NOTE.--By detaching or connecting the firing battery, which is effected by means of a plug, key, &c., they may be respectively rendered harmless, or dangerous. Thus friendly ships may pass over them in safety, whilst those of the enemy are debarred from so doing. On this account harbours, &c., protected by such mines are termed "Harbours of refuge." 2.--Fresh mines may be added to a system of such defensive torpedoes, thereby allowing an exploded mine to be replaced. NOTE.--This is a very important point in connection with a system of defence by submarine mines, as in the case of a deep water channel, a hostile vessel being sunk by one of them, would not become an obstruction, as, were the channel a comparatively shallow one would most probably be the result, and therefore it would be necessary to put a fresh mine in the place of the exploded one; this would also apply were a mine to be prematurely ignited, or if any portion of its firing apparatus were injured. 3.--At night, or in a fog, no vessel can pass through a channel, &c., so protected without affording a means of ascertaining her presence. NOTE.--This is also a very important advantage of a system of defence by electrical sea mines, affording as it does a complete safeguard against surprise. 4.--The power of obtaining proof, without going near it, by a system of testing that the electrical condition of the mine, &c., is perfect. NOTE.--This again is an extremely important point. For were a charge to become wet, one of the electric cables of the mine broken, or damaged, &c., it would instantly be made apparent at the firing station, and could be at once remedied. 5.--They can be raised for examination, or removed when no longer required, with ease and safety. Such are some of the chief advantages of employing the agency of electricity to effect the ignition of the charge in a system of defence by submarine mines. _Defects of Electrical Submarine Mines._--The following are the chief defects connected with the use of electrical mines:-- 1.--The number of wires that are required to be used with them. 2.--The necessity of employing specially trained men in their manipulation. In time there seems little doubt but that the former obstacle will be to a considerable extent overcome, but the latter must always be a flaw in an otherwise perfect system of coast defence by submarine mines. _Rules to be observed in using Electrical Submarine Mines._--In connection with a system of electrical submarine mines the following rules should be carefully observed:-- 1.--They should be moored in deep channels, that is to say, where the larger class of vessels would in attempting to force a passage be obliged to go.