In the case of the vessel S, though at C, she is on the prolongation of the line A m5, C, and therefore the key of the mine m5, 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 m5 exploded, but when the vessel s reaches the position N, that is over the mine m3, she being on the prolongation of the lines A m3, and B E, the key (m3) at A, and the key at B would both be pressed down, and therefore the mine m3 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 s1,) 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, m1, m2, &c. have been made.
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 [Fig. 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.
[Fig. 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.
To ascertain when this frame is level, a circular spirit level is attached thereto, a telescope d provided with one horizontal and three vertical cross wires, supported on Y's, admitting of vertical motion and attached to an upright e. A mill-headed screw f enables the telescope d to be raised or lowered; the telescope, which is rigidly connected to a vernier g, traversing over a graduated arc h, can be moved rapidly in a lateral direction by means of a rack and pinion arrangement i, and it can be clamped in any position by means of the screw h. Sights are fixed on the telescope in a vertical plane passing through its axis. To the outer rim of the frame of the arc, which is smooth, are secured the sights l l (shown on a large scale at Fig. 68), to give the direction of the mines. These sights are provided each with a brass point of V form, m, and a binding screw, n, in metallic connection with each other, but insulated by means of an ebonite plate from the rest of the metal of the sight. One end of a short piece of insulated wire is attached to the binding screw n, and the other passes through a hole in the base of the sight and projects below it; o is a brass tube rigidly connected to and moving with the upright carrying the telescope d, and projecting in front of this latter. A brass spring p (see Fig. 69) is attached to, but insulated from the outer extremity of this tube, and is so arranged as to make contact with the V point m on the sight, by means of a corresponding projection fitted to its under side. An insulated wire passing the tube o, the outer end of which is connected to a screw on the spring p, forms a metallic connection between this projection and the firing key.
At [Fig. 68] is shown an enlarged view of the front of the sight; in addition to the V projection m, and binding screw n, it is fitted with a capstan-headed screw to bear against the inner rim of the frame, and a thin wire upright t for giving the alignment of the mine, to which a disc is attached, on which the number of the mine is affixed.
When the distance between the station and the mine is only about one mile, an ordinary eyepiece is used in the place of the telescope d.
At [Fig. 67] is represented the arc employed at the converging station, which with the exception of there being no tube o, and only one sight, is precisely similar in construction to the one used at the firing station, and which has been described.