“Why! why, that’s the Tropic Bird!” exclaimed the scientist in astonishment, as they drew nearer rapidly to the vessel Nat’s keen eyes had espied.

“It is, indeed,” reiterated Mr. Tubbs, his red hair seeming to bristle. “Oh, the cowardly pack of rascals! I’d just like to run alongside and give them a bit of my mind.”

“They deserve it, certainly,” admitted the professor; “but I think we had better ignore them.”

But as they came close enough to the schooner to perceive her clearly, they saw that she carried her ensign reversed. This is a signal of distress which there is no ignoring at sea, and is the universal sign of imperative need on the part of the craft displaying it.

“We must see what they want,” declared Nat, setting his wheel over and changing the course of the Motor Rangers’ vessel.

“Got any fresh water?” hailed a voice, as they came alongside.

The man who uttered the appeal was a powerfully built fellow, with a plentiful crop of black whiskers, which gave him a ferocious expression.

“That’s Captain Ralph Lawless,” whispered the professor to Nat.

At the same instant, the skipper of the Tropic Bird appeared to recognize the professor.

“Why, surely that’s Professor Grigg?” he cried out, apparently in great astonishment.

“Yes, it is, you cowardly rascal,” burst out the professor, his anger overmastering his usually placid disposition. “What do you mean by deserting us in the manner you did? We might have perished if it had not been for these brave lads and their vessel.”

“Well, I’m sorry,” muttered the man, as the Motor Rangers’ vessel drew in close alongside, “but I couldn’t help myself.”

“Couldn’t help yourself?” echoed the scientist, still angry. “How was that, pray?”

“Why, I felt my schooner being drawn in toward the islands. If I hadn’t ‘cut stick’ when I did, we’d all have been lost, and I don’t see how that would have helped you.”

This answer mollified the professor somewhat.

“So now you are in distress?” he said.

“Yes. We have run short of water. Can’t those kids let us have some?”

“You’ll have to ask ‘those kids,’ as you call them,” said the professor, with some disgust.

“How much do you want?” asked Nat, who felt less and less liking for the captain of the Tropic Bird.

“Oh, a few gallons will do. I know an island not more than a day’s sail from here, where I can refill my tanks.”

At this point, another man—a short, stout fellow, like the captain—came bustling up.

“Hullo, there, professor!” he hailed in an impudent voice. “So you came out all right, after all. Are you coming on board?”

“I am coming on board to get my things, Mr. Durkee,” was the response, “but I am not going to continue my voyage on the Tropic Bird.”

The captain looked rather dismayed at this.

“Oh, come now,” he said, “let bygones be bygones. I should be in a fine fix if I sailed home without you.”

“You ought to have thought of that when you deserted us in that cowardly fashion during the magnetic storm,” rejoined the professor.

The deck of the Nomad was almost on a level with the top of the schooner’s bulwarks, so it was easy for the professor to step from one craft to the other. He now did so, disdaining the proffered aid of Captain Lawless and his mate.

Mr. Tubbs joined him, and the two went immediately into the after-cabin of the schooner, where they had lived while on board.

While they were collecting their belongings, Nat and Joe filled a twenty-gallon keg with drinking water, and it was hoisted to the schooner’s deck. It was really more than they could spare, but Nat was a generous lad, and figured that, if necessary, they could go on short allowance till the South American coast was reached.

During the time that the boys were about this work, Captain Lawless and his mate had been holding a consultation in the lee of the deckhouse, just aft of the foremast.

“It’s going to make lots of trouble for us if we arrive in America without the professor or that chap Tubbs,” said the mate. “Besides that, too, we’ll have lost our chance of sharing in that hunt for a lost city. There ought to be enough loot in that to make us both rich.”

“That’s so,” agreed the captain. “If what those papers of the professor’s say is right, that place must be paved with gold, and when it rains it must drop diamonds.”

“Pretty near,” grinned the mate, in appreciation of his superior officer’s humor. “I wish I’d had time to go over the papers more thoroughly before that kid’s craft overhauled us. That was a good guess of yours that they’d pick up the old gent and that chap Tubbs, and the reversed ensign was a good way to get ’em to come alongside.”

“Well, now that we’ve gone this far, we may as well take the next step,” observed the captain.

“And what’s that?” asked the mate, with a peculiar glint coming into his little rat-like eyes.

“Why, fix it so that it won’t be possible for old Grigg to make trouble for us in the States.”

“How?”

“Simple enough. We can easily overpower those kids, and as for the professor and Tubbs, we’ll lock ’em in the cabin.”

“Say, cap, you are a schemer!” observed the mate, in rather sarcastic admiration, “and then I suppose we’ll sail for home and be arrested and imprisoned as pirates?”

“Not at all,” was the reply. “We don’t need to go home. South America’s good enough for me. It’s Chile that the old cove is headed for, ain’t it?”

“So his papers said.”

“All right, then. We’ll make the whole bunch prisoners, land ’em on an island some place, and then we’ll sail on to Chile ourselves, and have a try at finding this old lost city. By the way, did you make a tracing of that map you found in the professor’s desk?”

“Did I? Well, I should say so. I’ve got it in my pocketbook now. That’s likely to mean dollars and cents to us later on.”

“That’s so. Now then, you go and tell the crew what we are going to do. They won’t cut up rough about it, especially if they think there is money in it.”

“All right. I’m off. But see here, how are you going to do it? Those kids look pretty husky.”

“Bah! What can they do against eight of us? If they get too obstreperous, a tap on the head with a marlin-spike will soon quiet them.”

While the two worthies of the schooner were cold-bloodedly discussing their plans to save themselves from the consequences of their cowardly act and at the same time enrich themselves, Nat and Joe, blissfully ignorant of any such proceedings, had hoisted the water keg on board.

This done, they started aft toward the cabin to join the professor and Mr. Tubbs. They found the two companions below, busily packing up their possessions. But at the instant they entered, the professor looked up from his desk, where he was sorting papers, with a troubled expression.

“What is the matter, professor?” inquired Nat politely.

“Somebody has been tampering with my papers!” he exclaimed. “I had them arranged in a peculiar manner. And see, this lock has been forced. Oh, that rascal of a captain! If we were in a civilized port, I’d——”

The professor’s angry tirade was interrupted in a startling manner. The door at the head of the companionway stairs was slammed abruptly to.

Warned by some intuition which he could not have analyzed, Nat bounded to the stairway and strove to reopen the door. But it resisted his stoutest efforts.

“It’s locked!” he managed to gasp, as the truth burst upon him.

“And we have been trapped by those two rascals!” exclaimed the professor.