But the water had by this time grown dark and oily-looking. The approaching squall would burst in all its fury in a few minutes. The work of saving the swimming lad must be accomplished within a brief few minutes, or not at all. "Hold on, my lad, we'll get you," hailed the ensign encouragingly, as the wherry drew closer and closer to the plucky boy. "Aye, aye, sir," hailed back Herc, expelling a thin stream of water from his lips and giving a cheerful grin; "but hurry up, for I've forgotten my lightning-rod, and it looks like thunder." But, just as Herc's easy rescue seemed a matter of certainty, the intentions of his saviors were interfered with in a startling fashion. It was Ned who saw the impending peril first. "Look! Look there!" he shouted. "What's that, sir?" "That" was a black, triangular object, moving through the water toward the unconscious Herc, who was treading water easily. The dark object came on at a rapid pace, the ripples parting on each side of it as it cut its way along. The ensign's reply to Ned's exclamation was a cry of alarm. "Give way!" he shouted. "We've got to get that man quickly, if at all." Ned looked his question. "It's a shark!" shot out the ensign, his face ashy-white and his lips sternly compressed; "these waters swarm with them." Ned was almost unnerved. The boat was still some feet from Herc, and the triangular fin was now close upon the lad. Suddenly its steady motion ceased, and it shot forward with a rush. At the same instant Herc perceived his peril, and gave one harrowing shriek, as he saw the terrible nature of the approaching peril. He swam desperately toward the boats, his countenance strained and lined with the effort and the horror under which he labored. "Crack!" The sharp bark of a service revolver sounded. "Crack! crack!" Again and again the reports reverberated, and the water behind Herc grew troubled and crimson. The fin vanished and only a small whirlpool remained to show where the mortally wounded shark had sunk slowly downward. In the stern of the wherry stood Ned, his face set and stern, and in his hand the navy revolver that had done the work. It was the ensign's weapon, which he had laid on the stern seat for his greater ease in moving about. Ned, casting about for some means of saving Herc, had suddenly spied it, and, on the impulse of the moment, had snatched it up and fired. "Well done, my lad," said the ensign in a voice that still trembled from the keen tension of the past few minutes. "Sir--I----" began Ned, somewhat alarmed, now that Herc was out of danger. He had committed what he knew must be a breach of discipline in seizing the officer's pistol. "You mean that it wasn't quite the thing to do to use my revolver," laughed the ensign. "My lad, I'm proud that it was put to such good service; glad that you were quick enough of wit to use it in the nick of time."