NOTE.--Mechanical submarine mines should never be used under these circumstances, as the difficulties of mooring them and keeping them in position would be very considerable, also a vessel being sunk in a very deep channel would not necessarily block it, and as a mechanical mine cannot be replaced, a gap would be left in the defence. 2.--They should be placed in the narrowest parts of the channel. NOTE.--The object of this rule is evident, fewer mines being required, and consequently in the case of electrical ones, a far less number of wires are needed, which gives an increase of simplicity, and consequently more effectiveness. This point should be observed in connection with mechanical, as well as electrical submarine mines. 3.--They should where practicable be moored on the ground. NOTE.--The advantages attendant on an observance of this rule are:-- _a._--Increased vertical effect. _b._--Avoidance of mooring difficulties. _c._--Less liability of shifting from its original position. _d._--Less chance of its being discovered and rendered useless by an enemy. _e._--By far heavier charges may be conveniently employed. 4.--Where possible, no indication whatever should be given of the position of the mines by their circuit closers, or in the case of small buoyant ones, by the mines themselves. NOTE.--In some instances this will be almost impracticable, as for example, where there is a very great rise and fall of tide. For instance, at Noel Bay in the Bay of Fundy, the rise is over fifty feet. Here, when circuit closers, or small buoyant mines are used, both of which ought never to be more than twenty feet below the surface, long before low water they would be found floating on the surface in full view. Many attempts have been made to overcome this difficulty, but as yet no really practicable means have been devised. 5.--The stations where the firing batteries, &c., are placed, should be in the defensive work likely to be held the longest, thus enabling the mines to be commanded up to the last moment. 6.--The electric cables should be laid in positions such that their discovery by the enemy would be extremely difficult, and almost impossible. NOTE.--This may be to a certain extent effected by leading them from the mines to the firing and observing stations by circuitous routes, and by burying them in trenches. 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.