Chapter I: "this Is Our Secret."

It was midnight. Johnny Thompson paced the deck of the _Sea Nymph_ alone. He would be doing this until daybreak. The tropical night was glorious. There was a faint breeze--just enough to ripple the waters where the phosphorescent light thrown off by a million tiny creatures rivaled the stars above.

"Spooky," he thought, meditatively. "Out here all alone with the night.... Natives over there." He faced the east, where dark green hills loomed out of the water. Over there was a small island. Johnny never had been there. Some time he'd get into a canoe and paddle over. Earlier in the evening he had seen a light, a white man's light, he had thought, without knowing why. He--

His thoughts were interrupted by someone moving, up forward. Or was there? He had supposed they all were asleep--the strange old man, bony and tall, with goggle eyes and heavy glasses, the tall young man and the blonde girl. They all had berths forward. The captain and mate were aft; the native crew, below deck. There was no need for any of the crew, now. The boat was anchored. Only he, Johnny Thompson, was needed, to keep watch for prowlers of the sea, or signs of a storm.

It was strange, this new job. He was not sure just what these people were planning--some scientific expedition, he thought. The ship's outfit was rather irregular, but he had been glad of the chance to sign up as watch. He loved the sea.

"Someone--" he said to himself, "--is moving, up there." He started forward, cautiously.

He had covered only half the hundred and twenty-five foot length of deck when suddenly he beheld the girl of the party.

"Walking in her sleep," Johnny thought, with a touch of alarm. But she wasn't.

"Hello!" She poked a hand from beneath her midnight-blue dressing gown. "It's too swell a night to sleep."

"Yes," Johnny agreed.

"You're not a regular watch, are you?" she asked.

"That--er--" Johnny hesitated. "That's not my regular job. Nothing is. Does that matter?"

"No, I suppose not. Anyhow nothing could happen, here."

"Plenty could happen," he contradicted, quietly.

"How do you know?"

"I've been in the Tropics before. Natives get ugly sometimes. They imagine white men are getting the best of them--which, for the most part, they are!" Johnny laughed. "Then there are storms," he went on. "Wildest place for storms you've ever seen. Once I drifted before a storm for thirty-six hours in a boat just about like this, only--" he hesitated, "it was different."

"Yes," the girl laughed, "it must have been, as there's not another boat quite like this in all the world, I guess. It--

"Look!" she exclaimed softly, pointing toward the distant island. "What's that strange light?"

"Light?" Johnny spun round. "Oh! Say--that _is_ strange! It's green. A green light."

"Like an arrow," the girl whispered. "Green arrow of the Tropics. Quite romantic! But what can it be?"

"It's not for us," said Johnny. "It--it seems to blink. Wait!"

Retracing his steps he went to a box of life-preservers where he had left his heavy field glass. He returned quickly to her side.

"Now," he invited, "have a look!" He held the glass in position for her.

"It--it _does_ blink," she murmured. "It's like an electric sign. Some lights go off; others go on!"

"Let's see." Johnny took the glass. "Why--it's some sort of signalling," he decided at once. "But not for _us_!"

Instinctively they turned to scan the sea.

"There's no other boat out there," said Johnny. "At least there wasn't any at sunset. If one had moved in, we'd see the light."

"If there were a light," whispered the girl, "how gorgeously mysterious it'd be. How--

"Look!" she exclaimed. "Do you see it? A green arrow out there on the sea?"

"No--oo," Johnny said, after a moment of gazing. "I can't see it. Must have been a reflection of that other light. That often happens, you--"

"No!" The girl said, emphatically. "There! I saw it again!"

"Perhaps I'm color-blind," said Johnny after another long look. "But I just don't see it!"

At that he turned around to continue his study of that land light.

"It's strange," he murmured. "I can't quite count the lights, but they _do_ go on and off. Irregularly, too. It must be a signal. But what are they saying?"

"And to whom?" the girl added.

"Well," she sighed a moment later, "we'll not learn the answer, at least not tonight. Because it's gone!"

"So it is," said Johnny, after a long look at the island.

"Sha--shall we tell them?" he asked after a moment.

"Who? Grandfather and Dave? Oh--why should we? It can't be anything that affects us! Let's keep it for our own little secret. Perhaps we'll solve the riddle--"

"All right," Johnny agreed, readily. There's a queer girl for you, he was thinking. She'd be lots of fun, though.

"Is the elderly man your grandfather?" he asked.

"Yes. Professor Casper's his name. Only wish I knew as much as he does. My name's Doris--Doris Casper." She put out her hand. "I--I'll be seeing you. Good night. And don't forget--it's our secret--sign of the green arrow!"

She was gone.

"Sign of the green arrow," Johnny whispered, softly. "Perhaps I _should_ report it to the professor. And then again--perhaps I shouldn't. It can't have a thing to do with this boat, and it's entirely out of my line of duty. The girl wants to share a secret. Most girls do, in fact. So why not?"

With that, for the present at least, the whole affair was dismissed from his mind.

Half an hour later he found himself sitting alone on the after deck, glancing away at those dim, mysterious shores, and thinking back over the events that had led up to this mildly exciting night.

Two months before, he had found himself in New York wanting a job, and not able to find one. After three weeks of trying he had grown somewhat bitter about the whole thing.

"I'm intelligent," he had said to a prospective employer. "I've always worked. I like it. Why shouldn't I have a chance?"

"Why not?" the grey haired man had replied sadly. "I've asked that question often, but I don't know the answer. I only know we can't use another man."

That very afternoon, while watching boats moving out to sea, Johnny had his chance, and took it. He caught sight of a young man, struggling toward a gang-plank under a heavy load.

"Give you a lift?" he had volunteered, courteously.

"Whew! Yes." The man mopped his brow. "Looking for a dime?"

"Not yet!" Reddening, Johnny impulsively jerked a few small bills from his pocket. "Not broke, yet."

"Oh!" The man looked at him with interest. "Say!" he exclaimed. "I shouldn't wonder if you'd do!"

"For what?" the boy asked.

"I'm off to the Spanish Main to take pictures--native life, ancient ruins, and all that. There's a lot of stuff to lug, and--" he hesitated, "perhaps a fight to step into now and then! Want to go?"

"_Do_ I?" Johnny grabbed the two largest bags.

"There's no money in it! Just experience and expenses."

"All right! What are we waiting for?" Johnny led the way up the gang-plank.

All that had been two months before and what wonderful months those had been! Sailing from island to island, they had taken pictures of quaint, native homes, of native women with flashing eyes, of ancient buccaneer cannon, fast rusting to nothingness. There had been three exciting fights, with men who had thought they were intruding. In one of these, a machete had come within a fraction of an inch of Johnny's ear. He seemed to feel the cool swish of it now.

Then, he thought with a sigh, those golden days had ended. Lee Martin, the photographer, had been called back to New York.

"You keep the stuff," Lee had said to Johnny. "You may be able to get some unusual pictures. If you do--send 'em home to me. I'll see what I can make out of 'em, for you."

Johnny had watched Lee's boat fade into the distance. Then, with heavy heart, he had marched back to his lodgings in Port au Prince, the capital of the Island Republic of Haiti.

That very day he had noticed the _Sea Nymph_, located the man in charge, and signed up as watch. His photographic equipment was in his stateroom. He had laid in a good supply of film packs and plates. Would he find opportunity to use them? Would he get some unusual pictures to send to Lee Martin? Time was to answer all these questions in its own way....

"It's a strange layout," he thought, as he took a turn about the deck. "I suppose I'll know what it's all about before long."

It was indeed a strangely equipped craft. A three-master, with an auxiliary motor for bad weather, the _Sea Nymph_ had been built for island trade. Since the bottom had dropped out of the sugar market, she had been lying idle in the harbor. Without making many changes, the elderly professor had equipped her for his purpose, whatever that might be. Johnny had not yet been told. There had been a hold at the boat's center, for sugar and other freight. This had been transformed into a tank--or swimming pool. Johnny could not tell which. Doris, garbed in a gay swim suit, had taken a morning plunge there, but he had a notion it was for some other purpose, also.

Strangest of all, close to the stern where it could be reached by the stout hoists, was a large, hollow steel ball. It was all of eight feet in diameter, and its walls were several inches thick. What, he had asked himself more than once, could that be for? But he had asked no one else. The natives would not know, and one simply did not ask such questions of an employer. Besides, Johnny had learned long before, it is a waste of time to ask questions which, in good time, will answer themselves....