A harbor which has been planted with mines to keep out enemy ships or blow them up if they try to enter it is often remined overnight by enemy submarines, and in this way a great deal of damage can be done to friendly shipping.
But whether the submarine is used for mining its own or enemy harbors, the outstanding feature of its work is that it does it all under water and therefore the operations cannot be seen by spying eyes.
As we said in [Chapter III], the mines are stored in a compartment in the hull of the submarine and this can be shut off from the other compartments by a bulkhead door. It also has a hatch opening through the hull to the water outside, just as the door of a kitchen opens into the back yard.
When the submarine has made its way under water to the place where the mines are to be planted, the hatch is opened and the mine compartment is allowed to fill with water. A mine layer in a diving suit can then get in and out of the flooded compartment, take the mines one by one through the open hatch, and place them in position.
Then there are mines which do not need to be set by a mine layer; these are simply dropped through the hatch into the water and adjust themselves as to depth. Other mines are made that are shot from the torpedo tubes by compressed air and these are used for mining an enemy harbor where the risks of having the submarine blown up is altogether too great to take the chance.
Kinds of Submarine Mines.—Submarine mines—that is, mines that are planted in harbors and other seaways, either as a protection from enemy ships or as an offensive measure to blow them up—really have nothing to do with submarine boats except that the latter are used to lay them. But you ought to know about submarine mines, anyway, and so we’ll digress a little and tell you something of them.
First of all, there are two distinct kinds of mines and these are, (1) contact mines, and (2) electrically controlled mines.
As you can tell by its name a contact mine is one that is exploded by the hull of a ship coming in contact with it—that is, by running against it. This is the kind of mine that is most often laid by the submarine.
An electrically controlled mine is one that is exploded by electricity. For this purpose a pair of wires are connected to it and these lead along the bottom of the harbor to two observation stations on the shore; when the enemy ship gets directly over the mine the observers close the electric circuit and the mine is exploded.
This is the way the Maine was sunk in Havana harbor in 1898, but the Maine was not an enemy ship. The result of this rash bit of foolishness led to the war between the United States and Spain, and the loss of her island possessions.