Tug, tug! Phil thought she saw Madge's air line give two desperate jerks. Two pulls at the line was the diver's signal for more air. Phil knew that without a doubt. Yet Philip Holt seemed to be pumping vigorously. At least, he had been only the second before when Phil last looked at him. Again Phil saw Madge's air line jerk twice. Tom Curtis and the two men in Captain Jules's boat were vainly trying to interpret some signals that Captain Jules was making to them. The two boats were at no great distance apart. "I am afraid something is the matter below, Phil," Tom Curtis turned to mutter hoarsely. But Phyllis Alden, who had been sitting near him a moment before, was no longer there. Phyllis believed she saw that Philip Holt was only pretending to pump sufficient air down to Madge. She may have been wrong. Who could ever tell? But Phil knew there was no time to discuss the matter. One minute, two minutes, five or ten--Phil did not know how long a diver at the bottom of the water can be shut off from his supply of fresh air and live. She did not mean to wait, to ask questions, or to lose time. Phil made a flying leap from the skiff that held her to the one in which Philip Holt sat by the air-pump. She landed in the water, just alongside the boat. Quietly, though more quickly than she had ever moved before in her life, Phil climbed into the boat and thrust Philip Holt away from the air pump. In the minute it had taken her to make her plunge she had seen Madge's signal again, but this time the line jerked more feebly than it had before. Phil set the pump to working again; the signal answered from below, "All is well!" The tender had recovered from his attack of faintness and resumed his work at Madge's airline. But Philip Holt sat crouched in the bottom of the boat, his face white with anger. What would Phyllis Alden's action suggest but that he was trying to suffocate Madge in the water below? Whether or not Philip Holt meant to stifle Madge Morton he himself never really knew. The impulse came to him as he placed his hands on her air-pump. It flashed across his mind that it was Madge who had tried to injure his prospects with Mrs. Curtis, and who had kept him from going down with Captain Jules to search for the pearls that he firmly believed would be found at the bottom of the bay. It was while these thoughts passed through Philip Holt's mind his pressure on Madge's air-pump had wavered. But Phyllis Alden had discovered it. She gave him no opportunity either for action or regret. CHAPTER XVI A STRANGE PEARL Madge felt herself in a great fairy world peopled with giants. Every thing below the water is magnified a thousandfold. Slowly she went down and down! The fishes splashed and tumbled about her, hurrying to get away from this strange, new sea-monster that had come into their midst.