_Countermining._--Countermining, that is, the destruction of submarine mines by the explosion of other mines dropped close to them, will under certain conditions prove of great use in clearing harbours of mines. This method could not be operated in waters properly guarded and swept by artillery fire. There are two distinct methods of laying out countermines, viz.:-- 1.--In a boat, which may be either towed, or hauled out to its destination, or may be steered, and controlled by electricity. 2.--By attaching them to buoys so that they are suspended at the proper depths, and then hauled out by means of a warp to an anchor which has been previously placed in position. Both of the foregoing methods have been successfully manipulated in practice, the first method, where the boat carrying the countermines is towed either by a pulling or steam boat being the most practicable one. A large amount of material would be required for clearing a channel by means of countermines: for example, if the mines to be attacked require 500-lb. gun-cotton charges to be used, 7-1/2 tons of the explosive, besides cables, buoys, &c., would be required to clear a passage about one mile in length and 200 feet in width. A ship's launch will carry about twelve of these 500-lb. countermines, with all the gear attached thereto. Experiments to ascertain the effect of countermining have been carried out in England and Europe for the last five years, some of which are given at length in the chapter on "Torpedo Experiments." During the Turco-Russian war, a portion of the Danube was swept in the ordinary and most simple manner by the Turks, and five Russian electro contact buoyant mines were picked up; one other exploded during the process of dragging it to the surface, but no injury occurred to those at work. _Destruction of Passive Obstructions._--To clear away booms, or other passive obstructions, if not possible to cut them away, they may be destroyed by outrigger boats exploding their torpedoes underneath, and in contact, or by attaching charges of gun-cotton at intervals, and then exploding them simultaneously. When a chain is horizontal, and therefore somewhat taut, a charge of 3-1/2 lbs. of gun-cotton (this explosive, being the most effective and convenient for such purposes, should always be used) will be found sufficient to destroy it, no matter what size, and whether the chain is in or out of the water, the charge being of course placed in contact with it. Great uncertainty must always attend the supposed clearance of a channel, or passage of submarine mines, as was exemplified during the American civil war, when most of the Northerners' vessels were destroyed while moving over ground which had been previously carefully dragged, and buoyed, and this fact, coupled with the tediousness and danger of performing such a service, proves the enormous value of a system of defence by submarine mines. FOOTNOTES: