LOCATION OF MINES.
The considerations involved in the location of mines are of two general classes, tactical and local.
Tactical considerations deal with the position of mines with reference to the other defenses. Local considerations deal with the width and depth of the channel, the swiftness of the current, the variation of the tide, and the relative importance of the harbor.
Where ordinary ship channels are unobstructed it is possible for modern battleships, with their high speed and heavy armor, to run by shore batteries, at least in the night or during a fog; hence the defense of such channels should not be left to guns alone.
On the other hand, where mines are unprotected by the fire of shore batteries it is possible for an enemy to remove or disable them.
Therefore guns and mines, the two elements of the fixed defenses of a harbor, are mutually dependent, and when the location of one has been decided upon that of the other must conform thereto.
Within the zone between 4,000 and 8,000 yards of the main defense the fire of heavy guns is destructive for warships, yet the latter are at such a distance that their rapid-fire guns will be of little effect against the batteries.
Moreover, at 4,000 yards vessels are just beyond the inner limit of mortar fire.
If possible, therefore, hostile vessels should be held in this zone by some obstacle. Such obstacle is afforded by a mine field.
On the other hand, attacks upon a mine field are most liable to be made by small boats at night. If the mine field be at too great a distance from the defenses, these boats will not be revealed by the mine searchlights. Furthermore, for protection against such attacks, the defense relies upon rapid-fire guns of relatively limited range.
Due to the above considerations the outermost mines are usually placed between 3,000 and 4,500 yards from the main defense.
In general, there should be in each main channel at least three lines of mines.