The means of such defence, should most certainly be inherent in the vessel herself, outward methods, such as nets, booms, etc., are to great extent impracticable, besides one of the above mentioned torpedoes, being caught by such obstructions would, on exploding, most probably destroy them, thus leaving the vessel undefended against further attacks. _Mechanical Mines._--Several ingenious methods have of late been devised for the purpose of obviating one of the principal defects common to all kinds of mechanical submarine mines, the most efficient and practical of which will be found fully described in the following pages, viz., the great danger attendant on the mooring of such mines; but as yet, no really practical mode of rendering mechanical mines safe, after they have once been moored and put in action, has been discovered, were such to be devised, a very difficult and extremely important problem of defensive torpedo warfare would be solved. _Electrical Mines._--In regard to electrical submarine mines, much has been done by torpedoists in general to simplify this otherwise somewhat complicated branch of defensive torpedo warfare, by adopting the platinum wire fuze, in the place of the high tension one, by the employment of Leclanché firing batteries, by the simplification of the circuit closer, and discarding the use of a circuit breaker, by altering the form of torpedo case, and whenever possible by enclosing the circuit closer in the submarine mine. The necessity of a very elaborate system of testing should, if possible, be overcome, for a system of submarine mines that requires the numerous and various tests that are at the present day employed, to enable those in charge of them to know for certain that when wanted the mines will explode, cannot be considered as adaptable to actual service. It must be remembered that the safety of many ports, etc., will in future wars depend almost entirely on the practical efficiency of electrical and mechanical mines. As yet, in actual war, little or no experience has been gained of the real value of a mode of coast defence by electrical mines, excepting from a moral point of view, though in this particular they have most undoubtedly been proved to be exceedingly effective. A submarine mine much wanted on active service, is one that can be carried on board ships, capable of being fitted for use at a moment's notice, and of being easily and rapidly placed in position by the ordinary boats of a man-of-war. It should be a self-acting electrical mine, with the circuit closing apparatus enclosed in the torpedo case, and capable of carrying about 100 lbs. of guncotton. This form of mine would be found extremely useful to secure the entrance to a harbour, etc., where ships might happen to be anchored for the night, or which might have been wrested from the enemy, etc. They should be capable of being placed in position and picked up again, in the shortest possible space of time.