1840.] The Galvanic Battery. 1169 



brought home. Over all a disc of sheet lead, must be soldered, k. and 

 the arrangements of the cylinder, with the exception of the loading, 

 are then complete. The latter is to be effected by setting the cylin- 

 der on one end, and pouring in the powder through an aperture, 

 about an inch in diameter, in the other. This aperture must after- 

 wards be filled by a wooden plug, and all the stray grains of powder 

 being carefully removed, a piece of sheet lead must be soldered over it. 

 It is an object of some importance to those conducting sub-aqueous 

 operations, to be enabled at any time to assure themselves that the 

 interior circuit of the wires is complete, without being obliged to with- 

 draw the priming apparatus itself. Col. Pasley recommends that this 

 should be done by introducing a portion of slightly acidulated water 

 into the circuit, and noting whether decomposition occurs ; but this is a 

 most dangerous plan, and ought never to be adopted. The decomposition 

 of a single grain of water, according to Faraday's recent researches, 

 requires a current of electricity sufficiently strong to keep a platinum 

 wire 1- 104th of an inch in diameter and eight inches* long, at a dull red 

 heat as long as the decomposition is in progress, the quantity of elec- 

 tricity maintained undiminished. The platinum wire we employ is 

 considerably thicker than the preceding, but still the risk of premature 

 explosion by Colonel Pasley's plan is very great. The danger is removed 

 by employing a very weak galvanic circle and a galvanometer, but as 

 the latter can seldom be met with in this country in its perfect form, 

 I may be permitted, before concluding this section, to describe a simple 

 substitute for it, employed during the operations against the "Equitable" 

 [Fig. XII.] safely and successfully. A piece of copper wire about 

 l-12th of an inch in diameter, and fourteen feet long, was coiled on a 

 rectangular wooden frame- work, a, 6" long, 3" broad, and 1" deep, 

 care being taken to preserve the metallic coils throughout from mutual 

 contact, h. The magnetic needle of a small theodolite was then 

 mounted on the point of a common needle, fixed in a thin wooden 

 stand. On placing this within the frame-work, with the coils passing 

 above and beneath it, and directing a galvanic current excited by the 



* The above length is stated merely to give some definite idea of the danger incur- 

 red ; but Faraday states that if the v^ire were a hundred or a thousand inches in length, 

 and the cooling circumstances alike, the same effect would be produced as if it were 

 no more than half an inch. 



