In the above case R would be about 24 feet, which in so far as the actual destruction of a ship is concerned, may be taken as correct, but if injury to a vessel's engines, boilers, etc., be also taken into consideration, and as the vessel would most probably be underweigh on such an occasion, this would be a very vital and important consideration, R would under those circumstances be more than doubled. Now the necessary interval for safety between such mines, according to torpedo authorities, is equal to 10 R, and should certainly be not less than 8 R, which in this case would give about 200 feet, therefore assuming the radius of destruction to be 50 feet, it is seen that there would be under those conditions a clear undefended space of about 100 feet between each couple of 500 lb. mines in the same line. 2.--The combined arrangement of submarine mines with forts and batteries, in such a manner, that every one of the former shall be well covered by the guns of the latter, and also that it would be impossible for an enemy's ships to get within effective range of the forts, or batteries, without moving over ground where mines were laid. NOTE.--This applies to the defence of the more important harbours, etc., in which case the submarine mines (which would be chiefly electrical ones) would only act as auxiliaries to the land defences. To effectually carry out the above, there can be no question but that they who plan the forts, etc., should also plan the systems of submarine defence. A harbour, river, etc., which it is necessary to protect by electrical submarine mines, etc., and where no land defences exist, should have its mines supported by a powerful ship or ships, as maybe thought desirable. _Success in Torpedo Warfare._--The two most important conditions essentially necessary to the successful employment of torpedoes, both offensive and defensive, are:-- 1. CERTAINTY OF ACTION. 2. SIMPLICITY OF MANIPULATION. Without the former this mode of Naval warfare is comparatively useless, while without the latter the former condition is rarely obtained, more especially in the case of offensive torpedoes. Submarine mines are divided into separate classes, viz.:-- 1. MECHANICAL MINES. 2. ELECTRICAL MINES. _Mechanical Mines._--By this description of submarine mines, is meant those whose charges are fired by mechanical means alone. _Mechanical Mines in the American Civil War._--During the civil war of America (1861-5), the Confederates depended almost entirely on mechanical submarine mines for the protection of their harbours, rivers, etc., and to this extensive use of such mines may be traced nearly the whole of the Federal disasters afloat.