The two lads, since the war began, had seen service in many waters. They had been on patrol duty off the west coast of Africa; they had served under the French flag when, under the tricolor, they had delivered a severe blow to the Austrian fleet in the Adriatic; they had trailed the German cruiser _Emden_, nicknamed the "terror of the sea," through the Indian ocean, and had been present when she was finally sunk by the Australian cruiser _Sydney_; they had taken part in sinking the German fleet in the South Atlantic, off the Falkland islands; they had been aboard a British submarine that sank three Turkish cruisers in the Persian Gulf; they had seen the capture of the German fortress of Tsing-Tau, in China, by allied British and Japanese troops, and finally they had been instrumental, while in London, of exposing a plot that would have been a severe blow to Great Britain, and of capturing a second German spy and a British traitor, who stood high in the regard of Winston Spencer Churchill, first lord of the Admiralty. Three times the ships to which they were attached had been sunk, and they had had many narrow escapes. Once Lord Hastings had gone down with their vessel, and the lads had believed him drowned, but he escaped and they encountered him weeks later. When their last craft had been sunk, upon the boys' suggestion, Lord Hastings had requested command of a submarine, and when they had once again set forth, it was in the D-16. And as the D-16 was something absolutely new in the way of submarines, a few words concerning it are necessary here. CHAPTER II. THE D-16. The British submarine D-16 was, at this time at any rate, the only under-the-water vessel that could remain under the sea indefinitely. The one real weakness of beneath-the-sea fighters had always been their inability to remain long under water, and for this reason they could operate only within a certain radius of their base. In discovering that there was a means by which a submarine could remain indefinitely under water, Sir John H---- had overcome this difficulty. The new invention had been tried on the D-16, and it had worked. According to Sir John, a submarine in order to remain submerged indefinitely, must be able to extract air from the water, as does a fish, for it is air that is needed most under water. Up to the time of Sir John H----'s discovery this had never been accomplished. Reasoning that it must be a peculiarity about the gills of a fish that permitted it to extract air from the water, Sir John had experimented along this line, and his experiments finally terminated successfully. When the D-16 had proved the practicability of Sir John's theory, the vessel had immediately been put into commission, and the construction of others begun.