On land, however, it seemed that neither side had gained much in the last few months. What advantage there was seemed to have rested with the Germans, for the Kaiser's mighty drive through the Balkans had completely crushed heroic little Serbia and had opened a path to Constantinople for the German hordes. Also it had brought Bulgaria into the war on the side of the Central Powers, giving them the armed weight necessary to keep Greece and Roumania from throwing their support to the Allies, much as they would have liked to do so. This was the situation, then, when the D-17 steamed slowly toward the German naval fortress of Heligoland. As Lord Hastings, standing by Frank at the wheel, gave the course, avoiding mines as well as he could by the chart that had somehow come into his possession, Jack retained his place forward in the glass-like cage, his eyes steadily ahead. For an hour, perhaps, he sat there, seeing nothing that indicated danger. Then, suddenly, his right hand shot out, his finger touched a little push-button; the throb of the engines ceased as if by magic and the D-17 seemed to halt in her track. Jack approached Lord Hastings. "Mine, sir," he said, quietly. CHAPTER II. WITHIN THE FORTRESS. "Good work, Jack," said Frank, approvingly. "What do you advise?" asked Lord Hastings. "Up or down?" "Depends on the depth, sir," replied Jack. "We'll have a look at the chart," said Lord Hastings. He spread it out before him and the three pored over it for several moments. "Here you are, sir," said Frank, at last, placing a finger on the chart. "Plenty of water," was Lord Hastings' comment. "Submerge another five fathoms, Mr. Templeton." Jack gave the order and then returned to his post. The D-17 submerged another five fathoms and proceeded at a snail's pace. Jack kept his eyes glued to the course ahead. Half an hour later the vibrations of the engine ceased. Then Jack made his way aft to where Lord Hastings and Frank were conversing. "Unless I am badly off in my calculations," said Lord Hastings, "we should now be able to come to the surface beyond the mine field." "And as soon as we do, some German warship will send us to the bottom," remarked Frank dryly. "So they will--if they see us," returned his commander. "We must make it a point that they do not see us. That's all." "All very well, sir," said Jack, "but if we go up now they are sure to see us--and they won't mistake us for one of themselves this time." Lord Hastings smiled quietly. "Then we won't go up now," he replied as he looked at his watch and noted that it was half past five. "It'll be dark in less than an hour. We'll remain here until then." This ended the conversation and the commander of the D-17 went to his cabin, leaving Jack in command.