696 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v 



two ends are brought to a distance of one or two millimetres in a kind 

 of box, c D, which is filled which powder, after having covered the 

 points with fulminate of mercury. " The first trial on a large scale," 

 says M. Du Montcel, " of the application of Ruhmkorff's induction 

 apparatus to mines was made in 1853 by the Spanish Colonel Verdu, 

 in the workshops of M. Herkm'ann, a manufacturer of gutta-percha 

 covered wire at La Villette. Experiments were made successively 

 on lengths of wire of 400, 600, 1,000, 4,SOQ, 5,000, 6,400, 7,600, 

 25,000, 26,00'0 metres", and the success was in every case complete, 

 whether the circuit was composed of two wires, or the return current 

 was carried by the earth. Only two elements of Bunsen's battery 

 were employed iii these experiments." (Exposd des Applications de 

 VElectricite, t. iii.) 



In order to explode large mines that is to say, chambers filled 

 with hundreds or thousands of kilogrammes of powder in several 

 cavities in communication with each other, so as to obtain their nearly 

 simultaneous explosion a commutator is used, the handle of which is 

 successively put into contact with the copper .wires connected with 

 each chamber. The explosions thus take place one after the other, but 

 at such small intervals that they might be thought simultaneous. The 

 employment of electricity for blasting mines is not only advantageous 

 iii the matter of security, it effects also a considerable economy (as 

 much as 60 per cent.) on the old method of trains, by the ease with 

 which, by its means, gigantic mechanical effects due to simultaneous 

 explosion can be produced. In 1854, in the works for hollowing 

 put the basin for the port of Cherbourg, it required the explosion 

 of six mines to detach at one blow a mass Of rock of 50,000 cubic 

 metres. 



The following is an exploding apparatus whose calorific power is 

 due to the development of induced currents and of the extra magneto- 

 electric current. Its invention is due to M. Breguet. An electro- 

 magnet has its poles opposite those of a powerful compound horse- 

 shoe magnet, so arranged as to have their poles turned in opposite 

 directions. The result of this, in the horse-shoe of the electro- 

 magnet, is a magnetisation which is made stronger by means of a 

 fixed armature. In front of this is a piece of soft iron, kept in 

 contact with the armature by an antagonistic spring, and which 

 can be separated suddenly by the rapid motion of the button 



