APPARATUS FOR FIRING BY OBSERVATION.
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, S1, S2, 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.
By the employment of electricity and a mirror, the great defect of this method, viz., the necessity of employing four people to manipulate it, would be remedied. The foregoing is a modification of Siemens's method of ascertaining distances at sea, &c.
Rules observed in Planting Mines.—In placing a system of submarine mines in position, the following are some of the chief points to be attended to, this work depending in a great measure on local circumstances, and on the method that is to be adopted in exploding and mooring them:—
1.—The plan of defence must be carefully laid down on a chart, on a scale of not less than six inches to the mile, and on this plan are to be marked the sites of the observing stations, the positions of each mine, circuit closer, and junction box, with their corresponding numbers, and also of the electric cables.