He had next searched about for a discarded cha.s.sis on which to mount his gears and motor. This search rewarded, he had proceeded to a.s.semble his car. And one fine day he sailed out upon the street with the "Humming Bird," as he had named her.

"Better call her 'Gravel Car,'" Joe had said when he saw that she had no body at all and that he must ride with his feet thrust straight out before him in a homemade seat bolted to a buckboard-like platform.

But when, on a level stretch of road, Curlie had "let her out," Joe had at once acquired an immense respect for the Humming Bird. "For," he said later, "she can hum and she can go like a streak of light, and that's about all any humming bird can do."

No further messages of importance having drifted in to him from the outer air, Curlie, an hour before dinner, made his way down to the street and, having warmed up the Humming Bird's motor, muttered as he sprang into the seat: "I'll just run out there and see what I see."

A half hour later, just as the first gray streak of dawn was appearing, he curved off onto a gravel road. Here he threw his car over to one side and, switching on a flashlight, steered with one hand while he bent over the side to examine the left-hand track.

There had been a light rain at ten that night. Since that time a heavy car with diamond-tread tires had pa.s.sed along the road, leaving its tracks in certain soft, sandy spots.

"Maybe that's him," Curlie murmured.

A little farther on, stopping his machine, he got out and walked along the road. Examining the surface closely, he walked on for five rods, then wheeled about and made his way back to the car.

"He was over this road three times last night. That looks like a warm scent. Can't tell, though. My friend might not have been in a car at all; might have been in a plane.

"We'll have a look at the very spot." He twirled the wheel and was away.

A half mile farther down the road, he paused to look at a map. "Not quite here," he murmured. "About a quarter mile farther."

The car crept over another quarter of a mile. When he again came to a halt he found himself on a stretch of paved road. "This is the spot from which the last message was sent. Tough luck!" he muttered. "Can't tell a thing here."

Glancing to his right, he sat up with a start. He had suddenly become aware of the fact that he was just before the gate of the estate of J.

Anson Ardmore, reputed to be the richest man of the city.

"Huh!" Curlie grunted. "Car must have stood about here when that last message was sent. Maybe it went up that lane. Maybe it didn't, too. J.

Anson's got a son, about my age I guess. Vincent they call him. He might be up to something. There's a girl, too, sixteen or so. Can't tell what these rich folks will do."

He stepped down the rich man's private drive, but here the surface of crushed stone was so perfectly kept that no telltale mark was to be seen.

He did not venture far, as he had no relish for being caught trespa.s.sing on such an estate without some good explanation for his conduct. Just at that moment he had no desire to explain.

As he turned to go back, he caught the thud-thud of hoof beats along the private drive.

Fortunately the abundant shrubbery hid him from view. Hardly had he reached the machine and a.s.sumed the att.i.tude of one hunting trouble in his engine when a girl rounded a corner at full gallop.

Dressed in full riding costume and mounted on a blooded horse, she swung along as graceful as a lark. As she came into the public highway she flashed Curlie a look and a smile. Then she was gone.

Curlie liked the smile even if it did come from one of the "four hundred."

"Gee! Old Humming Bird," he exclaimed as he patted his car, "did she mean that smile for you or for me? So there might be a girl in the case, same as there seems to be in that one over at the hotel? Girl in most every case. What if she sent those messages and I found her out? That would sure be tough.

"But business is business!" He set his mouth grimly. "You can't fool with old Uncle Sam, not when you're endangering the lives of some of his bravest sons at sea."

He threw in the clutch and drove slowly along the road. Twice he paused to examine the tracks made the night before. Each time he discovered marks of the diamond tread.

"That radiophone was mounted on a car," he decided; "I'll stake my life on that. Now if he keeps it up, how am I to catch him?"

CHAPTER III

A WHISPER IN THE NIGHT

The next night found Curlie in the secret tower room alone. Joe Marion was away helping to run down a case of "malicious interference."

It was curious business, this work of the radio secret service. Though he had been at it for months, Curlie had never quite got used to it. A detective he was in the truest sense of the word, yet how different from the kind one reads about in books.

He laughed as he thought of it now. Then as his tapering fingers adjusted a screw, his brow became suddenly wrinkled in thought. He was troubled by the two cases which had lately developed: the one at the hotel and that other, the station that moved. How was he to locate that powerful secret station in the hotel? How was he to discover the owner of that mysterious moving radio? He could not answer these questions.

And yet somehow they must be answered. He knew that.

The operator in the hotel was sending on 1200 meter wave lengths. State messages were constantly being sent across the Atlantic on 1200; messages of the greatest importance. There was a conference of nations at that moment going on in Europe. America's representative must be kept in constant touch with the government officials at Washington. If this person at the hotel persisted in sending messages on 1200 meter wave lengths an important message might at any moment be blurred or lost.

Not less important was the breaking in of this moving operator on 600.

This was the wave length used by ships and by harbor stations. Great steamships sometimes waited for hours to get a message ash.o.r.e on 600. If this person were to be allowed to break in upon them they might wait hours longer. Thousands of dollars would be lost. And then, as we have said before, the message of some ship in distress might be lost because of this person's interference.

"When, oh, when," sighed Curlie, "will people become used to this new thing, the radiophone? When will they learn that it is a great, new servant of mankind and not a toy? When will they take time to instruct themselves regarding the rights of others? When will they develop a conscience which will compel them to consider those rights?"

The answer which came to his mind was, "Perhaps never. But little by little they will learn some things. It is my duty not alone to detect but to teach."

He shifted uneasily in his chair, then held his ear close to the loud speaker tuned to 200. A message came floating in to him across the air, a mysterious whispered message.

"h.e.l.lo, Curlie," it said. "You don't know me, but you have seen me--"

Automatically Curlie's fingers moved the radio-compa.s.s backward and forward while his mind gauged the distance. His right hand scrawled some figures on a pad, and all the time his ears were strained to catch the whisper.

"I have seen you," it went on, "and I like your looks. That's why I'm talking now."

For a second the whisper ceased. There was something awe-inspiring about that whisper. As he sat in his secret chamber away up there against the sky, Curlie felt as if some spirit-being was floating about out there in the sky on a fleecy cloud and pausing now and then to whisper to him.

"I saw you," the whisper repeated. "You are in very grave danger. He is a bold and treacherous man. It's big, Curlie, _big_!" The whisper rose shrilly. "But you must be careful. You must not let him know the place where you listen in. I don't know where it is. But I do know you listen in. Be careful--careful--careful, c-a-r-e-f-u-l-" The whisper trailed off into s.p.a.ce, to be lost in thin air.

Wiping the beads of perspiration from his face, Curlie sat up. "Well, now," he whispered softly to himself, "what do you know about that?

"One thing I do know," he told himself. "I'd swear it was a girl's whisper, though how you can tell a girl's whisper is more than I know.

Question is: Which one is it--hotel station or the one that moves?"

For a moment his brow wrinkled in thought. Then with an exclamation of disgust he exclaimed:

"That's easy! I've got their location!"

He figured for a few seconds, then put a pencil point on a certain spot on his map.

"There!" he muttered. "It's the hotel, the exact spot."

Suddenly he started. There came the rattle of a key in the door.