Page:Popular Science Monthly Volume 88.djvu/51

Popular Science Monthly by the ship and causes the main charge to explode.

In order to make a mine field as effective as possible loose ropes are sometimes connected between different mines with the object of getting the ship's propeller entangled in the rope and thereby drawing the mine towards the ship and exploding it.

Mines of the type described are easily laid. When stowed away on the deck of a mine-laying ship the mine rests on the anchor which at the same time forms a little carriage to be run along the deck and simply dropped over the stern of the ship at the right moment.

Whether mines have actually been laid by submarines is, of course, known only to the naval authorities. Patents have, however, been taken out within the last few years for specially designed mines to be laid by submarines and also for providing submarine boats with a series of chambers on each side for holding and launching mines. These chambers are disposed between double walls of the submarine and are made to form a smooth outline with the hull of the boat. This provision makes it possible to carry a double cargo.

Unanchored automatic or floating mines must be dead in an hour. They are used to some extent in naval battles and are very cheap in construction. In some mines of this type clockwork is used which after an hour throws the firing gear out of action while in another type delay-action devices for opening valves to admit water are employed so that the mine is sunk after a definite time interval.

To some extent chemical methods are employed to fire the charge, in floating mines, but a disadvantage is that the explosion does not take place instantaneously as is the case with a mechanically fired mine. A glass tube is attached to the mine which is broken when struck by a ship; water enters and by coming in contact with sodium or potassium fires the charge. Other chemicals such as sulphuric acid have also been used to fire the charges in floating mines.