1170 The Galvanic Battery. [No. 108. 



insertion of a circle, composed of a single piece of copper and zinc, 

 each 2" by 1', into a glass of very slightly acidulated water, through the 

 wire, the needle was immediately affected. Sometimes it merely 

 trembled violently on its pivot ; at other times its deflection was con- 

 siderable, and it never failed to indicate the passing of a current when 

 the circuit was complete. It is only therefore necessary to make the 

 apparatus to be tested a part of the circuit, and if it does not interrupt 

 the circulation of the current, the same appearances will be observed. 



The cylinder is lowered to its position near the wreck to be de- 

 stroyed by means of rope slings and guys. It is usually slung under 

 the bows of a vessel, having davits or fixed pulleys, through which the 

 suspension ropes are led. At the time appointed for lowering the 

 cylinder, this vessel is moored directly over the wreck, the bows being 

 just over the spot destined for the cylinder, and the lowering is 

 effected by gradually allowing the suspenders to glide over the davits 

 till it is felt that the bottom of the river has been reached. A very 

 simple and ingenious plan for insuring the descent of the cylinder in a 

 horizontal position has been suggested by Capt. Bowman, and consists 

 in marking the two suspending ropes at intervals of a foot or eighteen 

 inches apart. By noting these marks, which are alike at corresponding 

 distances in each suspender, the position of the cylinder can imme- 

 diately be seen, and if one end is lower than the other, the proper 

 correction can be made. 



On connecting the main conductors with the priming wires, the 

 greatest possible care must be taken to ensure perfect contact through- 

 out the junctions. Over each junction a piece of wax cloth or canvas 

 should be wrapped, so as to prevent any contact of the conducting 

 wires at those points, and it is sometimes advisable to lash a rough 

 wooden case over the whole of this part of the apparatus. 



Section VII. — On the Theory of the Galvanic Battery. 

 The first step towards the establishment of Galvanism as a branch 

 of physical science, was made by Galvani, Professor of Anatomy at Bo- 

 logna, in the year 1790. The accidental observation of certain muscu- 

 lar contractions in the limbs of a frog lying in the immediate vicinity 



