The groups composing a line of buoyant mines are not usually planted in prolongation of each other, but with a space for the passage of friendly vessels, and also for the movement of the planter when at work upon adjacent groups. Groups of ground mines may be placed in prolongation of each other or between the groups of buoyant mines, as they will always be below the hulls of passing vessels.
5. The mine planters and other boats with the necessary equipment for planting and maintaining the planted mines.
6. The range-finding system, the same as or similar to that used for the guns, enabling accurate plotting of the positions of the individual mines, and consequently permitting vessel tracking and observation firing.
7. The searchlights, for illuminating the mine fields at night.
8. The rapid-fire guns, for the protection of the mine fields.
CHAPTER II.
MATÉRIEL OF THE SYSTEM.
The generating set.—This consists of a D. C., shunt-wound generator driven by a kerosene oil engine, or of a direct-connected gasoline set. (For method of operation of a Hornsby-Akroyd oil engine, [see Appendix 2].)
The storage battery.—This is a 40-cell chloride accumulator, with a normal charge and discharge rate of 5 amperes. The voltage may be taken at 2 volts per cell; the internal resistance is negligible. Directions for setting up, care, and usage of the storage battery are given in [Appendix 3]. The 5-ampere battery is the standard equipment at the present time, but the new installations will have batteries with a normal charge and discharge rate of 15 amperes.
The motor-generator, D. C.-A. C.—This is a D. C.-A. C. (60-cycle, single phase) machine, running on D. C. voltage (80-110) and designed to give one-half kilowatt at 80 volts. To insure against breakdown two of these motor-generators are supplied to each casemate.
Starting switch.—This is a 4-point lever switch and is used to start the motor-generator and to accelerate it to full speed. To insure against breakdown two of these motor-generators circuit to the fourth point. Resistances are connected between the points, as shown in [figure 1]. The contact made at point 1 is not broken as the lever is moved to its successive positions. It is seen that the total resistance is 8 ohms; it is all in the armature circuit when the switch blade is in the first point; 4 ohms when in the second point; 2 ohms when in the third point; none when in the fourth point. The operation of closing the lever short circuits in turn the resistances 4, 2, and 2.