The battery is the motive power of the machine, occupying the same relative position to it, as the boiler does to the locomotive; for, though it does not produce any immediate result on the works, yet the part it performs in the undertaking is essential. While travelling, Mr. Cooke found great inconvenience to result from the spilling of the acid solution used in Smee’s batteries; and, from this, he was led to consider if the substitution of fine white Shanklin sand, saturated with the diluted acid, would not avoid this difficulty. Experiments having confirmed the truth of his conjecture, the change was permanently arranged, and it was subsequently found so advantageous, that the same method was tried in the permanent batteries, and, in like manner, the result has proved satisfactory. At present, the generator resembles, in its principal features, the one known as Wollaston’s trough; and it is so arranged, that the series of plates of copper and amalgamated zinc, arranged for the evolution of the electric fluid, admit of being placed in a corresponding series of cells, filled with well-washed and dry sand. The _United Service Gazette_ states, that all that is necessary in order to use the instrument is, slightly to moisten the sand with diluted sulphuric acid. The conducting wires are, at their ends, of less diameter, and are so arranged as to form the coiled magnets. Those in the diagram are seen in connexion with the works; the electric current, taking the course indicated by the arrows, occasions the deflection of the needle. The following engraving represents the interior of the machine, and shows the means by which the magnet is connected with the electric current. The parts lettered _a_ are the key-shafts, which, on being turned to the right or left by a handle, pushes one of the springs, _c_, from its point of contact, _d_, and, by changing the course of the electric current, produces a corresponding change in the position of the needle. [Illustration]