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
unscrewing 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