_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 _m__{1}, _m__{2}, _m__{3}, &c., and _n__{1}, _n__{2}, _n__{3}, &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 _s__{1} 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 m__{1}, _A m__{2}, _A m__{3}, &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, _m__{1}, _m__{2}, &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. [Illustration: RUSSIAN SUBMARINE MINE, FIRING BY OBSERVATION. PLATE XVI] In the case of the vessel _S_, though at _C_, she is on the prolongation of the line _A m__{5}, _C_, and therefore the key of the mine _m__{5}, is pressed down at _A_, yet not being on the prolongation of the line _B_, _E_, the key at _B_ is not pressed down, therefore the firing battery is not thrown in circuit, or the mine _m__{5} exploded, but when the vessel _s_ reaches the position _N_, that is over the mine _m__{3}, she being on the prolongation of the lines _A m__{3}, and _B E_, the key (_m__{3}) at _A_, and the key at _B_ would both be pressed down, and therefore the mine _m__{3} exploded, and the ship destroyed. In the case of a vessel passing through an interval between any two mines at such a distance as to be out of the radius of destructive effect of either of the mines belonging to the first row (which is shown at _s__{1},) only the key at _B_ would be pressed down, and thus the vessel enabled to pass safely through, but only to come to grief at the second or third row of mines, provided they have been properly placed, and separate though similar arrangements as in the case of the line of mines, _m__{1}, _m__{2}, &c. have been made.