Submarine mines may be divided into two classes; those for harbour defence, and those for use in the open sea. Harbour defence mines are almost invariably electrically controlled; that is, they are connected with the shore by means of a cable, and fired by an electric impulse sent along that cable. In one system of control the moment of firing is determined entirely by observers on shore, who, aided by special optical instruments, are able to tell exactly when a vessel is above any particular mine. The actual firing is carried out by depressing a key which completes an electric circuit, thus sending a current along the cable to actuate the exploding mechanism inside the mine. A hostile ship therefore would be blown up on arriving at the critical position, while a friendly vessel would be allowed to pass on in safety. In this system of control there is no contact between the vessel and the mine, the latter being well submerged or resting on the sea floor, so that the harbour is not obstructed in any way. This is a great advantage, but against it must be set possible failure of the defence at a critical moment owing to thick weather, which of course interferes seriously with the careful observation of the mine field necessary for accurate timing of the explosions. This difficulty may be surmounted by a contact system of firing. In this case the mines are placed so near the surface as to make contact with vessels passing over them. The observers on shore are informed of the contact by means of an electric impulse automatically transmitted along the cable, so that they are independent of continuous visual observation of the mined area. As in the previous system, the observers give the actual firing impulse. The drawback to this method is the necessity for special pilotage arrangements for friendly ships in order to avoid unnecessary striking of the mines, which are liable to have their mechanism deranged by constant blows. If the harbour or channel can be closed entirely to friendly shipping, the observers may be dispensed with, their place being taken by automatic electric apparatus which fires at once any mine struck by a vessel. Shore-controlled mines are excellent for harbour defence, and a carefully distributed mine-field, backed by heavy fort guns, presents to hostile vessels a barrier which may be regarded as almost impenetrable. A strong fleet might conceivably force its way through, but in so doing it would sustain tremendous losses; and as these losses would be quite out of proportion to any probable gains, such an attempt is not likely to be made except as a last resort.