Only the rustle of leaves and the startled tweet-tweet of some bird surprised in his sleep disturbed the utter silence of the woods.

"Ghostly," whispered Joe, then he lapsed into silence.

With his slim legs stretched out before him, Curlie was soon asleep, all but his ears. Joe insisted that those ears never slept.

A half hour, an hour, an hour and a half dragged by. Joe had gone quite to sleep when Curlie suddenly dug him in the ribs and uttered the shrilly whispered warning:

"Hist! There she blows!"

A flashlight was snapped on. Curlie's fingers flew from instrument to instrument. The voice of the mysterious operator could be heard. Now rising, now falling, it filled the woods with echoes, yet the speaker was more than a mile away, as near as the boys could guess.

The words spoken by him were now of no importance. Location was everything.

"Same place," exclaimed Curlie, "exactly the same! You know where! Drive like mad!"

Instantly the car lurched forward. Coming out of the bush on two wheels, she sent a shower of gravel flying as she rushed madly down the road.

Quick as they were, the quarry had been quicker. As they rounded a corner, they caught the red gleam of a tail-light disappearing at the next turn.

"Heck!" said Curlie, then, "Let her out! Show him some speed."

The motor of the Humming Bird sang joyously. Fairly eating up the road, she took the corner with a wide swing. But when they looked down the long stretch of highway there was no red tail-light to be seen.

"Heck!" said Curlie again, "he's reached the next crossroad and turned the corner. Can't tell which way he went. It's a hard, dry gravel roadbed--won't tell a thing. Best we can do is to rattle along up there, then sit it out for another listen-in."

Disappointed but not disheartened, Curlie adjusted his instruments, then sat in breathless expectation.

He did not have long to wait, for again the voice in the loud speaker boomed out into the night.

"Huh," he grumbled a few seconds later, "he's got three miles lead on us. To the right. Quick, give her the gas."

Again they were off. For two miles and a half straight ahead they raced.

The Humming Bird quivered like a leaf, instruments jingling in spite of their lashings.

"Make it all the way," said Curlie, as Joe slowed up. "He's not there.

Given us the slip again."

Six times this program was gone through with. Not once in all that time did they catch sight of that tail-light.

"Some car he's got!" said Curlie when the farce was ended. "Bet he never even guessed he was being chased. But you wait; we'll get him yet."

When they were once more in the secret tower room Curlie plotted the route of the mysterious operator.

"Only significant thing about that," he commented, when he had finished, "is that he starts and finishes within a quarter of a mile of the same place as on the other two nights."

"And that place--" suggested Joe.

"Is near old J. Anson's driveway."

"Looks mighty suspicious to me," said Joe.

"Does to me, too; but, as I have said before, you can't raid a man's private castle on any such flimsy proof as that. You've got to have the goods.

"Tell you what," he said after a moment's silence, "sometimes our natural ears and eyes are better than all these instruments and wires.

I'm going out there to-morrow night alone and on foot."

"Might work," said Joe thoughtfully, "but whatever you do, you must be careful."

"Careful?" said Curlie scornfully. "There are times when a fellow can't afford to be careful. This thing's getting serious." He glanced over a second message from the head office of his bureau. It was couched in no gentle terms. He was told that this intruder must be caught and that at once if he, Curlie Carson, wished to hold his position as chief of the secret tower room station.

CHAPTER VI

A REAL DISCOVERY

Darkness found Curlie again on the edge of the Forest Preserve. This time he was on foot and alone. Apparently he carried nothing. His right hip pocket bulged, the handle of a flashlight protruded from his coat pocket, that was all.

He did not pause at the spot where they had hid their car the night before, but continued down the main road for a half mile farther. There he plunged into the forest, to continue his journey under cover. Eleven o'clock found him concealed in a clump of bushes in the woods that lay opposite the millionaire's driveway.

"If they come to-night," he whispered to himself, "I'll know whether they belong on that estate or not, and if they do I'll know who it is.

Anyway, I'll know it's one of J. Anson's folks. And we'll see if it is a boy or the girl?"

The question interested him. He had no relish for getting a girl into trouble, especially that frank-faced, smiling girl he had seen on horseback.

"But the thing must stop," he told himself sternly, taking a tight grip on something in his hip pocket.

The night was clear. He could see objects quite plainly. The trees, the shrubbery, the stone pillars at the entrance to the driveway, stood out in bold relief. For a time he sat staring at them in silence. At last he closed his eyes and slept, as was his custom, all but his ears.

He was startled from this stupor by a sudden flash of light which made its presence felt even through his eyelids.

As his eyes flew open, he found himself staring at two glowing headlights. The next instant he had flattened himself in the gra.s.s.

"Wow! Hope they didn't see me!" he whispered.

A low-built, powerful car had come purring so quietly down the driveway of the estate that it had rounded a sudden curve before he had been aware of its presence.

Now, with undiminished speed, it turned to the right, entered the public highway and sped straight on.

As Curlie rose from the gra.s.s to stare after it, a low exclamation escaped his lips. Supported by high parallel bars, which were doubtless in turn supported by strong guy wires, were the aerials of a radiophone.

The whole of this rose from, and rested upon, the body of the powerful roadster.

"And I missed them!" he exploded, then:

"No, I didn't. They're stopping."

It was true. Some eighty rods down the road the car had slowed up. He had no means of telling what they were doing but felt quite warranted in supposing they were sending a message.