"Well, good-night, old man," said Merritt, as Donald, who relieved him, came on duty, "and good luck." "I'll keep a good watch out, all right," was the earnest response. "It's our only way to get poor old Rob back." "I'm afraid so," sighed Merritt, leaving the place with a despondent air. As Donald had said, it was a chance--but what a long, seemingly hopeless one! Donald, left alone, began sending out calls, and every little while he paused for an answer out of space to his appeals. As he pressed the sending key the blue, lithe spark leaped and crackled between its points like a fiery snake. Then all would become silent again as he listened for an answer to his call. Once he caught a steamer bound north and carried on quite a conversation with its operator. He felt quite lonesome when he closed down his sending apparatus with a parting "good-bye." It was very still about the encampment. So still, in fact, that the boy began to feel more and more lonesome. He longed for someone to talk to; but he knew that chance would not come till Tubby, his relief, appeared. The stout youth was almost due when Donald suddenly got into communication with a steamer called the _Cambria_, bound north from New Orleans to New York. He put his customary query about the green motor boat. "A green motor boat?" came back the reply. "Yes," flashed Donald. "With one signal mast and a high cabin?" "Yes! yes," shot out Donald, pounding the key excitedly. "Have you seen such a craft?" "We sighted her this evening." The boy's fingers shook as he wrote down the reply with flying pencil on the scratch pad at his elbow. "Down off some islands about Lat. 80 deg., Long. 33 deg.," came the answer. "She was coming straight toward us and then all of a sudden she headed away. Seemed like she didn't want to get near us. Is that all?" "Yes; good-bye, and thank you," flashed back Donald exultantly. His fingers had hardly left the key before he was startled by a soft footfall behind him. The boy wheeled like a flash and then almost fell off his chair. Facing him, with an ugly-looking revolver in his hand, was Barton, the machinist. There was a mean sneer on his sinister face as he snarled out: "Let me see that message and let me see it quick." "I've got no message for you," responded Donald, determined not to let the man know that he had information of the green motor boat's whereabouts. "That's a lie," snarled Barton; "don't monkey with me. I've got this gun and, jingo, I know how to use it, too." CHAPTER XXIII. THE DEPTHS OF OLD OCEAN. Just as Donald, who was no match physically for the burly machinist, was pondering what to do, the door which was open became filled by a rotund figure. It was Tubby.