 

 from it can be carried a considerable distance. In the low 

 tension method there are emploj^ed similar magnetos of low 

 tension, or chemical batteries. In this method the two wires 

 which extend into the charge are connected, or bridged, at 

 their ends within the priming with a short piece of fine plat- 

 inum or similar wire. This wire offers considerable resist- 

 ance to the passage of even a small current, so that it rapidly 

 becomes heated to incandescence and thus ignites the prim- 

 ing, which in turn explodes the detonator charge. 



