_c_{1}_, _c_{2}_, _c_{3}_ and _b_{1}_, _b_{2 3}_; and if they can be put at some distance, the position of the torpedo is determined with great accuracy by the intersection of the lines of sight of the two telescopes. Electric wires connect the stations and the torpedoes in the same manner as we have before described. Such methods of firing torpedoes are no doubt the most efficient, for the destructive charge may be sunk so far below the surface that not a ripple or an eddy can excite an enemy’s suspicion, or the channel appear otherwise than free and unobstructed, while friendly ships may pass and repass without risk; for the current which determines the explosion only passes when the two sentinels complete the circuit by simultaneously depressing their keys. Attempts have often been made to convert the torpedo into an offensive weapon, by causing vessels containing explosive charges to drift by currents, or otherwise, into contact with the enemy’s ships. The results have been always unsatisfactory, as there is great uncertainty of the machine coming into contact with its intended mark. Besides, it is easy to defend vessels against such attacks by placing nets, &c., to intercept the hostile visitors, especially if the attack is made by day, and by night the chance that a torpedo drifting at random would strike its object is very small indeed. One condition essential to the success of such attacks is that the approach of the insidious antagonist may be unobserved. Accordingly divers schemes have been projected for propelling vessels wholly submerged beneath the surface of the water, so that they may approach their object unperceived, and exert their destructive effect precisely at that part of the vessel where damage is most fatal, and where an ironclad vessel is most vulnerable, namely, below the water-line. Vessels have been built, propelled by steam and so contrived that their bodies are wholly submerged, only the funnel being visible above the surface. These _quasi_ submarine ships carry small crews, and are fitted with a long projecting spar in front, at the end of which is carried the torpedo. [Illustration: FIG. 109.—_Explosion of Whitehead’s Torpedo._ ] [Illustration: FIG. 110.—_Effect of the Explosion of Whitehead’s Torpedo._ ]