This torpedo, intended to reach a vessel at anchor and surrounded by a boom of logs, consists of a case of powder or dynamite resting loosely on a small shelf attached to one extremity of a heavy pine beam, ballasting it in such a manner that it will float on end with the top just above water. The torpedo is attached to the beam by a loose chain bridle, the ends of which are stapled respectively to the lower end and just above the centre of gravity of the beam. The shelf itself is hinged, and is held in position by a catch operated by a small bell-crank lever on top of the beam. This beam on coming in contact with a boom is canted slightly, and the lever being tripped the shelf drops from under the torpedo, which, hanging by its bridle, cants the beam almost horizontally, thus allowing it to slip under the boom and float on against the ship’s side, where the torpedo explodes on coming in contact. DEFENCES AGAINST OFFENSIVE TORPEDOES. Vessels at anchor in protecting themselves against torpedoes establish three separate lines of defence. The outer line consists of guard-boats; the boats of the ship, either carrying machine and boat guns or having their crews armed with rifles, patrolling the approaches to the ship at such a radius as may be well guarded by the number of boats employed. The second line consists of a boom of logs or spars arranged around the ship at a distance of from twenty to fifty feet, having in addition, wherever practicable, heavy nets which hang down below the level of the keel. The third and inner line consists of the machine-guns and small-arms of the crew, the double hull and compartment construction of the vessel itself, and powerful electric lights which at intervals sweep the water in the vicinity of the vessel, lighting up every approaching object. DEFENSIVE TORPEDOES. Torpedoes of this class may be described generally as being either heavy cases of explosive material resting on the bottom of shallow channels and fairways, and usually exploded at will by electrical fuses, or smaller torpedoes anchored and either buoyed or supported at from three to twelve feet below the surface. The first class of torpedoes is of crude and varied construction, being simply heavy cases of boiler-iron of a capacity for from 100 to 200 lbs. of dynamite or 1000 to 2000 lbs. of powder, caulked water-tight and sunk generally within easy range of a heavy battery, in a position where a vessel would probably stop or anchor to open fire, or else in a shallow channel where a ship must pass directly over it. The buoyant torpedoes are of different form; the most successful types being _The Frame-Torpedo._