_Application of the Intersectional Arc Method._--The application of the method of firing by observation, by means of intersectional arcs fitted with telescopes, is shown at Fig. 70. _C_, _D_, and _E_ are three of the larger kind of arcs, one being used for each row of mines at the firing station _A_. At the converging station _B_, one of the smaller arcs is used for each row of mines, as shown at _F_, _G_, and _H_. _S_, _S__{1}, _S__{2}, are the signalling apparatus, the _F_ terminals of which are connected to the sights _l_, _l_, _l_, Fig. 69, of arcs _C_, _D_, _E_. Firing keys _a_, _a_, _a_ at station _A_ are connected to each arc, and to three of the cores of the cable connecting the two stations _A_ and _B_, respectively. At the converging station _B_, three firing keys _b_, _b_, _b_ are connected to earth and to three cores of the connecting cable respectively. The remaining core of this cable is connected to the recording instruments _d_, _e_. The action of the arcs, &c., will be readily understood from the diagram at Fig. 70. This arrangement does not interfere with the action of the circuit closer, as all that is effected by the observing arc circuit is to put the signalling battery current at the converging station _B_ to earth instead of at the circuit closer. _Prussian System of Firing by Observation._--The principle on which this system is based, depends upon the proposition that if _c d_, in the triangle shown in Fig. 71, be always kept parallel to _H B_, then _A c_, _c d_, _d A_ bear exactly the same proportion to each other as _A B_, _B H_, _H A_ do to one another; so that by means of the small triangle _A d c_, the lengths of the sides of the large triangle _A B H_ can be obtained, and hence the position of the point _H_, the base _A B_ being of course known. In Fig. 71 at _A_ there is a slate table representing the roadstead, and upon it the exact position of every torpedo is laid down, corresponding to their position in the roadstead. At _A_ and _B_, 500 yards apart, telescopes having cross wires are placed; at _A_ a long narrow straight-edged strip of glass _A d_ is arranged to move in unison with the telescope at _A_; and by the application of dynamo electricity, a similarly constructed piece of glass _c d_ moves in exact unison with the telescope at _B_, and having its pivot at _C_; that is to say, _C d_ keeps parallel with _B H_, the line of sight of the observer at _B_. Then if the observers at _A_ and _B_ have got a ship in their telescopes, the point of intersection _d_ of the two pieces of glass _A d_ and _C d_ gives the position of the ship on the slate table at _A_, and when this point _d_ comes over the position of any one mine on the slate, it is known that the ship is over that particular mine in the harbour, and she may be destroyed accordingly, by throwing the firing battery into circuit.