1840.] The Galvanic Battery. 1163 



an apparatus would seldom, if ever, be necessary. It is in removing 

 obstructions to river navigation, where strong tides and currents are to 

 be contended with, making it essential that the conductors employed 

 should be reduced to their minimum length, that self-acting dis- 

 chargers may be employed to the greatest advantage. We are 

 indebted to Dr. W. B. O'Shaughnessy for an ingenious plan for such a 

 [Fig. VI.] discharger, of which the following are the details. The dis- 

 charger consists of two distinct parts, having distinct offices, one being 

 intended for completing the circuit, and effecting the ignition of the 

 powder, the other for breaking the circuit, should any accident delay the 

 explosion, and so rendering it perfectly safe to approach the Battery, and 

 if possible ascertain and remove the cause of failure. These two objects 

 [Fig. VI.] are effected by causing wires to pass, as shewn in diagram No. 6, 

 into four glass tubes partially filled with mercury. Over these, fixed in 

 a small wooden framework, is placed a watch, for the hands of which a 

 thin piece of sheet copper is substituted. This is fixed on the arbor 

 of the hands, and each extremity carries, suspended from a short arm, a 

 copper wire, bent like the letter U. The length of the legs designed 

 for completing the circuit is so regulated, that on the copper hand 

 being set to any specified number of minutes, they will not come in 

 contact with the mercury in the tubes till that time has elapsed. Mean- 

 while the legs of the other wire have been immersed in the mercury of 

 their tubes, but if the circuit is completed without ignition, then 

 the copper hand continuing to traverse the dial of the watch in four or 

 five minutes more lifts them out, breaks the metallic continuity of the 

 circuit, and thus effectully prevents all risk in approaching the Battery. 

 This form of discharger was successfully employed on the occasion of 

 the first explosion of the " Equitable" in the river Hooghly, but it was 

 then apparent that the utmost delicacy was requisite in making its 

 adjustment. This delicacy and minuteness of detail must always 

 prove serious defects in any form of apparatus employed in practical 

 operations, in which so many risks of derangement are incurred, and in 

 the case of the watch-discharger, the expense is an additional objection. 

 The idea of employing such an apparatus, having however been sug- 

 gested, there was comparatively little difficulty in designing a form of 

 it which should not be liable to the preceding objections, and one was 

 accordingly contrived, which was successfully employed throughout the 

 remaining operations for destroying the " Equitable." The principle of 



