"But why don't they come on out?" asked Will, anxiously.

"Perhaps they have found Bert and don't want to leave him!" suggested Ed.

While the boys watched the red light, which seemed to glimmer from the very extremity of the cavern, it turned to blue!

"Now I've got it," cried Will almost dancing up and down in his excitement, "you know what that means, don't you?"

"I can't say that I do!" replied Ed.

"It seems to me that the Portland Boy Scouts are not very well posted,"

laughed Will. "One of the boys--which one, I don't know, of course--is talking to us in the Morse code!"

"Still I don't understand," said Ed.

"The red light means a dash," Will explained, "and the blue light means the dot. Now we'll see if we can catch what the boy is saying."

"But where does he get the red and blue lights?" asked Ed.

"From red and blue caps screwed over the electric searchlight," was the reply. "All of our electrics are provided with these signal caps."

"There, the light is red again!" cried Ed.

"I'll show how it works," Will said, bringing out his own flashlight and uns.c.r.e.w.i.n.g the false cover from the loading end.

Directly he had the blue and red caps out, and then the red one was fastened over the eye of the searchlight.

"There, you see!" Will exclaimed turning on the light. "We've got a beautiful red light and that means a Morse dash."

"I see," answered Ed. "And when you turn on the blue, that means a dot.

I learned the Morse code, of course, when I was admitted to the Boy Scouts, but I never knew that it was used in that way."

"I wonder if he sees this?" asked Will as he swung the red light back and forth in the growing twilight.

"We'll have to wait and see," replied Ed. "Of course, he'll answer if he knows we're here!"

Swiftly the light changed from red to blue and from blue back to red again. This took place several times and then Will said:

"Now, count!"

"Red," said Ed. "Red again. Red again."

"That's 'O'," exclaimed Will. "I guess we've got him at last!"

"Now there's another red," Ed went on. "Now there's a blue. Then one more red. Oh, this seems to be easy!"

"That's 'K'!" cried Will. "O. K., don't you see? O. K. That means that he knows we're here!"

"Glory be!" shouted Ed. "The boys are all right or they wouldn't be signalling. I hope they've found Bert!"

Will signalled back "O. K.," and then the lads turned back up the rivulet, the idea being to cross over to the north side.

"I want to find out why the boys don't show themselves instead of signalling," Will explained. "There must be some good reason."

After a walk of half a mile upstream the boys found it possible to cross without wading, and then they turned down toward the mouth of the cavern where the lights had been seen.

As they did so, two figures detached themselves from a group of trees which stood not far to the east and followed stealthily along behind them.

If the lads could have heard the conversation carried on at that time between Cameron and Fenton, they would have proceeded on their way with less confidence.

"Just what we've been looking for!" chuckled Cameron.

"We surely have them trapped now!" replied Fenton.

"They'll naturally step into the outer cavern to see why their chums don't walk out, and when they do so, we'll hold them up with our guns until we can build up a barrier which will keep them in."

"One of the boys certainly must understand the code we are so anxious about," Fenton observed. "That's the kid we want. We've certainly got to find out what that message contains! If the people in the east are trying to steal our plans, we certainly ought to know it!"

The boys, however, heard nothing of this talk and pa.s.sed on down the north side of the creek. As soon as they came opposite the cavern, in sight of the light once more, they stopped and began signalling.

As they did so, Cameron and Fenton came nearer and waited anxiously for the lads to enter the cavern.

"I'd like to know what all that signalling means!" said Cameron.

"Boy Scout signals," replied Fenton.

"You can't read them, can you?" asked the miner.

"Of course not," replied Fenton, "I'm no Boy Scout!"

The boys continued to signal back and forth until the situation was fairly well understood. Will and Ed knew that Bert had been found and that all three were barricaded in the cave.

They were disposed to make their way to the rescue of the boys without further delay, but George advised them to wait until it became darker, as Cameron might return at almost any moment. The news that Bert had regained consciousness was very welcome and, confident of their ability to thwart the plans of the miner, the boys looked forward to quiet hours in the cabin.

Of course the boys had no suspicion that their enemies were close at hand watching every movement. Cameron and Fenton became impatient, after a time, and began advancing slowly toward the boys, who were now not very far from the mouth of the outer cavern.

Something better than an hour pa.s.sed, and then George signalled from the interior of the cavern that it might be well for the boys to come up and begin the work of removing the rocks which barred their egress.

"Sneak In," George signalled. "Don't show yourself more than you have to. Cameron may be about! It may be that he has seen our signals already!"

Sandy replied that he had not discovered any indications of the presence of the miner, and the two boys advanced to the shelf of rock which faced the opening. It was nine o'clock then.

"What's that strange noise?" asked Will as they moved along the shelf.

"You've got me!" replied Ed, "The ground's tipping!"

There came a deafening crash and the whole face of the cliff fell away!

When Will and Ed regained their feet and looked through the dust which was rising over the scene, they saw that there was no longer any cavern in view. The rock on which they stood was sliding down the slope.