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.
Just as barbed-wire entanglements on land are blown out of the way by small charges of high explosives, so mined areas of the sea can be cleared by “counter-mining.” One or more strings of linked-together mines, of a small, easily-handled type, are carefully placed by light-draft vessels in the waters already planted by the enemy. When these are exploded together, the concussion is enough to destroy any anchored mines near at hand, either by setting off their exploding-devices or causing their cases to leak, so that they will be filled with water and sink harmlessly to the bottom. Or a channel may be cleared by “sweeping” it with a drag-rope towed along the bottom by two small steamers, exploding the mines or tearing them up by the roots. Very effective work of this kind has been done by the small steam-trawlers used by the North Sea fishermen, and if anything of the sort is ever necessary in American waters we may be thankful for the powerful sea-going tugs now towing strings of barges up and down our coasts.
U. S. Mine-planter San Francisco.
But even a light field-piece on shore can shell and sink the sort of small, unarmored craft that must be used for mine-sweeping. When a fleet attacks a channel or harbor entrance properly defended by both mine-fields and batteries, each supporting the other, there comes a time when the naval forces must wait till troops can be landed to drive away the forces protecting the rear of the batteries, so that the mine-sweepers can advance and clear a channel for the superdreadnoughts. The most striking example of this is the holding of the Allied fleet by the Turks at the Dardanelles.
There, too, effective use is being made of the latest, which is an adaptation of the oldest type of torpedo: the drifting mine.[23] This twentieth-century improvement on Bushnell’s “kegs charged with powder” floats upright, with a vertical-acting propeller on top and another on its bottom, and a hydrostatic valve set to maintain it at any desired depth. Should it rise or sink, the change in pressure will cause the valve to act on the principle already explained in connection with the Whitehead torpedo (see page [44]). Controlled by the valve, the little compressed-air motor attached to the vertical propellers will cause them to make a few revolutions, just enough to keep the mine at a constant depth beneath the surface of the Dardanelles, as the four-mile-an-hour current carries it down against the Anglo-French fleet. Within a few hours of each other, during the furious bombardment of the forts on March 18, 1915, the French battleship Bouvet was struck by one of these drifting mines and went down stern-foremost, then H.M.S. Ocean was sunk by another, and the Irresistible forced to run ashore to escape sinking, only to be pounded to pieces by the guns of the forts. A feature of this type of mine is that its size and shape enable it to be launched through a torpedo tube, either from a surface craft or from a submarine.
Ordinary contact-mines, without anchors and attached to floats that held them a few feet below the surface of the water, are sometimes dropped overboard from a vessel closely pursued by an enemy. A small mine so dropped by a German light cruiser returning from an attempted raid on the English coast, early in the war, was struck by the pursuing British submarine D-5 and sent her to the bottom. The D-5 was running awash at the time and only two officers and two seamen were saved.