It would naturally be supposed that a submarine carrying eight or more torpedoes is about as destructive an engine as human ingenuity could devise; but not so, for there are other munitions of warfare with which it is equipped that make it even deadlier.

Now, the word munitions means war materials of every kind—except, of course, men and money; but on a submarine the munitions are limited to (1) the automobile torpedo, which we have described in the last chapter; (2) rapid-fire machine guns, and (3) submarine mines.

In the present war Germany has undertaken the gigantic task of sinking all the merchantmen, not only of the Allies but of the neutral countries as well, and it is this U-boat policy which has brought her into a state of war with nearly every civilized country on the face of the earth.

As the torpedo is such a costly weapon, it would not do to waste it on sinking any but the largest ships, or those carrying valuable cargoes; but the same policy of Germany, which is to starve out the Allies, also calls for sinking the smaller craft, such as trawlers and unarmed merchantmen, and this is done by means of rapid-fire machine guns.

These guns are mounted fore and aft on the deck of the submarine, and they are used to bring down small ships and also to enable the undersea craft to defend herself against submarine chasers, airplanes, or occasionally when she is surprised by a destroyer or other armed ship which may get within firing range and attack her before she has time to dive.

The submarine mine is as old as, or older than, the submarine boat itself; and it was probably the idea of fixing a bomb onto or planting a mine under a ship that led to the actual building of the first underwater craft.

The mine is not, however, an explosive destroyer which is used by the submarine boat; for she could never get close enough to an enemy ship in these days of electric eyes and ears to make use of one. What the submarine does do is to play the peaceful rÔle of planting the mines—not with the idea of making two grow where only one grew before but of making one ship less where there was one ship more, and this is done by blowing up the enemy ships which may chance to pass over them.

Arming the Submarine with Guns.—From what has been said above, you will see that by the use of rapid-fire guns poor prizes can be easily and cheaply sunk; further, a small craft always makes a bad target for a torpedo and thus the odds of making a miss and losing a torpedo are very considerable. The use of rapid-fire guns is, then, a very good stroke of business, everything considered.

Again, when a submarine is chased and has no time to dive, she is in a bad way, for she is not fast enough to escape, nor can she stop long enough to aim a torpedo at her foe. It is easy to see, then, why her armament[23] should and does include a couple of rapid-fire guns.

The Need of a Quick-Action Gun.—With these conditions staring them in the face, the designers of submarines have worked hard to equip their craft with not only rapid-firing but, what is equally as important, quick-action guns.

To get a rapid-fire machine gun was easy, for this type had been built by the Maxims these many years; but to get one that could be swung or lowered under the deck so that it would not offer resistance to the water when the submarine was running submerged, and then brought up and into action quickly, was quite a different thing, and inventors are still at work on it.

By this I do not want you to take it that submarines are not provided with rapid-fire and quick-action disappearing guns at the present time; and should you or I take a look at them we would say that they are 99 per cent. perfect and that there is no need for further improvement. But that is because we are neither designers, nor builders, nor officers of undersea craft; and do not forget, either, that better things are always in order.

Now, there are two kinds of rapid-fire, quick-action guns used for submarine armament and these can be told from each other by the way they are brought into action. They are (1) the spring action gun, and (2) the compressed-air action gun.

The Spring Action Gun.—This type of submarine gun is made by the Krupps of Essen, Germany, who now are building the U-boats for the Central Powers.

This gun is mounted on the deck, over an opening in it called a deck-well, and this arrangement allows the gun to be stowed away in it. This is done to get rid of the resistance the water would have on the gun if it remained fixed on deck when the boat is running submerged.

Although the deck-well is closed with hatches it is not watertight, and the fittings, such as the sights and range finders, are taken off to keep them from rusting when the gun is in the well.

A diagrammatic sketch of the gun and its housing is shown in Fig. 46; the gun is mounted on a swinging frame by trunnions, and the frame is fixed to a pedestal. A strong spring in the bottom of the well supplies the power needed to throw the gun up and bring it into position.

When housed in the well the gun lays on its back, as shown by the dotted lines, and this presses down on the spring. The tension of the spring is enough to force the gun up and into place the instant the catch which holds it is released.

The sights and other attachments are fixed on the gun after it is brought into position and the whole operation of raising the gun from its well and putting on the fixtures takes just 20 seconds.

These guns have a bore of 75 millimeters[24]—equal to 3 inches—and 88 millimeters—equal to 3½ inches—respectively, and the shells they use weigh in the neighborhood of 25 pounds.

The Compressed Air Action Gun.—This kind of submarine gun is used on British undersea boats. It is mounted over a deck-well but inasmuch as the gun is lowered into the well with all the fittings on it the well has to be made watertight.

If you will look at Fig. 47, you will see how the gun is brought into action. When compressed air is turned on in the cylinder, it forces the plunger on which the gun is mounted up above the level of the deck. The gun is fitted with fixed sights and a range finder, and is ready for use the moment it has been brought into place above the deck, the whole operation taking only 5 seconds.

The guns of the Allies’ submarines have bores of 3 and 4 inches, and each one weighs nearly a ton. As a protection from enemy aircraft they have a wide range and can, if needs be, fire a shot at 90 degrees, that is, straight overhead.

How a Submarine Lays Mines.—The dangerous work of laying mines in an enemy harbor by a submarine is not as spectacular as torpedoing ships but it is an important part of the business of undersea craft.

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.

How the Mines Are Made.—The contact mine.—This kind of a mine consists of a steel shell, or container, which holds the charge of high explosive, the trigger mechanism which explodes it when a ship strikes it, and the weights and cables needed to sink the mine to the right depth. Air chambers are also placed inside the container, to give the mine buoyancy; otherwise it would sink at once to the bottom of the sea.

In the contact mine the detonator is formed of an arm, or lever, called a striker, and this projects through the shell to the outside. When a ship strikes the arm it drives a firing pin against a percussion cap and explodes it, and this in turn fires the explosive charge in the mine. (See Fig. 48.)

How the Contact Mine Works.—The way the mine works is like this: Before it is taken out of the compartment of the submarine it is adjusted to float at the proper depth.

When the mine is in the water a weight that it carries is released and sinks to the bottom; the cable which connects the weight and the mine together holds the mine in the right position, while the air tanks in the mine make it rise as high as the cable will let it go, as shown in Fig. 49.

After the mine is planted, the submarine steals silently and invisibly away. When an enemy ship—or any other, for that matter, for the contact mine is no respecter of ships—strikes the trigger of the mine, it explodes the charge, and there is one less ship to sail the turquoise seas.

The Electric Controlled Mine.—To the end that a harbor can be planted with mines which will blow up enemy ships and yet be harmless to friendly ships, mines fired by electricity from the shore were invented.

Mines of this kind are made about like contact mines in that they contain an explosive charge and a detonator, but they are much larger and far more powerful, for they have to be laid on the bed of the harbor, and instead of being fired by a pin striking a percussion cap they are fired by an electric spark.

From each mine laid in the harbor a pair of insulated copper wires run to the shore, and this electric circuit connects with two shore stations at a distance apart and which with the mine form a triangle. The electric circuit has a key in it at each shore station, and to explode the mine both of these keys must be pressed down at the same time.

Each shore station has a telescope fixed in position so that its line of sight (shown by the dotted lines in Fig. 50) passes directly over the mine. Now when an enemy ship approaches, each observer will see it when it sails on or across his line of sight and he will press his key. But it is necessary that both observers see the ship at the same time that it is at the point where their lines of sight cross, and consequently directly over the mine, and when this happens they will both press their keys, the circuit will be closed, the electric current will fire the mine, and the ship will change her course to one straight up and speed with all possible haste to the port whence no ship ever clears again.