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of such mines could be discovered by- 

 spies. 



The mines which have been chiefly- 

 used 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 pro- 

 vision must be made to prevent the pre- 

 mature 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 acci- 

 dentally — a difincult 

 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 mech- 

 anism, 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 an- 

 chor in which the cable, one end of 

 which is connected to the mine, is un- 

 wound from a drum suitably braked and 

 mounted in the anchor casing. The ro- 

 tation of the drum is controlled by a 

 plumb weight attached to a short sound- 

 ing line. \\^hen the plumb weight reach- 

 es 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 auto- 

 matic anchor in water from the moment 

 of dropping the mine overboard to the 

 final moment of mooring 



Popular Science Monthly 



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 sound- 

 ing 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 imme- 

 diately below the mine with the plumb 

 weight hanging about nine feet below 

 the anchor. 



Position 2. The 

 barrel is unwind- 

 ing its cable and 

 the anchor is de- 

 s c e n d i n g 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 ro- 

 tate. 



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 bot- 

 tom 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 op- 

 erative, but as soon as it comes to the 

 surface the water pressure is gone and 

 the mine cannot be fired. The percus- 

 sion 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 



