In the principal wars that have subsequently occurred, though this form of submarine mine has been to a certain extent used, it has generally been only as an auxiliary to the more effective electrical torpedo, and owing to the deterrent effect produced by the numerous torpedo successes that characterised the American Civil War, on Naval Commanders, etc., few vessels have been destroyed by their means, the effect of the employment of defensive torpedoes having been almost wholly a moral one. _Mechanical Mines for Coast Defence._--The experience hitherto gained, with regard to the employment of mechanical mines for coast defence in actual war, proves that they will be found exceedingly valuable in the following positions:-- 1.--In combination with booms or other obstructions placed in defence of narrow channels, etc., which are intended to be completely blocked up. 2.--In shallow water on the flanks of electrical mines. 3.--In protecting unfrequented bays, channels, etc., and a long line of seacoast, which may otherwise be entirely undefended. NOTE.--In this latter instance, though the mines may not be covered by any guns, still they will be of great use, in so far, that being mechanical ones, they cannot be rendered useless by the process of cutting cables, etc., but must be destroyed, which in time of peace is a work of considerable labour and danger, and, therefore, would in the time of war, cause at the very least, serious delay to an enemy desirous of effecting a landing, etc., at a point so protected. There are numerous objections against their employment, the principal ones being:-- 1.--That they are all, more or less dangerous to place in position. 2.--That they cannot be tested when moored. 3.--That they are as dangerous to friend as to foe, when once placed in action. 4.--That an exploded, or known damaged mine cannot be replaced. NOTE.--The above objections, especially 2 and 3, constitute without doubt very serious defects in a system of defence by mechanical mines, and in the case of purely mechanical ones, it seems almost impossible to eradicate any of them, though, notwithstanding, under the particular circumstance before-mentioned, these species of defensive torpedo will be found extremely useful. _The Advantages of Mechanical Mines._--They possess a few advantages, which are as follows:-- 1.--They are comparatively cheap. 2.--They can be kept in store and ready for use at a moment's notice. 3.--They do not require specially trained men to manipulate them. 4.--Extempore ones can be easily and readily made. _Best Kinds of Mechanical Mines._--Among the very numerous and various kinds of mechanical submarine mines that have been devised the following may be considered as the most effective, and practicable of them all:-- 1.--FRAME TORPEDOES. 2.--BUOYANT MINES. This includes:--