_Firing by a Preconcerted Signal._--At Fig. 65 is represented a somewhat similar, though a much simpler plan of the foregoing system, by employing a preconcerted signal at the station _B_ in the place of the firing key and insulated cable, as in the former case. The only material difference in the arrangement of these two methods, is that in the latter case the pole of the firing battery at _A_, which in the former case was connected to the firing key at _B_, is put direct to earth. As will be readily understood, this latter system requires great coolness and nerve on the part of the operator at _A_, who has not only to watch the vessel passing across his intersections, but also to be on the alert to receive the signal from the observer at _B_. Should it ever be necessary to adopt this latter system, it will be found advisable to employ two men at station _A_, one to watch station _B_, the other to attend to the firing key and intersections. A separate signal-flag for each line of mines, and also a separate firing arrangement, would be required. As in many cases it would not be practicable to have a station in such an advanced position as at _B_, in Figs. 64 and 65, on account of the danger of its being cut off by an enemy, another combination becomes necessary. In this instance the station _B_ is placed on the opposite side of the river, &c., to that on which the station _A_ is placed, and a series of firing keys, instead of a single one, is here used, necessitating a multiple cable between the stations _A_ and _B_, in the place of single cored cable; the manner of manipulating this method is very similar to that previously described. _Firing by Intersectional Arcs fitted with Telescopes._--The foregoing methods of firing by cross bearings are replete with many serious defects, to remedy which, to a considerable extent, special arrangements have been devised, that is, the employment of intersectional arcs fitted with telescopes at the stations _A_ and _B_. Figs. 66 and 67 show the arrangements of these arcs, the former being the one used at the firing station _A_, the latter at the converging station _B_. At each station one arc is provided for each row of mines placed in position. The firing arc Fig. 66 consists of a cast iron frame _a_, with three feet _b_, _b_, _b_, these being provided with levelling screws.