of such mines could be discovered by spies.
The mines which have been chieflyused in the present war are automatic and mechanical, and are fired when the ship strikes against them.
It is by no means easy to design a satisfactory mine which shall have its firing gear carefully adjusted so as to insure explosion of the charge from the slightest shock produced by contact with the passing ship. At the same time provision must be made to prevent the premature firing of the mine either on land, on the mine laying ship, or when being launched into the mine field. Again, it is important that should one or two mines be exploded, the adjacent ones be not fired accidentally—a difficult problem, as the concussion of the water produced by the explosion tends to disturb other mines. Another essential condition is that the depth of immersion under the surface should be constant so far as the rise or fall of tides allows.
A mine consists of three parts: (1) the chamber containing the firing mechanism, the detonator and explosive charge: (2) the flotation chamber to give buoyancy to the mine, and (3) a detachable anchoring chamber provided with a winch having a paying out cable.
A mine is maintained at the desired depth in the water by means of an anchor in which the cable, one end of which is connected to the mine, is unwound from a drum suitably braked and mounted in the anchor casing. The rotation of the drum is controlled by a plumb weight attached to a short sounding line. When the plumb weight reaches the bottom of the sea the rotation of the drum is stopped and the mine is pulled down to the required depth. It is only necessary to determine at what depth below the surface it is desired to
Positions assumed by a mine and its automatic anchor in water from the moment of dropping the mine overboard to the final moment of mooring
anchor the mine and to throw into the water the complete apparatus, namely the mine and anchor, whereupon the whole apparatus will take up its proper position, the depth of submersion being determined by the length of the sounding line.
The diagram on this page illustrates the working of the automatic anchor:
Position 1. After having been dropped overboard the mine is at the surface of the sea with its attached anchor immediately below the mine with the plumb weight hanging about nine feet below the anchor.
Position 2. The barrel is unwinding its cable and the anchor is descending to the bottom of the sea owing to the force exerted by the plumb weight in keeping down a lever, so that the drum is free to rotate.
Position 3. The plumb weight reaches the bottom of the sea and the pull exerted on the lever ceases. This lever is now released and locks the drum, so that it cannot pay out any more.
Position 4. As no more cable can be paid out the anchor has sunk to the bottom of the sea and drawn the mine with it. It will be seen from the diagram that the depth of immersion depends on the length of the sounding line.
A safety device is generally introduced which is operated by the pressure of the water. The firing gear is locked by a spring which, however, is counteracted by the pressure of water. When the mine is submerged the firing gear is operative, but as soon as it comes to the surface the water pressure is gone and the mine cannot be fired. The percussion device employed is of the usual type for exploding charges of guncotton and does not differ from those ordinarily used.
The detonator is sharply struck by a ball or a lever when the mine is hit
