"No," said Curlie, thoughtfully, "you never can tell. That's about the only thing you can be sure of in this strange old world. You can always be sure that you never can tell. Thing that looks like one thing always turns out to be something else.

"Point is," he continued after a moment's deep thought, "somebody's getting past our guard. Slamming us right in the nose and we're not doing a thing about it. Don't look like we could. I've got a theory but you can't go searching the estate of the richest man in your city just on theory; you've got to have facts to back you up, and mighty definite facts, too."

"Yes, that's right," agreed Coles. "But what do you make out of all that babble about airplane, map, ship and much gold? Do you suppose it's some smuggling scheme, some plan to get a lot of Russian or Austrian jewels into the country without paying duty or something like that?"

"I don't make anything out of that," said Curlie rather sharply, "and for the time, I don't jolly much care. The thing I'm interested in is the fact that we're being beaten; that the air about us is being torn to shreds every night by some careless or criminal person; that we're getting a black eye and a reprimand from the department; that sea traffic is being interrupted; that lives are being imperiled and we can't seem to do anything about it. That's what's turning my liver dark black!" He pounded the desk before him until instruments rattled and wires sang.

"But how you are going to catch a fellow when he goes tearing all over the map," said Curlie, more calmly, "is exactly what I don't know. You go down and get a bite of chow. No, go on home and go to bed. I'll take the rest of the shift. I want to think. I think best when I'm alone; when the wires sing me a song; when the air whispers to me out of the night; when the ghosts of dead radio-men, ghosts of operators who joked with death when the sea was reaching up mighty arms to drag them down, come back to talk to me. That's when I think best. These whispering ghosts tell me things. When I sit here all, asleep but my ears, things seem to come to me."

"Bah!" said Coles Masters, shivering, "you give me the creeps."

Drawing on his coat, he slipped out of the door, leaving Curlie slumped down in his chair already all asleep but his wonderful ears.

For a full hour he sat lumped up there. Seeming scarcely to breathe, stirring now and then as in sleep, he continued to listen and to dream.

Then suddenly he sat up with a start to exclaim out loud:

"Yes! That's it. Catch a thief with a thief. Catch a radiophone with a radiophone. A radiophone on wheels? That's a game two can play at. I'll do it! To-morrow night."

Snapping up a telephone receiver he murmured:

"Central 662."

A moment later he tuned an instrument and threw on a switch; "Weightman there?" he inquired. "Asleep? Wake him up. This is Curlie Carson. Yes, it's important. No, I'll tell you. Don't bother to wake him now--have him over at the Coffee Shop at five bells. The Coffee Shop. He'll know.

Don't fail! It's important!"

He snapped down the receiver. Weightman was the radio mechanic a.s.signed to his station. He would have unusual and important work to do that day.

He slumped down again in his chair but did not remain in that position many minutes.

From one of the loud speakers came a persistent whisper:

"h.e.l.lo. h.e.l.lo, Curlie, you there?" the girlish voice purred, the one that had whispered to him before. "I saw you to-night. That was dangerous. Why did you do it? Nearly got me in bad. Not quite. He almost got you."

The whisper ceased. Adjusting the campus coil Curlie sat at strained attention.

"I wish I knew you were listening," came again. "It's hard to be whispering into the night and not knowing you're being heard."

Curlie's fingers moved nervously over a tuner k.n.o.b. He was sorely tempted to tune in and flash an answering "O.K.," if nothing more.

But, no, he drew his hands resolutely back. It was not wise. There was danger in it. This might be a trap. They might locate his secret tower room by that single O.K. Then disaster would follow.

The whisper came again: "You're clever, Curlie, awfully clever. The way you doubled over and turned yourself wrong side out was great! But please do be careful. It's big, Curlie, big!" again the whisper rose almost to speaking tone. "And he is a terribly determined man; wouldn't stop at anything."

The whisper ceased.

For a moment Curlie sat there lost in reflection, then he muttered savagely: "Oh! get off the air, you little whispering mystery, you're spoiling my technique. Your very terrible friend didn't send any message to-night and the one he sent before hasn't got us into any trouble. I've got to forget you and go after this moving fellow who sends 600."

As if in answer to his challenge the loud speaker to his right, the one tuned to 1200, began to rattle. Then, in the full, determined tones of a man accustomed to speak with authority there came:

"Calm night."

Three times, over five thousand miles of air, this great voice bellowed its message.

The silence which followed was ghostly. Cold perspiration stood out on Curlie's brow.

It was not necessary for him to calculate the location from which this message was sent. He knew that it had come from the hotel. And it had.

"Next thing," he told himself with a groan, "the International Service will be on my back for letting that lion roar. I ought to turn that over to the police; but I won't, not just yet."

CHAPTER V

IN THE DARK

As the clock in a distant college tower struck the hour of eleven the following night, a flat looking car with a powerful engine stole out into the road that ran by the Forest Preserve. It was the Humming Bird.

Joe Marion was at the wheel. Curlie sat beside him.

On the back of the car was a miscellaneous pile of instruments all securely clamped down. Above there hung suspended between two vertical bars a square frame from which there gleamed the copper wires of a coil.

To catch a radiophone on wheels, Curlie had reasoned, one must mount his radio compa.s.s on wheels and pursue the offender. How well it would work, he could not even guess, but anything was better than sitting there helpless in the secret tower room listening to this person tearing up the air in a manner both unwise and unlawful.

So here they were, prepared to make the test.

"Of course," Curlie grumbled, "now we've got the trap set, the ghost may decide not to walk on this particular night. That'll be part of our rotten luck."

"Most ghosts, I'm told," chuckled Joe, "prefer to walk when there's someone about, for what's the good of a ghost-walk when there's no one to see. So our radio ghost may show up after all."

Curlie lapsed into silence. He was reviewing the events which led up to this thrilling moment. When the message on 600 came banging to his ears with great power on that first night, he had carefully platted the various locations of the person who had sent the messages. There had been some criss-crosses shown but, in the main, a line drawn through these points had formed an oblong which on the actual surface of the ground must have been some ten miles in length by six in width. One interesting point was that the first and last messages of that night had been sent at points not a quarter of a mile apart.

"Which goes to show," he reasoned, "that this fellow started from a certain point and made his way back to that point, just as a rabbit will do when chased by a hound. And those two points, the start and the finish, are close to the driveway into the million dollar estate. But of course that doesn't prove that the car came from there. Any person could drive to that point, begin operations, race over the square and return to the point."

Coles Masters had platted the points for the second night. A line drawn through these points made a figure quite irregular in form, which was, however, composed of rectangles.

"Which proves," he told himself, "that our friend, the lawless radio fan, drives an auto and not an airplane. An auto follows roads, which for the most part in this section form squares. He pa.s.sed along two or three sides of these squares and this makes up the figure.

"There's only one thing in common in the two night journeys," he continued. "The start and finish are at almost exactly the same spot, near the entrance of that great estate."

He tried not to allow these facts to cause him to hold undue suspicion against the inhabitants of that mansion, but in this he experienced some difficulty.

"The thing for us to do," he had said to Joe, "is to run out there and back our car into an unfrequented, wooded road running into the forest preserve. We don't dare go too near the original starting place. If we're seen with this load of junk it will give us dead away. Thing is to be ready to move quickly when he lets loose with his message. Ought not to be more than a mile away, I'd say. He's got a powerful car. You can tell that by the fact that he sent a message at this corner, then raced over here, four miles distant, and got another message off in eleven minutes, which is quick action."

They backed into the gra.s.s-grown road of the Forest Preserve, then settled down in their places to wait.

The night was dark. There was no moon. Clouds were scurrying overhead.