“ 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. ”

“ The firing station can be on a vessel moored near the mine field, but is more usually on shore, where it can be made absolutely secure against any form of attack. But the observing stations must be on shore to give stability to the observing instruments, they cannot be entirely protected as they must have a small opening facing the mine field, but can be made very inconspicuous. ”

“ The cable is carefully insulated and protected with a layer of steel wires. An earth return is used for the electrical circuit.
The employment of mines in any defence must depend entirely on the general character of the defence adopted, which will itself depend on the size and importance of the harbour to be defended and other details (see Coast Defence) . The role of mines in a defence is to act as an obstacle to detain ships under fire and compel them to engage the artillery of the defence. Thus mines find their greatest usefulness in the defence of harbours with long channels of approach. ”