_Mode of Converting into an Electro Contact or Observation Mine._--To convert this mine into an electro contact one, it is only necessary to connect the wires _w__{1} and _w__{2} to other wires leading from the shore; also by replacing the horns _D_, _D_ by solid brass screw plugs, the mine may be converted into an ordinary observation one. In this case the two wires _w_ and _w__{1} attached to the fuze _f_, terminals would have to be connected to the observation instruments on shore. _Turkish Vessel sunk._--It was by means of one of these electro mechanical mines, that the Turkish gunboat _Suna_ was sunk at Soulina. Firing by observation, that is to say, effecting the ignition of an electrical submarine mine at the precise moment of a hostile vessel being vertically over it, through the agency of one or two observers stationed at a very considerable distance from the mine, should, with the very perfect self-acting circuit closers that exist at the present time, be resorted to only in very exceptional cases, or in connection with the self-acting system. There are two defects, which are common to all methods of firing submarine mines by observation, and these are:-- 1.--At night time, or in foggy weather, it cannot be employed. 2.--It is necessary to employ at least two observers, at a considerable distance apart, who to effect a proper action at the right moment, must work in perfect unison. These defects alone are sufficient to explain the preference given to a self-acting method of closing the electric circuit at the precise moment of a vessel being in position over a mine by those governments who have adopted electrical submarine mines as a means of coast defence. _Methods of Firing by Observation._--There are several methods of firing by observation, of which the following are the ones principally used:-- 1.--By pickets or range stakes. 2.--By cross bearings. 3.--By intersectional arcs fitted with telescopes. 4.--The Prussian system. _Intersection by Pickets or Range Stakes._--In narrow channels and at short distances, this system of ascertaining the relative position of a hostile vessel and a submarine mine may be used, provided that skilled and careful men are employed to work it. Two or more pickets or stakes are arranged in front of the firing station in such a manner that a vessel passing up the channel on the prolongation of these stakes will be over a mine. This arrangement should of course always be considered as an extempore one; it was used on several occasions by the Confederates during the American civil war.