A many-fluked grapnel is often used to drag for the cables of such mines as are exploded from the shore. In some cases the fished-up cable fails to break, so that it is well to have an explosive charge in the grapnel, with an arrangement for detonating it from the vessel by which the grapnel is operated. During the American Civil War, search was made for mines with chains and hawsers. This is the method which is now usually employed when no other means are at hand. It was the one used by the Russians when they first started searching for the mines which the Japanese placed in waters about the harbor-mouth at the siege of Port Arthur.
The Russians used two boats in dragging for mines, with a hawser stretched between them. It was soon found that little could be accompliThed with this arrangement on account of the fact that the hawser in motion tended to rise toward the surface, and that it formed an acute angle in the middle, thus reducing greatly the area swept. The first of these diffi culties was partially overcome by attaching weights to the cables where they were fastened to the sweeping-hawser. A further improve ment, partially overcoming the second difficulty, was in placing floats at the ends of the cables, suspending weights from each float to the proper depth, and attaching the hawser to these weights. The floats were so constructed and attached as to cause them to sheer out, thus giving the sweep the necessary spread.
From this time dates the organization of the first mine-reeping flotilla ever employed. It searched and cleared the channel on a regular plan, affording ships a safe ingress and egress. No Russian ships went out of the harbor until the mine-sweeping flotilla had first searched the channel. The use of floats and weights necessi tated a contrivance of great bulk, difficult to cast and haul in, and too heavy to permit of any but the slowest speed. To overcome these difficulties, use was made of the well-known theory that a pent-shaped log, towed behind a boat, can be kept at any given depth with any given speed by adjusting the tow-line to the proper length.
Sjostrand, a Swedish inventor, used this theory in producing his mine-sweeper, which consisted of a drag-hawser attached at the ends to logs or kites kept at the proper depth by regulating the length of cable and the speed of the vessel. The kites were constructed and attached in such manner as to cause them to sheer out and maintain a wide spread of sweep. This device was in reality only a mine-searcher. An improvement converted it into a mine-re mover as well. The hawser was so attached to the cable at one end that, upon encountering a mine-mooring, it would break from the cable. At the loose end was fixed a grappling-hook containing an explosive charge. The mooring having been encountered and the hook end freed thereby, the hawser was dragged along the mine cable until the hook or catcher grappled and held it. The charge was then exploded from the ship and the mine destroyed or brought to the surface.
The removal of mines brought to the sur face is most easily accomplished by firing into them; for this work, small-calibre guns firing shell are provided. Small-bore rifle bullets make holes so small that the water enters very slowly into the mines. Guns firing shell pro duce much larger holes, and there is always the possibility of exploding the mine with a fragment. The great importance which the ex penence of war attaches to all means of com bating mines has led to the production of a great many appliances, but none of them are practicable, so that the mine-sweeping appli ance of Swedish origin remains the most ad vanced and the best. This sweep is useful only against mines which are anchored, since it at tacks the moorings. Against free mines, there is no protection except a sharp lookout. The only real protection against drifting mines lies in the development of naval aircraft, from which the location of mines can be established, especially drift mines, which float on or near the surface.