In the foregoing pages of this chapter will be found the requirements and conditions essential to a perfect system of electrical submarine mines for the defence of a harbour, river, &c.; in the following pages a general description of the component parts of such defensive torpedoes, under the following heads--Form and Construction of Case; Electrical Fuzes; Electric Cables; Watertight Joints; Junction Boxes; and Mode of Mooring, will be considered. _Form and Construction of Torpedo Case._--The case of a submarine mine should be capable of fulfilling the following conditions:-- 1. It must be able at great depths to withstand a great pressure of water, and remain perfectly watertight. NOTE.--This in the case of a charge of gunpowder being an imperative necessity. 2. As a buoyant mine, it must be capable of affording a considerable excess of buoyancy, by which it may be rendered stationary when moored. NOTE.--This is generally obtained by having an air space within the torpedo, thus requiring a much larger case in which the charge is enclosed than would otherwise be necessary, causing increased difficulties in transportation, mooring, and raising them for examination, &c. 3. When explosive agents which require a certain time for thorough combustion are used as the charge, such as gunpowder, picric powder, gun-cotton (not fired by detonation), &c., a much stronger case is necessary to obtain the full explosive effect than would be the case were detonated charges, under the same conditions, employed. NOTE.--This is an extremely important point, for if a weak case is employed with a charge of gunpowder, &c., fired by a fuze primed with powder only, a portion of it on being fired would generate a sufficient quantity of gas to burst the case, thus blowing out the remainder of the charge before its ignition had been effected. 4. It should be of such a form that the complete ignition of the charge is obtained by the employment of the least number of fuzes possible to effect this result. NOTE.--This point is especially to be observed when gunpowder is the explosive agent. The various forms of defensive torpedo cases may be classed under the following heads:-- 1.--Spherical shape. 2.--Cylindrical shape. 3.--Conical shape. _Spherical Shape._--This form of case is theoretically the very best one possible to devise, but on account of the difficulty of constructing it, and its comparative costliness, such a form may be put aside as being impracticable. _Cylindrical Shape._--Torpedoists in general have hitherto adopted the cylindrical form of case as being the best adaptable for both ground and buoyant mines containing a heavy charge.