A MIDNIGHT ADVENTURE.

Half an hour after her start, the _Viper_ glided alongside the island from which Merritt had seen the signals go up the afternoon before. He could not forbear to take a glance at Barton as the ensign ordered the engines stopped.

The machinist was stooping over the motor to hide his agitation; but by the trembling of his hands Merritt could tell that the fellow was apprehensive of something that might involve himself. As soon as the anchor dropped, the motor boat's dinghy was drawn up alongside and the ensign and Merritt boarded it. The others were left on board the _Viper_ with whispered orders from the officer to watch Barton's every move.

The island was a small one, and from its highest point it was possible to see all around it. To Merritt's bitter disappointment, however, no sign of another motor boat was in sight. Their quarry had flown.

"There's but one thing to do," declared the ensign; "we must make for that small hut over yonder and search it thoroughly. It may yield a clue of some kind."

A short walk brought them to the hut which had been the scene of the stirring events of the preceding night. Hardly had they entered the door before Merritt gave a start of surprise and a swift exclamation.

"Look! Look there!" he cried. "There's Rob's hat!"

Sure enough, lying in a corner was the boy leader's campaign hat, which he had lost in the scuffle with Mike and Gyp.

"Well, that shows conclusively enough that he was here last night, and from that upset table and the general look of things, I should imagine there had been a pretty lively sc.r.a.p here," commented the ensign.

"But where can Rob be now?"

"Probably fearing discovery if they remained here, the men who have taken the plans and the models carried him off, too."

"How will it ever be possible to obtain a clew as to where they have gone?"

The ensign's answer appeared enigmatical.

"Could you describe the motor boat you saw off here yesterday?"

"Well, she was of a very remarkable color--a light green, with a signal mast sticking up amidships. Then, too, her cabin was unusually high."

"Good. Such a boat as that ought not to be very hard to locate."

"I don't quite understand."

"Well, then I'll explain. These waters are fairly well traveled, and by working our wireless we may be able to get into communication with some boat similarly equipped, which may have seen that green motor boat."

"Cracky, that's a good idea," cried the admiring boy; "let's go back and try it at once."

"Yes, it's small use our waiting about here. The rascals overreached us by getting away as quick as possible. I suppose they didn't want to run any chances of discovery."

The return to the _Viper_ was quickly made, and the motor boat was driven back to the Submarine Island at top speed. Barton tried with all his might to overhear what was said in the bow of the boat where the Boy Scouts had gathered; but the ensign was careful to keep his voice low, and then, too, the noise of the engines precluded the machinist from catching a word, hard as he strained his ears.

Under the tutoring of Hiram Nelson, the wireless scout, the others had all become fair operators. It was agreed that day and night one of them should be at the apparatus, seeking for news of the green motor boat.

It was the ensign's opinion that the craft would not put into a port immediately, fearing a hue and cry, but would cruise about or hide in some little frequented part of the coast. But he hoped that if the wireless "caught" some vessel that had spoken to her, he could at least obtain a line on which direction she had taken.

The first "session" at the wireless was taken by Hiram, then came the others in rotation; but when at ten o'clock that night Donald, who had learned wireless on his father's yacht, came on duty, there had come no word from the air of a green motor boat. Several ships had been spoken to, but not one reported anything to give the boys hope.

"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 s.p.a.ce 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.