7.—They should not be thrown away on boats.
Notes.—As they can in all cases be fired by will, even when circuit closers are used, this rule is easily observed. But to prevent an enemy's boats from rendering the mines useless, a line of small torpedoes might be placed in advance of the large ones, or the circuit closers themselves might be charged.
At night, or in foggy weather it will be necessary to employ guard-boats, electric lights, &c., to protect them against damage by an enemy's boats, &c.
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.