"Nevertheless," replied Frank, "the second might not be a miss. Keep your eyes open." "I'll keep them open, never fear," said Jack. "Now, you go to bed." "All right," said Frank, and left his friend alone. CHAPTER VIII. HELIGOLAND. "Off there," said Lord Hastings, pointing over his shoulder, "lies Heligoland, one of the strongest harbors in the world, and regarded by the Kaiser and his subjects as impregnable. A raid by an enemy has been deemed as impossible by strategists." "Nevertheless," said Frank drily, "it is not impossible, as the Kaiser and his subjects will find out." "As I understand it," said Jack, "Heligoland is a natural stronghold." "To a certain extent, yes," replied Lord Hastings. "Heligoland, as you know, is an island, and nature has done her best to make it immune from attack. To nature's work has been added the brains and brawn of the best German strategists and workers. An attack by a hostile battle fleet could have but one result--failure. But, so far as I have been able to determine, there as yet has been nothing devised that will ward off the attack of a submarine." "Except mines," said Frank. "True. But it is certain there can be no mines in the harbor proper, for they would be an eternal menace to the German fleet. Of course the entrance is strongly guarded by mines and the powerful guns of the forts. But it is our business to get beneath these and torpedo a few of the enemy's vessels before we are discovered." "And then?" asked Jack. "Why, then," said Lord Hastings, "we shall make off as fast as possible to return at some future date, perhaps, and pay our respects once more. As I see it, there is but one thing that is likely to cause us any trouble." "And that, sir?" asked Frank. "The enemy's submarines," replied his commander. "But we have the heels of them, sir," said Jack. "True," replied Lord Hastings, "but the trouble is we are liable to run into a net of them, and in that event we would have to fight. To my knowledge, there has never been a battle of under the sea vessels, and what the result might be it is impossible to determine." "Well," said Frank, "we shall have to take a chance. That's all." "That's all," agreed Lord Hastings. Lord Hastings turned to the chart of the harbor of Heligoland and bent over it eagerly. The lads peered over his shoulder. "We are down as deep as it is safe to go," said the commander of the submarine. "The chart shows that the water is not so very deep here, and as it is all the guide we have, we must be careful." He turned to Jack. "Proceed at five knots," he commanded. Jack gave the necessary command, and the D-16 slowed down perceptibly. For many minutes there was intense silence, broken at last by Lord Hastings. "I should say," he said, "that we must now be in the harbor. I am positive we have passed under the mines safely." "Then shall we go up, sir?" asked Frank eagerly.