A, Mine-Planter; B, Mine being dropped overboard; C, Plummet-line extended; D, Anchor sinking; E, Plummet touching bottom; F, Mine submerged and anchored; G, Battleship striking mine; 1, The “Striker”; 2, Charge of Explosives; 3, Air-space, for Buoyancy; 4, Mine-case; 5, Anchor; 6, Plummet. ]
Mines are almost never laid singly but in groups, the area of water so planted being called a “mine field.” A secret, zigzag channel is often left clear for the benefit of friendly craft. The rows of mines are usually “staggered” or placed like the men on a checker-board, so that if a hostile vessel passes through an opening in the first row she will strike a mine in the second. Another device is to couple together the mooring cables of two or more mines so that a ship passing between them will draw them in against her sides.
Contact may cause explosion in any one of several different ways. The head or sides of the mine may be studded with projecting rods like the striker on the nose of a Whitehead, to be either driven directly in against a detonating charge of fulminate or else open the jaws of a clutch and release the spring of a firing-pin. Such external movable parts, however, are too prone to become overgrown and clogged with barnacles and the like. A more modern way is to have the shock of the collision with the ship’s hull dislodge a heavy ball held in a cup inside the mine. The fall of this weight sets in motion machinery which fires the detonating charge. Or the device may not be mechanical but electrical, as in the type of mine that, when drawn far enough over to one side by a vessel passing over it, spills a cupful of mercury. This stream of liquid metal closes an electric circuit, so that an electric current passes through a piece of platinum wire embedded in fulminate and heats it red-hot, with obvious results. This current may be obtained either from a storage-battery carried in the mine itself, or through a wire running down the mooring cable and over the bottom to the shore. Most shore-control mines are so designed that they can either be fired by observation, or else turned into electro-contact mines of the above-mentioned type by arranging the switches in the controlling station. It is also possible to have the contact serve to warn the operator on shore by ringing a bell and indicating the position of the intruding ship in the mine-field.