The French admiral was greatly astonished, but when Lord Hastings told him of his course and assured him that the D-16 was capable of all he claimed for her, the admiral was delighted. "Then you may really be of assistance to me," he said. "I shall be glad to aid in any way possible," declared Lord Hastings. "You have but to command me. Consider me under your orders for the next twenty-four hours." "In that event," said the French admiral, "I wish that you would try and creep into the harbor and learn what is going on. 'Forewarned is forearmed,' you know." "I shall be glad to do so." Lord Hastings bade the admiral goodbye and returned aboard the D-16 immediately. When Frank and Jack learned that there was work ahead of them, both at once became very enthusiastic and could hardly wait to be on their way. "We may as well submerge right here and then advance," said Lord Hastings. The order was given and the D-16 disappeared from the sight of the other ships. Then she moved forward slowly. Frank, at his place in the lookout compartment, kept his eyes wide open for the sign of an enemy, or of the enemy's mines. Suddenly a dark object appeared directly ahead of him and, swerving quickly, dashed by before he could give the alarm. Immediately he informed Lord Hastings and the D-16 was brought about quickly and headed after the object. "I don't know whether it is an enemy or not," said Lord Hastings, "but we can afford to take no chances. We'll have to go after it." The D-16 dashed on, but after half an hour saw no sign of what all believed to have been an Austrian submarine. "Might as well go up and take a look about," said Lord Hastings. "We should be among the French fleet again." And among the French fleet they were, as they learned as soon as they bobbed up on the surface. But now the air of quiet that had been prevalent before the D-16 submerged was changed. The peaceful appearance of the French fleet, which had been lying quietly in the water, was gone. As the three officers stepped upon the bridge, hoarse cries of command came to their ears. Battleships began to move from their moorings, and all were cleared for action. "Great Scott!" ejaculated Frank. "What do you suppose is the matter?" "I don't know," said Lord Hastings, but Jack cried: "Look!" Lord Hastings and Frank gazed in the direction indicated, and both cried out in alarm and amazement. Not half a mile away a French cruiser of the first class was sinking by the head. Members of her crew were throwing themselves into the sea, and boats from other ships were standing by to pick them up. "What do you suppose is the matter? Explosion?" asked Frank. "Looks like it," answered his commander. "I----" But Jack supplied the answer. "The Austrian submarine that passed us!" he exclaimed. "She is responsible for this." "By Jove!" exclaimed Lord Hastings. "You must be right." At this moment the French Admiral signalled Lord Hastings.