=4. Hydroplanes.= A brief mention should be made of hydroplanes, or sea-planes, which, although usually equipped only with the pontoons that enable them to alight upon the water, are equally fitted for a landing on terra firma by the super-addition of wheels. Most of the British attacks upon the German airdromes, encampments, and fortified lines on or near the Belgian coast have been made from hydroplanes. These machines have proved of great value for patrolling the coast against submarines. The aviators can see the submarines at a certain depth under water, and following in pursuit, they attack them by dropping special bombs, which, like those used by destroyers when passing over a submarine, are so constructed as to explode at a certain depth by water pressure even if they do not strike their target. The force of the explosion is sufficient within a radius of many metres to disjoint the plates of the submarine. =5. Balloons--Zeppelins.= At the outset of the war the Germans had a marked superiority in dirigible balloons. They had then already completed their particular type of rigid dirigible balloon, the Zeppelin, which they have since improved and multiplied to the full extent of their ability. Only at sea have they used them for strictly military purposes, observing very advantageously by their means the British fleet and flotillas. In the fighting on the Western Front they have used their Zeppelins but once, in the course of their attempt upon Verdun, when an effort was made to destroy the Paris-Verdun railroad track. Two days before the attack they dispatched a couple of the great airships on this errand, but one was brought down and the other was driven away before they could accomplish their mission. All the other Zeppelin expeditions on the Western Front have been made not against combatants but against towns. While they have caused a great many casualties among the civilian population in the Allied countries, a large number of them have been shot down. France has used for distant expeditions a few non-rigid dirigibles and has lost several of them. England, for the protection of the Irish sea and the Channel, uses some very swift little dirigibles which are very easily handled and form an excellent submarine patrol, but as fighting units they are worthless and are obliged to flee from hostile airplanes. CHAPTER III TRENCH ORGANIZATION 1. General remarks. 2. General plan of an intrenchment system. Trenches. First and second lines. Trenches of attack. Artillery. Wires. 3. Mines and counter-mines. 4. Special railway troops. Transportation by roads. 5. General remarks on transportation. 6. Camouflage.