The mines were dropped every three hundred feet in lines five hundred feet apart, as it was unsafe for the mine-layers to steam any closer to one another than that. The mines were of the type shown in Fig. 22 and automatically adjusted themselves to various depths. The depth of the water ran down to twelve hundred feet near the Norwegian coast. Never before had mines been planted at anywhere near that depth. It was dangerous work, because the enemy knew where the mines were being planted, as neutral shipping had to be warned months in advance. The mine-layers were in constant danger of submarine attack, although they were convoyed by destroyers to take care of the U-boats. There was even danger of a surface attack and so battle-cruisers were assigned the job of guarding the mine-layers. The mine-layers steamed in line abreast, and had one of them been blown up, the shock would probably have been enough to blow up the others as well. Enemy mines were sown in the path of the mine-layers, so the latter had to be preceded by mine-sweepers. Navigation buoys had to be planted at the ends of the lines of mines and the enemy had a habit of planting mines near the buoys or of moving the buoys whenever he had a chance. But despite all risks the work was carried through. The barrier was not an impassable one. With the mines three hundred feet apart, a submarine might get through, even though the field was twenty-five miles broad, but the hazards were serious. Before the first lines of mines had been extended half-way across, its value was demonstrated by the destruction of several U-boats, and as the safety-lane was narrowed down the losses increased. It is said that altogether twenty-three German submarines met their doom in the great mine barrage. U-boat commanders balked at running through it, and U-boat warfare virtually came to a standstill. According to Captain Bartenbach, commander of submarine bases in Flanders, three U-boats were sunk by anchored mines for every one that was destroyed by a depth bomb. CHAPTER XV SURFACE BOATS The war on the submarine was fought mainly from the surface of the sea and from the air above the sea, and naturally it resulted in many interesting naval developments. As described in Chapter XIII, the first offensive measure against the U-boat was the building of swarms of speedy motor-boats which drove the invaders away from harbors and into the open sea. To follow the U-boats out into rough water larger submarine-chasers were built, but even they could not cope with the enemy far from the harbors. MOTOR TORPEDO-BOATS The Italians made excellent use of speedy motor-boats in the protected waters of the Adriatic Sea. One type of motor-boat was equipped with two torpedo-tubes in the bow. Small 14-inch torpedoes were used, but as each torpedo carried two hundred pounds of high explosive, the motor-boat was a formidable vessel if it crept in close enough to discharge one of these missiles at its foe.