_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.