"Don't do it, Madge!" expostulated Phil; but she was too late; Madge had dived and was swimming along almost completely under the water. She swam in the darkness cast by the shadow of the boat as it passed within a few yards of them. Like a flash she lifted her great rubber ball. She had better luck than she deserved. The ball came out of nowhere and landed in the center of the group of three young people on the yacht. It fell first on the deck, and then bounced into the lap of the offending Mabel. It was hard work for the waiting girls not to laugh aloud as naughty Madge came slowly back to them. A wild shriek went up from on board the yacht. "Oh, dear, what was that?" one girl asked faintly, when the first cries of alarm had died away. "Where is it? What was it?" growled a masculine voice. "Are you really hurt, Mabel? You are making so much fuss that I can't tell." Mabel had dropped back in a chair. She was white with fear and trembling violently. "It is in my lap," she moaned. "It may explode any moment--do take it away!" The owner of the yacht, Roy Dennis, turned a small electric flashlight full on his two girl guests. There, in Mabel's lap, was surely a round, globular-shaped object that had either dropped from the sky or had been thrown at them by an unknown hand. Roy had really no desire to pick it up without seeing it more clearly. The other girl was less timid. She reached over and took hold of Madge's ball. Then she laughed aloud. Oddly enough, her laugh was repeated out on the water. "Why, it's only a rubber ball!" she asserted. Ethel Swann, who was one of the old-time cottagers at Cape May, ran to the side of the boat. "See!" she exclaimed, "over there are some boys swimming. I suppose they threw the ball on board just to frighten us. They certainly were successful." She hurled Madge's ball back over the water, but Roy Dennis's small yacht had gone some distance from the group of mischievous mermaids and he did not turn back. "If I find out who did that trick, I surely will get even with them," muttered Roy. "I don't like to be made a fool of." "Don't tell Jenny Ann, please, girls," begged Madge, as the four girls clambered aboard the "Merry Maid." "It was a very silly trick that I played. I should hate to have the cottagers at the Cape hear of it. I don't suppose I shall ever grow up."