The thing he saw was an enormous mass without the severe angular shape of the great dock. Its outline rose crude and shapeless, as well as he could trace it among the canopy of stars, and gave not the slightest intimation as to what use it could be.

As they stared, the speed of the _Vulcan_ slackened sensibly. The faint rippling of water under the prow ceased. The breeze fell away into a dead blanket of heat. It was as if a sweatbox had been cooped over the crew.

"The thing's cut loose from us," said a weary voice.

Hogan laughed shortly: "Everybody out--fifteen minutes for refrishmints."

"Yonder goes that thing!" cried Galton. "Hi can see it!"

Indeed, by peering carefully, Madden could follow the slender outline of the mysterious craft that had towed the _Vulcan_ to this uncanny spot. It had now left the tug and was gliding away to the great misshapen fabric that sprawled on the sea.

Every eye strained to see the outcome of this strange maneuver, when suddenly from the gliding vessel there shot a dazzling light that spread over the bulky mass. Under the beating illumination every detail of the huge vessel stood out garishly. She was immense, with a broad flat prow like a railway ferryboat. She stood high in the water and seemed to have three promenade decks around her.

There was no mast, no rigging, no outside gearing. One squat funnel amidship told that she used steam for some purpose, and out of this funnel black masses of smoke rose slowly in the motionless air. She resembled no craft Madden had ever seen.

Notwithstanding her enormous size, everything about the vessel impressed Madden that she was built for secrecy. She was squat, considering her length and breadth. It was as if her designer were trying to make a craft invisible at sea. As near as Madden could determine in the strange light, she was painted a pale sky-blue. During the day, no doubt, she melted into the sky like a chameleon.

As the smaller craft approached its huge mate, its circle of light contracted until it finally concentrated into a dazzling white spot centered on the prow of the monster. This spot diminished to an intense point, like an electric arc between carbons. A sharp reflection of this point streaked the water between the tug and the mysterious vessels.

Then, under the unbelieving eyes of the crew, the little vessel ran completely into the larger one and was gone. The light vanished instantly. Utter blackness fell over the dazzled eyes of the watchers.

There were gasps, explosive curses of bewilderment, amazement. The little boat had disappeared into the larger. Impossible! Gaskin began his shrill laughter again. Then he gurgled in the darkness as if somebody's fingers had clamped his windpipe.

Madden's mind attacked more violently than ever the incomprehensible motives behind this inscrutable mystery. What was the key to this incredible affair? In the midst of his mental struggle, he felt a hand on his arm, Caradoc said in his ear,

"What do you say we get in the small boat and pay them a visit?"

"It's a big risk. I daresay we'll get our heads blown off."

"I had thought of that," agreed Caradoc.

"Come on," said the American, and the two moved across the deck to see if they could still use the dinghy, which had been trailing along all this time.

Nearly an hour later, the two boys in the dinghy approached the puzzling craft with muffled oars. As Madden and Caradoc drew near, the vast size of the strange ship grew more striking. The faint impression of light which they had first received grew stronger and Madden saw that the decks were illuminated by long bands of diffused light, although he could not guess its origin.

On the lowest deck, the American made out the small figure of a man marching back and forth with a gun.

At this sight, both boys stopped rowing, lifted the oars from tholes and began paddling noiselessly, canoe-fashion.

"That must be the accommodation ladder," whispered Madden, "where the guard is."

"Who are they afraid will board them?" queried Caradoc. "Mermaids?"

"It is a strange precaution to take in the Sargasso," agreed the American. "It is going to make our entrance difficult."

They ceased paddling now and drifted silently toward the monster.

"I wonder if they aren't smugglers," hazarded Caradoc,

"Must be up-to-date, to use submarines--a submarine would defy detection, wouldn't it?"

"And rich--nobody but millionaire smugglers could get together all this paraphernalia."

"I'll venture insurance is at the bottom of this fraud, Caradoc,"

hazarded Madden. "These swindlers insure a cargo, bring it to this place, reship it, sink the vessel, or repaint and rebuild it, then collect the insurance money--do you remember the log of the _Minnie B_?"

"No, I didn't read it."

"It stated her cargo had been reshipped--reshipped from the Sargasso.

The entry may have been for the benefit of Davy Jones. Anyway, they are methodical scoundrels."

The lads fell silent as the hugeness of this nefarious business gradually dawned on them. For insurance swindlers and smugglers to work on such a large scale, very probably the organization branched over the whole civilized world. This vast shapeless vessel was a spider at the center of a great network of criminality.

"Say, the Camorras are mere infants in crime compared to these men,"

shuddered Leonard. "I suppose they murder the crews--drown 'em."

"They would have to get 'em out of the way somehow."

"Then Malone and all the tug's crew are..."

There was an expressive silence.

After a while Caradoc whispered, "Well, shall we try to get aboard?"

"Wouldn't do any good."

"It won't do any good to stay here."

"No, we can't hide on the tug always, and we can't run her engines.

_You_ don't know anything about marine engines, do you, Caradoc?"

"Very little. I couldn't run one."

For several minutes, the two adventurers sat in silence, watching the small erect figure of the guard pace and repace his short path.

Presently Madden said:

"I've thought of one chance, Caradoc, to escape being starved or murdered."

"Yes, what's that?"

"It--it's almost too wild to propose, but it's all I can think of. As far as I know it's absolutely our last chance."

"Go on, go on," urged the Englishman impatiently. "I don't know of any way out whatever."

"If we could slip aboard there and--and--well, kidnap somebody who knows how to run our engines, bring him back to the tug, fire up and make a race to South America--but there's no sense to a scheme like that.

Captain Kidd himself wouldn't be up to it."

A long silence followed this ultimatum, then Caradoc said, "Oh, it's possible, I suppose. The mathematical formula of possibility would work out about ten million chances to one that we lose."