One of the causes which led to the failure of offensive submarine weapons, when employed on active service, seems to be due to the fact that, owing to the extremely small radius of the destructive effect of such weapons, it is absolutely necessary for complete success to explode the mine in actual contact with the attacked vessel; to ensure which, at night time, in an unknown harbour, with the position of the vessel attacked somewhat uncertain, and even without the additional obstacles of guard boats, booms, electric lights, &c., is a service of infinite difficulty, and one which may easily terminate in a failure. The foregoing would more especially apply to the spar torpedo attack, but in an attack with the Whitehead fish, or towing torpedo, there would be an additional cause of failure, viz., the complicated nature of their manipulation. Torpedoes may be divided into four classes, viz.:-- 1.--Drifting or floating torpedoes. 2.--Towing torpedoes. 3.--Locomotive torpedoes. 4.--Outrigger or spar torpedoes. _Drifting or Floating Torpedoes._--By "drifting" or "floating" torpedoes are meant all those submarine machines which are dependent on the tide or current of a stream for their action and motion. During the American civil war this mode of attacking vessels was constantly employed by the Confederates, and though not successful in destroying any of the Federal ships, was the means of considerably hampering the movements of their river flotillas. Drifting torpedoes might be advantageously used for the destruction of pontoon bridges, booms, &c., and in this way, had the Turks in their late war used them, the Russians would have found the crossing of the Danube a matter of infinite danger and difficulty; in fact, by a systematic use of such weapons, combined with a little dash on the part of the Ottoman flotilla on the Danube, that river should have been to the Russians an impassable barrier. To use these torpedoes most effectively, especially against a single vessel, a thorough knowledge of the force and direction of currents should be gained before proceeding to undertake an operation in which these submarine weapons are used. Another point to be remembered is, that if such a torpedo were started with the flood, for example, towards an enemy, and did not explode, there would be a chance of its being returned to the starting-place by the ebb tide. In this class the following torpedoes seem the most practicable:-- 1.--Lewis's drifting torpedo. 2.--McEvoy's drifting torpedo. 3.--American extempore drifting torpedo.