The How and Why of Submarine Mines
��At right: Dropping a mine. The cable is wound on a dnim having an escapement like that of a watch
��Below: Towing out electric cables to the mines which are controlled from the observatory en shore
���NOT the least important weapon for defending our coasts and our har- bors from the attacks of hostile war- ships is the submarine mine. Though it is perhaps the least ominous looking of naval defenses, if it is once hit by an invading warship, its tremendous explosive power will sink the ship in less time than any other weapon could.
The shock of the striking vessel automat- ically explodes the mine and the violently expanding gases from the powder crush in the hull of the ship in a twinkling.
Submarine mines are made of an outer casing of steel and an inner charge of high explosive. A separate chamber, con- taining nothing but air, gives the mine buoyancy. The mines are carried in small light-draft boats to the positions previous- ly mapped out. The boat simply drops the mines with their anchors overboard.
��When the cable is all --»».' paid out, the buoyant
mine will be floating about ten feet below the water's surface.
V The mines lying far out-
side of a harbor contain their ignition system with- in themselves. This may be a number of percussion- pins placed all around the outside of the mine. When a ship strikes one of these, the heat of the impact ignites the powder instantly. Or, an internal igni- tion system may be employed. When a ship hits the mine it rolls it over. This releases an iron ball which is ordinarily held in a small cup in the center of the mine. The ball falls out of the cup and pulls upon a string, releasing a trigger and firing off the charge.
There is another type of mine in which chlorate of potash is placed in the bottom and an open vessel of acid is placed above it. The mine is anchored well under the water. Ordinary wave action does not affect it. Disturbed by a passing ship, however, the acid spills over upon the chlorate causing a violent explosion.
The mines which are near the land are generally controlled from the shore through an elertric cable from the observatory.