276 ELECTRIC LIGHTING. 



Everybody knows of the dangers miners are exposed to 

 when gas, issuing from the beds ofxoal, comes into contact 

 with the flame of a lamp. A dreadful explosion takes place, 

 and fires the whole drift. These sad accidents are called 

 explosions of fire-damp. Now as the electric light is inde- 

 pendent of a supply of air, for it can be produced in a 

 vacuum, the danger of fire-damp will obviously be avoided, 

 by enclosing each lamp in hermetically-sealed globes, placed 

 in the drifts where the miners are working. It will, however, 

 "be necessary to exhaust these globes, lest the dilation by heat 

 of the enclosed air should break them. There is then not 

 the least danger to be feared, for the luminous points are 

 thus completely separated from the external air. 



In order to avoid the considerable cost of setting up the 

 electric light, Dumas and Benoit conceived the idea of sub- 

 stituting the light of the inductive spark in a vacuum; they 

 therefore arranged the vacuum tube spirally, and placed it in 

 an outer tube, provided with copper fittings for suspending. 

 The exhaustion is made on Morren's gas, in order to obtain a 

 fine white light. I have spoken at length about this kind of 

 illuminating tubes in my account of Ruhmkorff's induction 

 apparatus (5th edition), and to this I refer the reader. 



The electric light produced by the Alliance machines was 

 successfully used in 1863 by Bazin for lighting the slate 

 quarries of Angers. A machine with 4 discs was capable of 

 lighting a gallery 60 metres long, by 50 metres wide and 

 40 metres high. The machine was near the opening of 

 the shaft, and the electric current was transmitted by wires 

 150 metres long. In spite of the diminution of intensity oc- 

 casioned by this great length of wire, the illumination proved 

 so satisfactory that the workmen expressed their delight by 

 loud applause. These advantageous results were confirmed 

 on several different occasions, and it was found that the 

 effective labour of the workmen was increased by a fifth or a 

 sixth a net advantage of 15 or 20 per cent, to add to the 

 comfort of the workmen, which it was desirable to secure 



