]160 The Galvanic Battery. [No. 108. 



military mining, where the charges are carefully calculated to produce 

 certain definite effects, the limit of danger may generally be known, or 

 at least easily determined, but where large sur-charges are employ- 

 ed, experience is still required to shew accurately the extent to which 

 their effects will reach. On this point Capt. Fitzgerald, in his report 

 on the operations for destroying the '' Equitable," remarks, *'the limit 

 of actual danger with a charge of 2050 lbs. of powder, in a depth of 

 thirty feet of water, may from such experience as this single instance 

 affords, be calculated as something beyond 120 feet. At 200 feet it is 

 conceived that a person in a substantial boat would be perfectly safe, 

 alike from the effects of the waves and the fragments of the wreck ;" 

 but should the charge be fired directly from a boat, he recommends 

 *^ that for charges of the above description, the main conductors should 

 not be less than 250 feet in length." These remarks must of course be 

 received with the caution required by the limited experience on which 

 they are founded, but they will furnish some standard to which, under 

 similar circumstances, reference can be made. The depth of water most 

 materially influences the limit of danger, which rapidly diminishes as 

 that increases. In Colonel Pasley's operations at Spithead, when the 

 depth was about ninety feet, the limit of danger appears to have been 

 scarcely beyond fifty or sixty feet, although the charges ranged between 

 2000 and 2500 lbs., and instead of a lofty column of water being 

 thrown up, the elevation of the surface over the charge appears to have 

 been but slight, and the visible disturbance comparatively trifling. 



In long conductors, it is impossible to have the wires continuous 

 throughout, and the proper formation of the junctions is of essential 

 consequence to their efficiency. The ends of the wires to be connect- 

 ed should either be strongly brazed together, or if this may be impracti- 

 cable, they should be twisted together in the smallest possible twists 

 for a length of at least six inches. A few turns, or imperfect contact, 

 should never be considered sufficient, as such connections diminish the 

 igniting power of a battery most seriously, while, on the other hand, 

 well made junctions do not perceptibly affect it. To prove this, a thin 

 wire, l-20th of an inch in diameter, and 100 feet in length, was taken, and 

 the minimum number of cells required to ignite dry saltpetre cloth as- 

 certained. When the experiments commenced, five junctions existed 

 in the conductor itself, while there were two more at the poles of the 



