In the heart of the rock, underground passages, chambers, and galleries have been excavated, and the whole island has been turned into an impregnable fortress. The many batteries are invisible from the sea, and the plateau on top of the island has been made bomb-proof. Only on the north side of the island can the cliffs be scaled by an invader, and the possible landing-places are all commanded by guns. On the highest point of the island--245 feet above the sea--are a lighthouse and a wireless station. Hangars for Zeppelins have been built on the plateau. These sheds are very cleverly constructed. They can be revolved so that the air-ships in them can be brought to the entrance, head to the wind, and, if necessary, they can be sunk into a valley out of sight of the sea. There is a large harbour for destroyers and submarines at the eastern end of the island, and also a small dockyard for repairing light craft. When Heligoland passed into German hands a Russian soldier said that thenceforth a blockade of the North Sea German coast would be extremely difficult. A British blockading fleet would not only have to expect attack from the front, but both its flanks would be constantly threatened. Thus the German vessels would be able to slip out, make raids on the estuaries and ports of the east coast of Britain, and attack British ships in their own waters. We shall see later that this prophecy came true. Meanwhile the Germans strewed their own coast with mine-fields, and thus made it almost impossible of attack. * * * * * Immediately war broke out our Grand Fleet disappeared. It melted into space, as it were, and nothing was seen of it but the ships patrolling the coast. But though a thick veil was drawn over its movements, it made itself felt at once. It forced the Germans to keep their most powerful ships in harbour, and it put an end to all talk of invasion. In the year 1910 Sir Arthur Wilson, who was then First Sea Lord of the Admiralty, said that the really serious danger that we had to guard against in war was not an invasion of our shores, but the stoppage of our trade and the destruction of our merchant shipping. Our overseas trade is extremely important, and the destruction of our merchant shipping would, as you know, rob us of our food and compel us to starve or surrender. The Germans know this very well, and just before the war they sent out cruisers and armed liners to fall upon our peaceful merchant ships and sink them. [Illustration: Sir John Jellicoe on board his flagship, the Iron Duke. _Photo, Alfieri._]