We found in the Diesel engine that when air is highly compressed it becomes very hot. When compressed air is expanded, the reverse takes place, the air becomes very cold. The air that drives the motor of the torpedo grows so cold that were no precautions taken it would freeze any moisture that might be present and would choke up the engine with the frost. And so an alcohol flame is used to heat the air. The air-motor is started automatically by release of a trigger as the torpedo is blown out of the torpedo-tube. By means of gearing, the motor drives two propellers. These run in opposite directions, so as to balance each other and prevent any tendency for the torpedo to swerve from its course. The torpedo is steered by a rudder which is controlled by a gyroscope, and it is kept at the proper depth under water by diving-rudders which are controlled by a very sensitive valve worked by the weight of the water above it. The deeper the water, the greater the weight or pressure; and the valve is so arranged that, should the torpedo run too far under, the pressure will cause the diving-rudders to tilt until the torpedo comes up again; then if the torpedo rises too high, the valve will feel the reduction of pressure and turn the rudders in the other direction.
The business end of a torpedo is a "war-head" packed with about four hundred pounds of TNT. At the nose of the torpedo is a firing-pin, with which the war-head is exploded. Ordinarily, the firing-pin does not project from the torpedo, but there is a little propeller at the forward end which is turned by the rush of water as the torpedo is driven on its course. This draws out the firing-pin and gets everything ready for the TNT to explode as soon as the firing-pin is struck. But the firing-pin is not the only means of exploding the torpedo. Inside there is a very delicate mechanism that will set off the charge at the least provocation. In one type of torpedo a steel ball is provided which rests in a shallow depression and the slightest shock, the sudden stopping or even a sudden swerve of the torpedo, would dislodge the ball and set off the charge. Hence various schemes, proposed by inventors, for deflecting a torpedo without touching the firing-pin, would have been of no value at all.
GUNS ON SUBMARINES
As torpedoes are expensive things, the U-boats were supplied with other means of destroying their victims. The Germans sprang a surprise by mounting guns on the decks of their submarines. At first these were arranged to be lowered into a hatch when the boat was running submerged, but later they were permanently mounted on the decks so that they would be ready for instant use. They were heavily coated with grease and the bore was swabbed out immediately when the boat came to the surface, so that there was no danger of serious rust and corrosion. The 3-inch gun of the early months of the war soon gave way to heavier pieces and the latest U-boats were supplied with guns of almost 6-inch caliber and there was a gun on the after deck as well as forward.
The U-boats depended upon radiotelegraphy to get their orders and although they did not have a very wide sending-range, they could receive messages from the powerful German station near Berlin. The masts which carried the radio aërials could be folded down into pockets in the deck. From stem to stern over the entire boat a cable was stretched which was intended to permit the U-boat to slide under nets protecting harbor entrances, and in later boats there were keen-toothed knives at the bow which would cut through a steel net. During the war German and Austrian U-boats occupied so much attention that the public did not realize the part that the Entente Allies were playing under the sea. America, Great Britain, France, and Italy made good use of submarines, operating them against enemy vessels, blockading enemy ports, and actually fighting enemy submarines.
A STEAM-DRIVEN SUBMARINE
The British in particular did splendid work with the submarine and developed boats that were superior to anything turned out by the Germans. For instance, they developed a submarine which is virtually a submersible destroyer. It is 340 feet long and it can make a speed of 24 knots on the surface. The most remarkable part of this boat is that its engines are driven by steam. Its boilers are fired with oil fuel. There are two smoke-stacks which fold down when it submerges. Of course when running under-water the vessel is driven by electricity and it makes a speed of 10 knots. It carries three 4-inch guns, two forward and one aft, and its displacement submerged is 2700 tons as against 800 tons for the largest German submarines.
A SUBMARINE THAT MOUNTS A TWELVE-INCH GUN
Still more remarkable is the big "super-submarine" designed by the British to bombard the forts of the Dardanelles, but unfortunately it was built too late to be used there. This submarine carries a gun big enough for a battle-ship. It is of 12-inch caliber and weighs 50 tons. Of course a big gun like that could not be fired athwart the submarine. It might bowl the little vessel over, even though it was a 1700-ton submarine. The gun is mounted to fire fore and aft, with a deviation of only a few degrees to one side or the other, so that the shock of the recoil is taken by the length instead of the beam of the submarine. It fires a shell weighing 620 pounds and a full charge is not used, so that the extreme range is only about 15,000 yards. This submarine monitor would have been a very difficult target for the Turkish gunners to hit.