"Torpedoed by Austrian submarine," read the signal flags. Frank hastened to get out the D-16's flags, and in response to Lord Hastings' command, signalled the flagship: "She escaped us, but we'll get her." The flagship signalled "good luck" and Lord Hastings gave the command to submerge. "Unless I am much mistaken," he said, "the Austrian will make for the open sea. Probably she will make for the Mediterranean and attempt to sink some of our merchant vessels. They may have established a base some place." "I wouldn't be surprised if you are right, sir," said Jack. "But we'll get her." "We will," said Lord Hastings. "We'll get her if I have to chase her around the world." The D-16, with her periscope protruding slightly above the water, dashed on at full speed. CHAPTER XXIII. THE CHASE. Lord Hastings stood at the periscope. Frank was on duty in the lookout compartment. Every man was at his post and ready for instant action. Torpedoes were in the tubes and every man stood at attention, for there was no knowing at what moment they might come upon the enemy, and Lord Hastings had no mind to allow the enemy to fire the first shot. Suddenly, as Frank peered intently into the murky water, a blinding light dazzled his eyes; then, as suddenly, it was gone and all was black before him. The lad immediately summoned Lord Hastings, to whom he related this strange occurrence. The latter thought some time before replying: "There is but one possible explanation. There can be no such light beneath the sea except that furnished by the hand of man. The Austrian submarine must have some device similar to ours." "There can be no other explanation, sir." "And the result is that we shall have to be doubly careful," said Lord Hastings, as he returned to his post at the periscope. Suddenly Jack, who stood near Lord Hastings, was startled to see his commander leap back and utter a loud exclamation. "There she goes!" he cried. "Push her to the limit, Mr. Templeton." In response to Jack's order the D-16 leaped ahead faster than before. "Has he sighted our periscope, sir?" asked Jack. "No, I do not think so, but for a submarine, she is moving with great speed." "But not so fast as we are, sir?" "I should say not quite. We appear to be gaining a trifle." "She must be another surprise the enemy has been waiting to spring on us," said Jack. "Undoubtedly; and the only thing that gives us the advantage is that we know about what she can do and she has no idea what we can do; in fact not even that we are here." "But she is bound to spot us unless we submerge," said Jack. "True," said Lord Hastings, "but if we submerge she is likely to escape us unless we use our searchlight. I would avoid letting her know we are at least on anything like even terms, if possible."