56 THOMAS COMMERFORD MARTIN 



the employment of dangerously high pressures; and in the 

 third place, there is objection to employing underground 

 an open-flame lamp, such as the ordinary arc lamp has always 

 been. Hence, while a large number of mining plants utihze 

 arc lamps, these are to be found above ground, while the 

 lighting below the surface depends upon incandescents. The 

 advantages claimed for incandescent electric lighting are: 

 The flexibility of the system, making it possible to move 

 circuits readily from one part of the mine to another, the ab- 

 sence of fumes and smoke, less danger of fire, decreased cost, 

 generally better lighting, and reliability of the light under all 

 the variations of temperature and barometric pressure. 



It follows that the electric lighting in mines is usually 

 of a composite type, the arc lamp being used at the surface 

 in buildings, yards, sidings, outworks, etc., while the incan- 

 descent lamp is used in the mains, levels, tunnels, etc. At 

 one time, in order to accomplish this dual purpose, it was 

 necessary to install two types of generating dynamos — one 

 for the high pressure arc Hghting and the other for the low 

 pressure incandescent lighting; but the later developments 

 and improvements in arc lights have made it possible to oper- 

 ate them on the same circuit as the incandescents and in 

 conjunction with motors, and hence recent years have seen 

 the installation of standard types of direct current dynamos 

 for all services, operating at voltages of 100, 220, 550 volts, etc. 



One feature of electric lighting worthy of note, but not 

 of great importance, is the use of portable miners' lamps. 

 A great many efforts have been made in this direction with 

 electricity, the lamps depending for their supply of current 

 upon either primary or storage batteries. In either case, 

 the drawbacks have been the weight, delicacy, high cost, and 

 uncertainty of the apparatus, and the fact that such lamps 

 are not usually of the safety type by which the presence of 

 choke damp or other dangerous gases is revealed. In con- 

 nection with these portable electric lamps for miners, small 

 bulbs and filaments are used, so as to reduce the consump- 

 tion of current to a minimum, thus making the batteries 

 last longer. 



