By the employment of electricity and a mirror, the great defect of this method, viz., the necessity of employing four people to manipulate it, would be remedied. The foregoing is a modification of Siemens's method of ascertaining distances at sea, &c. _Rules observed in Planting Mines._--In placing a system of submarine mines in position, the following are some of the chief points to be attended to, this work depending in a great measure on local circumstances, and on the method that is to be adopted in exploding and mooring them:-- 1.--The plan of defence must be carefully laid down on a chart, on a scale of not less than six inches to the mile, and on this plan are to be marked the sites of the observing stations, the positions of each mine, circuit closer, and junction box, with their corresponding numbers, and also of the electric cables. 2.--The position of each mine having been determined, should be marked off by buoys. 3.--The utmost care should be taken to lay the electric cables, so that they shall be as far as possible away from the mines in the vicinity of which it may be necessary to take them, so as to lessen the liability of injury to them, by the explosion of the latter. 4.--The electric cables should be laid parallel, and never be allowed to cross directly over each other, otherwise the operation of underrunning them will be much complicated, also a certain amount of slack should be allowed to facilitate in picking the cables up for repair, &c. 5.--Every manner of device is to be used to conceal the electric cables, such as laying dummies, making detours inland, &c. 6.--All marks indicating position of the mines to be removed, after the mines have been placed in position. 7.--The identity of each cable and mine to be very carefully preserved throughout, by means of a number. 8.--A number of electro contact mines should be placed in advance of the leading line of mines, at irregular intervals, to prevent the enemy, having once ascertained the position of one mine of a line, from knowing within limits the position of the others of that line. [Illustration: SYSTEMS OF DEFENCE BY SUBMARINE MINES. PLATE XVIII] In connection with a system of defence by electrical submarine mines, the following batteries are required:-- 1.--Firing battery. 2.--Signalling, or shutter battery. 3.--Testing battery. 4.--Telegraph battery. _Firing Battery._--The firing battery should be suited to the nature of the fuze employed, and should possess considerable excess of power to enable it to overcome accidental defects, such as increased resistance in the various connections, or defective insulation in the line wire, &c.