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. [Illustration: U. S. Mine-planter _San Francisco_.]