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CHAPTER II.—THE SIGNAL IN THE SKY.

Frank left the cavern in a moment and walked along the beach toward the campfire. His thought was to gather embers and fresh fuel and build up a blaze at the end of the cave which would reveal every inch of the interior. He was certain that Jack had not left the place, and decided that he had fallen into some hidden opening which had escaped his own investigation.

As he bent over the remains of the fire he heard a rattle of small stones, and, looking up, saw Pat coming down the declivity from the plateau where the tents had been set up. The incline was steep, and at times Pat was rolling rather than walking. He was in his shirt sleeves and bareheaded. At last his red head pitched toward the lake like a meteor in downward flight.

Frank rushed forward and caught him as he struck the beach, thus saving him from an impromptu bath. Pat struggled to his feet in an instant, rubbed his legs and arms to see if any bones had been broken, and then turned his head and looked up the incline.

“Talk about shooting the chutes!” he exclaimed. “I wonder what time I made coming down?”

“Sure you’re not hurt?” asked Frank anxiously.

“Every inch of my body has three bruises, one on top of the other,” Pat replied, “but I guess I’m able to walk. Say, but that was a roller-coaster glide!”

“Why did you try such a foolish caper?” asked Frank.

“Why, I saw you boys here,” was the reply, “and started down. You know the rest, as the yellow-covered books say. What you boys doing here, wasting your time, with the bacon burning to a crisp?”

“We came here to investigate,” was the reply, “and Jack went into the cavern, and vanished—just vapored into thin air. I’m going to build a fire in there and see if I can’t condense him!”

“Well,” Pat said, listening, “he may have vanished physically, but his voice appears to be on deck yet.”

Three sharp calls came from the cavern, and both boys dashed inside. There was no doubt now that Jack’s voice, at least, had condensed, for the shouts coming from the back of the cavern were both hearty and imperative.

“Hi, there!” Jack called. “Pry this stone out of the doorway!”

“Where are you?” demanded Pat. “Which one of the walls do you want us to push in? You’re a nice chump, getting in a scrape like this!” he added, with a laugh which must have been exasperating to the unseen boy.

“You’ll find a crevice where the back of the cave joins the south wall,” Jack said, his voice coming faintly to the ears of his chums. “Put your fingers in and pull. The blooming door opens outward. Hurry! It’s stifling in here!”

After burning nearly all the matches they had in their pockets, and scorching their fingers on the short sticks, Pat and Frank discovered the crevice spoken of and inserted the ends of their fingers.

“Pull!” yelled Jack. “Pull, you loafers! It is moving!”

In a moment the south half of the back wall swung out so suddenly that both boys were thrown from their feet and Jack, who had been pushing with his whole strength, came tumbling on top of them as they lay on the floor of the cavern.

“What sort of a combination is this, anyway?” demanded Pat, struggling to his feet. “If I get any more bumps to-day I’ll be taking something that belongs to some one else. I’ve had my share.”

Frank sprang to the opening as soon as he could disentangle himself from the collection of arms and legs and looked in. All was dark and still inside, and a gust of dead air struck him in the face. Pat, leaning over his shoulder, laid a hand on the rock which had opened so strangely, and the next instant it closed softly, sliding into the opening like a door operated by well-oiled machinery.

“Now you’ve done it!” Frank exclaimed, disgustedly, as Pat threw himself against the stone in a vain effort to force it open again.

“No harm done,” Jack exclaimed. “There’s only a stinking cavern in there. Wow! I can feel snakes and lizzards crawling on me now! Come! Let us get into the open air. Stifles like a grave in here.”

The boys hastened outside and stood meditatively before the shining waters of the lake, each one trying to think clearly concerning what had taken place. They believed themselves—or had believed, rather—miles away from any trace of civilization, and yet here was a practical door of rock at the end of a cave almost under the great divide.

“We’ve found something,” Frank said, at length. “That thing in there never happened. Human hands fashioned that door for some secret purpose. And it wasn’t Indians, either.”

“I guess we’ve run up against a band of train robbers,” suggested Jack, with a grin.

“Probably the entrance to some deserted mine,” Pat put in. “This region has been searched for gold for fifty years. I’ve heard of mines being concealed by moving stones.”

“Well,” Frank said, after a short silence, during which all listened for some indication of the immediate presence of the men who had been seen to row around the green point a short time before, “whatever the game is, we’ve got to remove every trace of our visit. When they come back they probably won’t notice the tracks we have made, for there were plenty about before we came here, but we must gather up all the match-ends we left in there and leave the door as we found it.”

“I found it open and walked in,” Jack said, “and then it closed. Whew! I felt like I was being shut up in a tomb!”

“How large a place is it in there?” asked Pat.

“Don’t know,” was the reply. “I had no matches with me, and so could not see a thing.”

“Then we won’t have to open the door again to clean up any muss,” Frank said, moving toward the entrance to the cavern.

“I wouldn’t go in again for a thousand dollars,” Jack cried. “If you leave it to me, the place is haunted. I heard groans in there.”

Frank paused at the entrance and turned back. His matches were about gone, and so he took a burning stick from the fire, added two dry faggots to it, waited until the three burst into flame, and then entered the cave.

To gather up the half-burned matches which had been scattered over the floor was the work of only a moment.

“Now you’ll have to open the door, if you leave it as I found it,” Jack said, looking in from the mouth. “Pat will help you.”

“Come on in, both of you,” Frank directed.

“Not me!” cried Jack. “I hear bones rattling!”

The boys thought he was joking at first, but it soon appeared that he was in sober earnest, so Pat and Frank, by exerting their entire strength, managed to open the door without his assistance.

“You’re afraid of the dark!” Pat taunted, as the boys gathered around the fire again.

“I’m not half as afraid of the dark as you are of an aeroplane,” Jack replied. “If I ever see you going up in a ’plane, I’ll go in there alone.”

“Don’t