depend upon the heating powers of an electrical current to ignite certain explosive substances, preferably gun-cotton. Although there are many modifications of the electrical fuse, they almost without exception agree entirely in general make. Two current wires are necessary, called terminals, and having their inner ends separated, in order that some substance offering a high resistance to the passage of the current may be inserted between them. The terminals are of copper wire; the resistance material or bridge is generally of fine platinum wire, although other substances are used in different types. The terminals are as a rule separated by a small block of hard wood which forms an insulator and a support. Around the bridge a wisp of gun-cotton is generally wrapped, although here again other explosives are used in modified forms. This combination is called an exploder, and it is inserted in a fuse-case or magazine containing powder or gun-cotton. The exploder and magazine together form the fuse. One of the terminals is generally attached to the exterior of the torpedo, which forms an earth; the other one is connected with an insulated wire or cable leading to the electrical battery. These fuses are made to explode either at will or on contact. If they are to explode at will, the firing circuit as a rule is not completed until the moment desired for explosion. If they are to explode by contact, the general manner of use is to insert in the circuit at some part a resistance-coil of sufficient force to weaken the current below the firing point. At the moment of contact, the resistance coil is automatically cut out and the torpedo is fired. Another way is to use what is called a circuit-closer, which normally leaves the circuit broken, but on being touched the circuit is closed and the torpedo is fired. [Illustration: American Dynamo-Electric Fuse.] [Illustration: English Dynamo-Electric Fuse.] One advantage of the electrical fuse is the facility with which the whole circuit may be examined at any time to test its condition. It is simply necessary to send a very weak current through the circuit, and, the resistance being known, the length of the circuit can be measured electrically and the position of flaws may at any time be located. The modifications of the different fuses depend in a great manner upon the type of generator of the current, as voltaic batteries, dynamo- and magneto-electric machines are used according to the position of the torpedo. CLEARING CHANNELS OF TORPEDOES.