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:—

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.

Firing by Cross Bearings.—The simplest method of so determining the relative position of a vessel and a submarine mine, and exploding it at the right moment, is that in which observers are placed on the prolongation of the mines. This mode is shown at [Fig. 64], where m1, m2, m3, &c., and n1, n2, n3, &c., are the mines; A and B, the points in prolongation of the mines where the observers are stationed; D the firing battery, and s, and s1 two hostile vessels.

At the stations A and B firing keys are placed, at the former one for each separate mine, perfectly distinct and insulated from each other, at the latter a single key. The pivot points of the series of keys at A are connected by separate wires to one pole of the firing battery D, the other pole of which is connected by a single cored insulated cable to the pivot point of the key at B; the contact points of the series of keys at A are connected by separate line wires as A m1, A m2, A m3, &c., to the different mines, while the contact point of the key at B is put to earth. Thus it will be seen that, in the case of the row of mines, m1, m2, &c., unless the key at B, and the key at A, of either of those mines are both pressed down at the same instant, no current can pass, and therefore none of those mines can be exploded.

[RUSSIAN SUBMARINE MINE, FIRING BY OBSERVATION.]