CHAP, viii.] THE ELECTRIC LIGHT. 693 



Fig. 446. A capillary tube twisted into a spiral is placed in a 

 glass cylinder ; two platinum wires communicating with the bobbin 

 are cemented to its two extremities, between which the successive 

 discharges take place, as in Geisler's tubes. The lamp is attached 

 to a box containing the induction apparatus and the battery. 

 (Fig. '447.) 



This system of illumination preserves the miners from all 

 danger. In fact the luminous beam is produced in a vacuum 

 without any communication with the air contained in the glass 

 cylinder, and much less with the air of the mine, besides which, 

 if the apparatus is broken, the entrance of the air immediately 

 destroys the spark, and with the extinction of the light all danger 

 of fire disappears. 



III. BLASTING IN MINES. TORPEDOES; 



The blasting of chambers in mines by the old methods is often 

 a dangerous operation, and the accidents caused by it from 

 time to time are unhappily too serious for us not to attempt to 

 prevent them. In order to set fire to the powder inclosed in the 

 chambers the process was as follows : Communication with the 

 interior of the mine was effected by longer or shorter trains 

 of powder placed on the surface of the ground, or by canvas tubes 

 full of powder, technically called fuses. Then at the end of 

 the train was placed tinder lighted at the end, outside the mine, 

 the dimensions of this tinder being calculated so that the workmen 

 in charge of the operation might have time to get away. It was 

 useless to insist on the danger of too sudden a kindling; often 

 it was the delay of the kindling that caused the accidents, espe- 

 cially if several mines were blasted at the same time and it was 

 not known in which the explosion had not yet taken place ; or 

 lastly, if any trains were supposed to be gone out, when, in reality, 

 they were not. 



By making use of currents, and of the spark which is pro- 

 duced at the moment the circuit is closed at a distance, all danger 



