At the risk of slipping off the platform, Lawless twisted his legs round the pillar supporting the bell and, leaning forward, caught hold of the chain and tugged at it with both hands. It came in quite easily at first, and then stopped with an abruptness that nearly jerked the Lieutenant into the sea. He could see nothing, but passing one hand down the chain he felt something at the end--something large and buoyant, with sharp square edges like a box or a tank. Also he knew by the feel that it was metal. Lawless forgot his own sufferings in contemplation of this puzzling discovery. What on earth, he asked himself, was the object of having a floating or submerged tank attached to this buoy? What did it contain? He pondered the matter for some minutes, and then, for the second time that night, had an inspiration. It must be a secret petrol tank for the use of German submarines! In a flash he divined the whole scheme. The bell-buoy, being such a well-known navigating mark, could not be mistaken for any other buoy in its vicinity. It was close to the English coast, and the tank attached to it could be replenished either by submarine petrol boats or by secret agents ashore--probably the former. On a dark night, such as this for instance, an enemy submarine could easily take on a fresh supply of petrol from the tank, while the latter, being submerged several feet below the surface, would not be seen in daylight. Doubtless, also, it was provided with an automatic arrangement whereby, when emptied of petrol, it became filled with sea-water, thus preventing it from floating on the surface and becoming visible. It was a most ingenious device, quite simple and, no doubt, an inestimable boon to belated enemy submarines which were running out of petrol. But the point was, having discovered this, how could he make use of it to the best advantage? Lawless was so engrossed by this problem that he forgot to wonder whether his scheme of muffling the bell would lead to his rescue until the sound of engines in the darkness aroused him from his contemplations. Then, realising that a boat of some sort was at hand, he shouted as loud as he could. There came an answering hail out of the darkness, and next moment the Lieutenant was momentarily blinded by the dazzling rays of a searchlight turned full upon him. "Ship ahoy!" he shouted. "Hullo there!" came the answer. "What the devil are you doing?" He recognised the voice of Sub-Lieutenant Trent and knew that it was the _Knat's_ searchlight which was dazzling him. Then, as the destroyer drew nearer, Trent, in his turn, recognised the soaked and huddled-up figure clinging to the platform of the buoy as that of his senior officer. "Stand by to jump aboard!" he yelled. With considerable skill he brought the destroyer round the buoy so close that her side grated against it, and Lawless, clutching at the deck-rail, swung himself on board.