Chapter III.--JUST A TYPEWRITER RIBBON.

"I don't understand what question he is asking," Jack said, "but I know how he makes the signal. He has an electric flashlight, and he tips the plane--the upper plane--forward, like he was plunging to the earth, and writes the interrogation mark on the under side with the flame of the flashlight. See? Then it shines through the canvas and we read it! Great idea!"

"That must be the way of it," Frank said, "but what does he want? And how does he expect us to answer?"

"If I was up there in the dark on a contraption like that," Pat said, "I'd be asking how I was going to find a landing place."

"Sure!" Frank cried. "Ned wants to know where we are, and whether it is safe for him to make a landing. Dunderheads! Why didn't we think of that before? He is passing now, and may not come back again."

The light flashed by at swift speed, whirled, ascended several hundred feet, and came over the plateau, repeating the signal. Then it settled down into a steady circling of the camp.

"He knows where we are, all right," Pat said. "What he wants to know is if it is safe for him to make a landing. If I ever go up in one of those things I'll drag a rope so I can climb down it."

"I'll tell him what he wants to know," Frank said, "if you'll get me a long stick on fire most of its length."

"Wigwag?" asked Jack.

"Sure!" was the reply. "Now," Frank continued, "build four fires, one on each edge of the plateau. That will show him how large the place is. Then I'll take the flaming stick and wigwag o.k. Ned'll understand that."

Pat watched the wigwag signal with interest.

"I saw foolish signs like those in the Philippines," he said, with a grin. "The natives use them to talk treason to each other. I've heard that the same method is used by the East Indians who talk from one mountain top to another faster than words on a wire. How does he make the o.k. signal?"

"O is one left, followed by one right," Jack replied, "and k is left, right, left, right. You won't think the signs are foolish when you see how quickly Ned reads them. See! He's shooting away now."

"Perhaps he thinks the signals are being made by savages," Pat said.

The aeroplane darted off to the west for half a minute, then whirled and came back. The boys could not see the great 'plane distinctly, but the lights which burned on the front were bright and clear, so they saw that the 'plane was sweeping toward the earth as it advanced in their direction.

"I don't believe many professionals would care to make a landing like this," Frank said, as the machine dipped and slid to the ground, exactly in the center of the plateau.

"Hello, Ned!" he yelled, as the aeroplane rolled over the smooth surface for an instant and stopped.

In a second the three boys were gathered about the machine, pulling at the hands and feet of the daring riders. Jimmie McGraw bounded to the ground as soon as he could cast off the lines which had held him to his quivering seat.

"Say," he cried, "you got a fire here? I'm most froze."

Indeed the little fellow's teeth were chattering.

"Cold?" echoed Pat. "We're melting down here. You're scared, that's what's the matter with you. You're scared stiff."

Jimmie made a run for the speaker but brought up at the fire where the supper had been cooked.

"Here's comfort!" he cried, extending his hands out over what was left of the small blaze. "The next time you get me up in the air I don't go! I've been freezing for an hour."

In the meantime Ned Nestor was caring for the aeroplane, looking after the delicate machinery and covering it carefully with a huge oil-cloth. Pat stood watching the work with a grin on his face.

"Are you thinking of giving me a ride in that thing?" he asked.

"Not to-night!" laughed Ned.

"Well, when you get ready for me to ride the air," Pat said, "just tell me the night before, and I'll shoo myself into the hills. If I'm going to fall off anything, I'll take the drop from something solid, like a mountain top."

"No danger at all, when you know how to operate the machine," Ned replied. "There's danger in running anything if you don't know how, even a sewing machine."

"Where did you pick it up?" asked Frank.

"He didn't pick it up at all," interposed Pat. "It picked him up."

"I found it at Missoula," was the reply, "all packed and stored away in a freight warehouse. I had to get it out at night, and so lost time. The people would have kept me there until now giving exhibitions if I had shown up during the day."

"But you did leave there in the daytime," urged Jack. "You were never in the air since last night."

"We left early this morning," was the reply, "and I was well up in the sky before many of the people saw me."

"I never knew you could run one," Frank said.

"Oh, I had some instructions from the Wrights," was the modest reply, "and, besides, there was an expert at Missoula who helped me get the machine together and contributed a few parting instructions."

"Then you've been in the air all day?" asked Pat.

"No, we stopped several times, of course, once on the right of way of the Great Northern railroad and filled our gasoline tanks," was the reply, "and rested there a few hours. Jimmie had to eat there, of course!"

"Eat!" came the boy's voice from the fire. "If I ever get a bite at food again it will drop down into the toes of me shoes! Here!" he shouted, as Pat produced a can of pork and beans and started to open it. "You needn't mind opening that! I'll just swallow it as it is."

"Bright boy!" laughed Pat, handing him a liberal supply of beans and fried bacon. "Now fill up on that and then loosen up on your impressions of the sky."

"I thought I'd make an impression on the earth before I got through," Jimmie mumbled, his mouth full of beans. "We went up so far that the mountains looked like ant hills, didn't we, Ned?"

"About 7,000 feet," was the reply. "You see," he added, turning to Frank, "I wanted to size up the situation before I landed. If there is anybody in this upturned country at all, our presence here is known. The aeroplane's chatter took good care of that. And, besides, our landing in the night, with the lights going, gave unmistakable evidence of something stirring."

"I should say so," Frank agreed.

"And so," Ned went on, "I wanted to learn if there were people about here, so I might visit them in the morning and put up the bluff of Boy Scouts playing with an aeroplane in the woods. We can't attempt anything in the mysterious line," he went on. "We've got to be entirely frank about everything except the business we are here on."

"Well," Frank said, "we found people here to-day and called on them."

"What sort of people?"

"Well, they seemed to have good broad backs," laughed Frank.

"They ran away from you?" asked Ned, in surprise. "I should think they would have proved inquisitive. Where were they?"

"Down by Kintla lake."

"Indians?" asked Ned.

Then Frank told the story of the visit to the shore of the lake and the cavern, taking good care to describe the surroundings as closely as possible. Ned laughed when the boy came to Jack's adventure in the hidden chamber.

"I say it is some deserted mine," Pat declared, when Frank had concluded the recital. "What else could it be?"

"Robber's nest!" suggested Jack.

Ned remained silent for a moment and then abruptly asked:

"What kind of footwear made those heelless prints?"

"You may search me!" Jack cut in.

"Must have been Indian moccasins," Frank observed.

Jimmie, who had been standing by the small fire, listening to the talk, now advanced to the little circle about the machine and uttered one word: "Chinks!"

"It is always Chinks with Jimmie," grinned Frank. "When there is a cyclone in New York the Chinks are to blame for it, if you leave it to him."

"What would Chinks be doing up here?" demanded Pat.

"Don't they get gold by washing it out?" asked Jack, with a nudge at Jimmie's side. "Perhaps they're going to start a laundry!"

While this chaff was in progress Ned stood looking thoughtfully in the direction of the lake. Not a word did he say regarding the sudden and brief communication Jimmie had presented.

"Any forest fires in sight?" asked Pat, finally breaking the silence.

"Not one," Jimmie answered. "I would have dropped into one if it had come my way. It was fierce up there!"

"It is rather cool when you get up a couple of miles," Ned laughed, "and Jimmie wouldn't listen to reason regarding his clothes. To-morrow I'll give one of you boys a ride, and you may see for yourself."

"Not me!" Pat exclaimed. "I'll stay below and help pick up the pieces."

"I should like to go," Frank said. "We may find the people we saw in the rowboat. When we become acquainted with them we may be able to learn something about that cavern."

"I would advise remaining silent about the cavern," Ned said. "It may be used for some criminal purpose, and we must not admit that we know of its existence. We are just carefree lads, here for an outing, remember," he added, with a laugh, "and we are due to make friends with everybody we come across."

"But you made us lug all this camping outfit up here," complained Jack, "so the men who steered the burros up the hills wouldn't know where we camped. What about that?"

"I thought it best to cut off all communication with the people below," explained Ned. "It may be that the purpose of our visit here is suspected. In that case some one from below might want to find us--for no good purpose. So we'll keep out of sight of the people in the towns, unless they see our aeroplane, and cultivate the acquaintance of the natives--if there are any."

"How about gasoline and provisions?" asked Pat.

"I have plenty of gasoline stored on the right of way of the Great Northern railroad," Ned replied, "enough to last us a month. It was piped into a hidden tank from an oil car by a train crew now out of the state. We are to get provisions at the same place, if we need more, for Uncle Sam fixed all the details for us. All we have to do is to find the fellows who are setting forest fires and bring them to punishment."

"We ought to locate every little smudge, with that aeroplane," Frank suggested.

"That is my idea," Ned replied. "Have you been keeping a good lookout on the lake since you left it?" he added, turning to Pat.

"Some one of us has had eyes on it every minute," was the satisfactory reply. "No one has returned, I'm sure."

"You're not thinking of going there to-night, are you?" asked Jack, with a slight shiver. "I wouldn't go in there again, even in broad daylight, for a million dollars!"

"Pat is afraid of the sky, and Jack is afraid of the bowels of the earth!" laughed Frank. "We'll have to tuck them both in bed before we can accomplish anything."

"You may all go to bed but one," Ned said, looking about the group, his eyes finally resting with a significant look on Frank's excited face. "I want to look through that cavern before anything is taken out of it."

Frank, knowing the meaning of the look he had received, went to his little tent for his revolver and his electric searchlight and was soon ready for the expedition. Jimmie looked sulky for a moment at being left out of the game, then his face brightened and he crawled into the tent that had been prepared for Nestor and himself and burst into a fit of laughter.

"I'll show 'em!" he said, stuffing the blanket into his mouth to suppress the sound of his merriment. "I'll teach 'em to put me in the discard."

"Any wild animals up here?" asked Ned, as the two started away down the steep declivity.

"Two Black Bears and three Wolves!" called Jimmie, from his tent.

This was a reference to the Boy Scout Patrols to which the boys belonged. Frank and Jack were members of the famous Black Bear Patrol of New York City, while Ned, Pat and Jimmie were members of the Wolf Patrol.

As the lad spoke Frank and Jack broke into growls which might well have come from the throat of the grizzliest grizzly in the Rocky Mountains, while Pat sent forth a wolf howl, which might well have been a signal to the pack.

"You may meet the real thing out here," warned Ned, turning back to look over the plateau, now shining in the light of a half-moon. "There are both bears and wolves in this region. When you meet them, don't wait for Boy Scout signs!"

"Oh, we'll initiate 'em, all right," Jimmie called from the tent, and Ned and Frank moved on down the declivity toward the lake.

It was still early evening, and the moon was low down in the east, so the valley where the lake lay was not touched by its light. Indeed, the plateau where the boys were would have been in the shadow of the mountain only for the dropping of the shoulder of the divide.

In half an hour the two boys, after several slides which were anything but pleasant, gained the beach. The campfire was now dead, and the locality was still save for the voice of a night bird and the occasional splash of a leaping fish. The mouth of the cavern loomed like a dark patch on the lower bulk of the mountain.

Making as little noise as possible, Ned and Frank crept into the cavern, advancing by the sense of feeling until they came to the very end before turning on one of the electric flashlights. The round eye of the flame showed a long, narrow, tunnel-like tube running directly east, under the mountain. The door of rock was as the boys had left it earlier in the day.

Ned examined that portion of the rock which had swung out into the first chamber with considerable care, as the story of the swinging stone had interested him greatly. All along the top, up to the center, he found the checks of a stone-chisel. Exactly in the middle an elevation of an inch fitted into a round cavity in the upper rock. At the bottom the same conditions were discovered.

"Rather a clever job," Ned said, "but I don't see how it was ever done."

"This door," Frank said, "is not exactly like the remainder of the wall in grain, so it must have been brought here from some other locality. Of course there was a hole between these two chambers, or the second one would never have been found. It would be easy enough to fit the stone door in by grooving out from the lower cavity and sliding the under pivot in."

"Sure," Ned replied, getting down to examine the lower part of the door more closely, "and that is just what was done. Then the groove was filled with concrete. Pretty classy work here!"

"And now the question is this," Frank went on, "what was the door fitted for? Why did the men who found the cave desire privacy? Is there gold in there? Have the men who have been setting fire to the forests established a home here? Is this the hiding place of a band of outlaws? You see there are lots of questions to ask about the two caverns," Frank added, with an uneasy laugh.

Ned closed the stone door and turned on both electric flashlights, making the place light as day where they stood. The inner cavern was as bare as the outer one save for dead leaves and grass which lay in heaps on the stone floor, and for half a dozen rough benches which were piled in one corner. At the farther end hung a gaudy curtain, once handsome, but now sadly spotted with mildew because of dampness.

"Here's the inner chamber," laughed Frank, drawing the curtain aside. "And it looks like it was the private office of the bunch, too," he added, as he turned the light about the walls.

There was a desk in the third cavern, a swivel chair, a small case of books, and a rusty safe, which looked as though it had not been opened for years. A current of fresh air came from the rear, and a small opening was soon discovered.

"That doubtless leads to some canyon not far away," Ned said. "Makes a pretty decent place of it, eh?"

"Good enough for any person to hide in," replied Frank. "Now," he added, "tell me what you think of it. Who cut this cavern, and who brought the furniture here? I'll admit that my thinker is not working."

"Nature made the caverns," Ned replied. "There is what geologists call a fault in the rock here. Owing to volcanic action, doubtless, the strata shifted, probably thousands of years ago, and when the seam appeared the broken pieces fell apart. These chambers show the width of the seam. There undoubtedly was a great earthquake at the time, and the lake below might have been dredged out at that time."

"Of course," Frank said, "I might have known that! Now, here's another question: How far does this seam extend under the Rocky Mountains? If it passes beyond these three chambers, why not make a fourth room for ourselves so as to be on the spot when the men who make headquarters of the place come back?"

"That may be a good thing to do," Ned admitted, "but, still, I would not like to be the one to lie in wait here. Suppose we try to learn something of the character of the people who come here? They seem to sleep on dry leaves and eat off benches. Rather tough bunch, I take it. Perhaps we have struck Uncle Sam's enemies the first thing!"

Keeping their lights on, and working as silently as possible, always with an eye to the outer cavern, the boys made a careful search of the inner chamber. The desk was not fastened, and a cupboard afterward discovered in a niche was open also. There were dishes in the cupboard and writing materials in the desk.

At the very bottom of the desk drawer Ned came upon a surprise.

"Not so tough as I supposed," he said, turning to Frank. "Here's a typewriter ribbon. The sort of people who set fire to forests and hold up trains are hardly in the typewriter class. What do you make of it?"

"Well," Frank said, with a chuckle, "if you'll tell me what the inhabitants of this place want of typewriter ribbons I'll tell you why they bring great tins of opium here. It seems that we have struck something more important than forest fires."