A sudden break in the rhythmic pulse of the engine reached Tom’s alert ears at this instant. Without speaking he hastened from the cabin to the engine-room, using, for this purpose, a door cut in the forward bulkhead. He found that one of the cylinders was missing fire and traced the trouble to a badly sooted plug.

While he was adjusting the trouble Jack stuck to his key. He would pound out his “S-K” call furiously for an interval, and then listen intently for even the faintest indication of a response. The lad tried various adjustments, of the potentiometer, which regulates the voltage and current supplied to the detector, and operated his receiving tuning coil in various ways. But though he tried for wave lengths from two hundred meters up to fifteen hundred, not a whisper came out of the void of silence about them.

“I’ll call once more,” said the lad to himself in a determined voice, “it’s our duty to do all we can and keep at it all the time. Of course, if the Sea King has met with a really serious disaster her wireless may be out of order and—Hullo! Here’s something coming now!”

Something was coming, sure enough!

As Jack clamped the receivers to his ears a hail of dots and dashes beat against his organs of hearing. Somebody was transmitting a message at a furious rate. Expert as the lad was, it was all he could do to make head or tail of it. His pencil fairly flew over the recording pad, and when he got through he had nothing for his pains but a sheet covered with figures, and again that annoyingly mysterious signature X.Y.Z.

Tom had returned to the cabin while Jack’s pencil was scurrying across the paper. He leaned over, the other lad’s shoulder and watched intently. When Jack stopped and affixed the signature X.Y.Z., he looked up at his cousin wonderingly.

“It’s X.Y.Z. again. He was sending like blue blazes, too. What do you make of it?”

“Blessed if I know. Using his cipher again, too, isn’t he? Say, Jack! See here,—X.Y.Z.,—whoever he is,—is within our radius right now—at this instant. Call him, and see if you can find out who or what he is and where his station is. If the Sea King is badly off he may be of great assistance to us.”

Jack switched his current over for sending out a call. With a puzzled frown on his face he adopted Tom’s suggestion.

“X-Y-Z! X-Y-Z! X-Y-Z!” he flashed out, and then added the signature “L-I.”

“Now to see if we get any result,” he said, adjusting the receivers to his ears and throwing the switch for the detection of a reply. He had not long to wait.

“L-I! L-I! L-I!—X-Y-Z!” came billowing through the ether, “what do you want?”

“We are proceeding to rescue of disabled yacht Sea King,” flashed back Jack. “Where are you? Can we rely on you for help?”

A long silence followed. Then the Continental code began to throb and beat in the receivers, once more.

But it was another question that came.

“Where is yacht Sea King?”

Jack flashed the bearings as he had received them earlier in the day, and then repeated his former question. But no reply came. For an instant the lad thought he had got out of tune with the wireless mystery, but although he ran the gamut of the tuning coil, nothing more came. For all that was further heard of him, X.Y.Z. might have been as intangible as the atmosphere out of which he had projected his questions.

For half an hour or more Jack persisted in his endeavors to reach X.Y.Z. again, but finally gave it up as a bad job. Grounding his current, he laid down his head band and swung in his chair.

“Lost him?” inquired Tom.

“I’d rather say that he lost us,” responded Jack, “it must have been a deliberate cut-out. One second he was coming strong and then—silence. How do you figure it, Tom?”

“I don’t attempt to. I give it up, unless X.Y.Z. is some sort of a wireless lunatic.”

Jack gave a rather mirthless laugh.

“Hardly. Or, if so, I begin to fear there is some method in his madness. You notice that he only seemed to want to find out the exact position of the Sea King?”

He indicated the writing pad on which the entire conversation was recorded, as was the young inventor’s wont.

Tom nodded.

“I see that plain enough. I am inclined to think. Jack, that you made a big mistake in giving that chap the location of the Sea King.”

“You do? Why?”

But as he spoke there came into Jack’s mind an uncomfortable recollection of what Jupe had said about wreckers.

“I don’t know just why,” was Tom’s frank response; “didn’t you ever have a feeling that somehow something you had done had been,—quite unintentionally,—a bad blunder?”

“I know what you mean. I wish to goodness we knew who this X.Y.Z. was,—or is.”

“Easy to find out.”

“Easy to find out!” echoed Jack with a fine note of scorn, “about as easy as—as——”

“Translating that cipher,” broke in Tom. “If we can read it we may have a good clew to Mister X.Y.Z. and his doings.”

Jack laughed aloud.

“Yes, ‘if,’” he said mockingly, “and if——”

“I think I can do it,” said Tom quietly.

“You do! Well, tackle it at once, then. I’m kind of worried, I don’t mind telling you, about that chap and his questions.”

Tom picked up the sheet of paper with the numbers inscribed on it in a seemingly hopeless jumble.

“I’ll take it to the engine-room with me and try to work it out and keep an eye on the motor at the same time. I like tackling propositions of this kind.”

“Yes, you always were a nutcracker at school; but I fancy you’ll find that the toughest yet.”

“I’m not so sure about that. Ciphers divide themselves up into groups pretty well, and I’ve half an idea that this is a very common one. Suppose you take a look at Jupe and take the wheel while he gets supper.”

“By ginger, I’d forgotten all about that till this moment.”

Jack glanced up at the clock affixed to the bulkhead.

“Almost five o’clock. Time has flown certainly. Well, good luck, Tom, with that mess of figures, and if you find out anything from them about X.Y.Z. you’re entitled to a big hunk of credit on a silver platter.”

Jupe, so Jack found, had kept the Vagrant on her course to a hair’s breadth. The old fellow had been a sailor in his younger days, and the waters they were now traversing were not unfamiliar to him. He hailed the news that he was to get supper with pleasure, however.

“Ah’ll cook yo’ boys as fine a meal as yo’ ebber sat down to,” he promised, as with a broad grin he surrendered the wheel and made aft to the galley, which was a small room forward of the cabin and between it and the engine-room.

It was an hour later that Tom appeared on deck with a knitted brow, and several sheets of paper covered closely with cabalistic figuring.

“Well?” said Jack.

“Well, I’ve worked it out, and——”

“You know who X.Y.Z. is, I hope?”

“Why, no,” was the response in a puzzled tone, “I don’t know who he is, but I’ve learned considerable of what he is,—and I don’t much like it.”