Minelaying and Minesweeping Mines

In the observation system, the exact position of the mines when laid is marked on a special chart, on which the track of any ship crossing the minefield can also be plotted. The operator in the shore station plots the track and when it is seen to be crossing the position of a mine he closes a switch and the mine is fired. The mines themselves are placed either on the sea bed or at depths well below the bottom of any friendly ships, and therefore such mines offer no obstruction to friendly traffic, though anchoring in the mine field must be prohibited for fear of damaging the cables. As the mines are not in contact with the ship at the moment of firing and as errors in observation must be allowed for, these mines must carry larger charges of explosive than mines fired by contact with a ship and they are usually fired in groups.

In the circuit closer system, each mine contains an apparatus which is actuated by any vessel or other heavy object bumping against the mine. When set in action the apparatus completes the electrical circuit through the detonators in the explosive charge and the mine fires if the main controlling switch on shore is closed. When it is not desired to fire, the main switch on shore is kept open and the circuit closer is restored to its open condition auto matically after the mine is bumped. Such mines, placed near the surface, require only a small charge, are effective in fog or mist, and are used where it is necessary to close waters entirely.

The "depth charge" used by surface craft in combatting sub marines is also controllable. It is a cylindrical bomb which is dropped near the spot and at approximately the depth where the submarine is believed to be. The depth at which it explodes is regulated by an adjusting screw which is pushed against the de tonator by water pressure. Concussion of the water from the ex plosion will damage or sink a submarine hundreds of yards away.

Non-controlled Mines.

In these the means of firing are contained in the mine itself ; they may be fired either electrically by means of a small local battery or mechanically by a spring, pistol or suspended weight. In all cases the impulse which actu ates the firing gear is the bump given by a ship or other heavy object striking the mine. When non-controlled mines have once been laid they are dangerous to friend and foe alike. Safety ar rangements are fitted to these mines to keep the firing apparatus out of action while the mine is being laid, to render the mine harmless in case it breaks away from its mooring and, in some cases, to render it inactive after a certain period of time. Mines that fail in these respects become as great a danger to friend as to foe. At the outbreak of war in 1914 the German Carbonit mine was unreliable in respect to its safety after breaking adrift while the British mines were, unfortunately, only fully efficient in the same respect. In the latter part of 1917 the new and improved British H2 mines came into use. This mine was similar in princi ple to the German Carbonit mine except for the method of taking up the predetermined depth.

The German method of depth-taking was for the mine, attached to the sinker, to drop to the sea bottom, where, after a short interval for the purpose of safety, the mine released itself from the sinker and rose to the depth for which it was set. In the British method, on release from the minelayer the mine remained on the surface while the sinker went down, unreeling the mooring wire until a small weight (hanging below the sinker a distance equal to the depth the mine was required to be below the surface) struck the bottom. The mooring rope then stopped unreeling and the sinker continuing to sink pulled the mine below the surface. An advantage of the British system was that heavy water pres sure on the mine-case was obviated; but, on the other hand, it did not obtain the exceptional accuracy in depth-taking which was noticeable in the German method. The United States used a mine of a novel type. It carried the usual charge of 3oolb. of trinitro toluene, but from each mine there extended antennae for a dis tance of 35ft. which, if touched by a metal ship, exploded the mine. The result was that the danger zone was largely increased.

Moored mines can be laid in any depth up to 1 oo fathoms, or even more in tideless waters, and they can be regulated to lie at any depth below the surface. Deep mines are used against sub merged submarines, and shallow ones against surface shipping. In a strong tide, with a long mooring-rope, the mine will bend over to the tide; so that mines, under strong tidal conditions, some times lie too deep to be harmful to surface ships. During the War in a position such as the Pentland Firth, mines were in fact only a danger at slack water, a period of minutes only. Again, the difference in height between high and low water is an important matter. If, for instance, the tidal range was exceptionally large, as in the Bristol Channel or Bay of Fundy, all types of ships could pass over a minefield at high water in perfect safety, pro vided no mines had been seen at low water on the surface. By The Hague convention drifting mines should become inactive one hour after they are first dangerous. Some uses for these mines are (a) for attacking ships sheltering in a harbour by allowing the tide to drift the mines in ; (b) for dropping in the wake of a ship or Squadron when being chased.

The most recent forms of drif ting mine are oscillating mines which maintain themselves at approximately a steady depth by mechanical means. The inventors claim for them the advantage that they cannot be swept up. Of oscillating mines a well known type is the Leon mine. The mine is slightly heavier than the water it displaces and therefore sinks slowly. On passing the pre arranged depth a hydrostatic valve switches on electrical power which drives a propeller and causes the mine to rise until at a certain depth the power is switched off, and the mine commences to sink again.