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:--
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:--
_a._--BARREL MINE. _b._--BROOK'S MINE. 3.--SINGER'S MINE. 4.--MCEVOY'S IMPROVED MINE. 5.--EXTEMPORE MINE.
_Frame Torpedoes._--This form of defensive mine is shown at Fig. 6. It consists of a frame work which is formed of four strong timbers _a_, _a_, _a_, _a_, these being kept parallel and only a few feet apart by means of cross timbers _b_, _b_. A cast-iron torpedo _c_, _c_, _c_, in the shape of a shell, is bolted to the head of each of the timbers _a_, _a_, _a_, containing about thirty pounds of fine grained gunpowder, and fitted with a percussion fuse, which is so placed that it would come into contact with a vessel striking against the framework, directly or not. One end of the frame is securely anchored, the other, that on which the torpedoes are fixed, is kept at its proper distance below the surface of the water by means of chains, _d_, _d_, and anchors. To prevent the frame from sinking when sodden with water, the uprights _e_, _e_, are provided.
This form of mechanical mine, which performs the double function of torpedo and obstruction, was much used by the Confederates, and found extremely useful, no passage was attempted to be forced by the Federals where these torpedoes were known to be placed.
_Stake Torpedo._--Fig. 7 represents another form of the frame torpedo.