The first mode is exceedingly simple, but except under very favourable circumstances cannot be relied on when firing by observation is the means adopted to explode a system of submarine mines. The second plan is practically easy to carry out, and by it a mine may be placed more accurately. To enable either of the above methods to be properly carried out, specially fitted steamboats, &c., are requisite. At Fig. 52 is represented a 42 feet launch fitted for laying down a submarine mine by the first of the two modes enumerated above. [Illustration: STEAM LAUNCH FOR MOORING SUBMARINE MINES. PLATE XII.] _a_ is the mine; _b_ is the electric cable carried from the drum _c_ to the charge, and connected for use; _d_ is the circuit closer, which is attached to the mine by its electric cable and mooring rope; _f_ is the mushroom sinker attached by means of its mooring chain to the mine, it is suspended by a slip rope _g_, which passes over a small crutch fitted with a sheave _h_; _i_ is a hollow iron derrick, and _k_ the tackle and fall for lifting mine into boat; this derrick is formed of an iron tube about 3 inches diameter, 3/8 inch thick, and 10 feet 6 inches long; it is attached to an iron tube mast of similar diameter and thickness to the derrick, but 12 feet 3 inches long, an iron chain 6 feet 6 inches long and 5/8 inch diameter, connects the derrick to the mast; _m_ is a leading sheave to keep the cable clear whilst it is being paid out; _l_ is a crab, for working the tackle _k_, &c., and _c_ is the drum on which the electric cable is wound. In connection with the defence of a harbour by a system of electrical submarine mines of large size, it will be necessary to employ a service of steamtugs, steamboats, mooring-barges, &c., specially fitted for such work. One of the great advantages of the hauling down method of placing mines in position, is, that the anchors, with the cables connected thereto, may be carefully and accurately got into position during the time of peace, and the mines themselves, which should be kept in store ready fitted for immediate use, need not be placed in position until they are actually required. The drums used for reeling a multiple cable on, are capable of holding half a nautical mile in length. That used for a single core armoured cable is similar to but smaller than the aforesaid drum, and is capable of stowing one nautical mile of such a cable. For transportation wooden drums are ordinarily used. CHAPTER IV. DEFENSIVE TORPEDO WARFARE--_continued_. _CLOSING the Electric Circuit._--In connection with the system of coast defence by means of electrical submarine mines, there are two distinct methods of effecting the closing of the electric circuit, and consequently, the firing battery being connected, the explosion of the mine or mines, which methods may be used separately, or in combination, and are as follows:-- 1.--The self-acting method. 2.--The firing by judgment, or observation method.