[Illustration: HARVEY'S TOWING TORPEDO. PLATE XXX] The legs of the slings should be so fitted that when stretched alongside the torpedo they extend 1 foot beyond the stem for the large torpedo and 8 inches for the small one; the four legs should be so fitted that when an equal strain is brought on them, the thimble should be on a level with the upper lugs, and the upper fore span form an angle of 80° to 85° with the side of the torpedo; this is shown at Fig. 106. This arrangement gives the best divergence with the least strain on the tow rope, and is suitable when the torpedo is kept at short scope, as well as when a long length of tow line is out. The mode of attaching the foremost and side levers is shown at Fig. 107. Before reeving the lanyards they should be well greased in the wake of the fair leads, but not where they are made fast. The lanyards should be made up like a reef point. Care should be taken that the short arm of the side lever _h_ is brought close into the fair lead, and its lanyard should be set up sufficiently taut to give a slight spring in the principal lever _b_ by the strain thus brought on it. This lever _b_ has a steel fish on the top, in order to prevent it taking a permanent bend. If the side lever lanyard is properly set up, the bolt will spring down about 1/8th of an inch when the safety key is withdrawn, owing to the spring in the lever, and the shrinking of the lanyard; this brings the muzzle 1/8th of an inch nearer the pin without disturbing the side lever. The bolt is so arranged that the torpedo can be fired by either of the following methods:-- 1.--Mechanically. 2.--Electrically at will. 3.--Electrically on contact or at will. _Mechanically._--In this case the bottom of the inner cylinder, as at _a_, Fig. 108, is fitted with the ordinary mechanical chemical fuze, ignition being effected by the breaking of the glass vessel containing the sulphuric acid on being forced into contact with the needle _n_, by the action of the levers on the torpedo striking a vessel. _Electrically at Will._--For this purpose a platinum wire fuze is used, one terminal being connected to earth through the bolt, the other to a wire leading up through the core of the bolt, and connected by means of an ebonite joint with a single cored electrical cable leading from the torpedo vessel. _Electrically on Contact, or at Will._--In this case, a resistance coil is inserted in addition to the fuze, and is so arranged that on the bolt being forced down a short circuit is formed, cutting out the resistance coil (about 20 ohms), and thus enabling the battery to fire the fuze, which, owing to the 20 ohms resistance in the circuit, it was previously unable to effect. Should the bolt so arranged be required to be fired at will, it is only necessary to put a more powerful battery in circuit, and so fire the fuze through the 20 ohms resistance.