SUBMARINE MINES  . A submarine mine is a weapon of

war used in the attack and defence of harbours and anchorages . It may be defined as " A charge of explosives, moored at or beneath the surface of the water, intended by its explosion to put out of action without delay a hostile vessel of the class it is intended to act against." It differs from the torpedo (q.v.) in being incapable of movement (except in the special form of drifting mines, which are not moored, but move with the tide or current) . But this subdivision into two distinct classes was not made till 187o . Prior to that date the term " torpedo " was used for all explosive charges fired in the water . Submarine mines may be divided into two main classes, controllable and uncontrollable, or, as they are often classified, " electrical " or " mechanical." In the first class the method of firing is by electricity, the source of the electric power whether by battery or dynamo being contained in a firing station on shore and connected to the mines by insulated cables . By simply switching off the electricity in the firing station, such mines are rendered inert and entirely harmless . In the second class, the means of firing are contained in the mine itself, the source of power being a small electric battery, or being obtained from a pistol, spring or suspended weight . In all mines of this class the impulse which actuates the firing gear is given by a ship or other floating object bumping against the mine . When mechanical mines have once been set for firing they are thus dangerous to friend and foe alike . Safety arrangements are employed to prevent the firing apparatus working while the mine is being laid, and clockwork is sometimes added to render the mine inactive after a certain definite time or in case the mine breaks away from its mooring . Their principal advantages, as compared with the electrically controlled mines, are cheapness and rapidity of laying .

" Controllable" mines are absolutely under the

control of the operator on shore, their condition is always accurately known, and if any break adrift not only is the fact at once known but the mines themselves are harmless . Another advantage is that when fired by " observation " as described below, they are placed at depths which will be well below the bottom of any vessels passing through the mine field . They can thus be used in channels which have to be kept open for traffic during hostilities . Electrical mines take rather longer to prepare and lay out than the other class, as the electrical cables have to be laid and jointed, and they require rather more skill and training in the operators employed to lay and fire the mines . Such mines represent the highest development of this form of warfare, and the details given below refer mainly to this class of mine .