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