we'll do our best to help you," promised Tom. "But now I think we
had better see what sort of a defense we can put up. We have our guns
and revolvers, and with these chairs and tables we can build a sort of
barricade behind which we can take refuge if those fellows do discover
our hiding place."

This was conceded to be a good idea, and soon a rude sort of fort was
made, behind which the adventurers could take their stand and fight, if
necessary, though they hoped this would not come to pass.

They remained quietly in the cave the remainder of that day, and, when
it was night, as they could tell by their timepieces--there was no
daylight--they divided the hours into watches, taking turns standing
guard.

Morning, at least in point of time, came without any disturbance, and
they made a cold breakfast. They hoped that Bill Renshaw would come, but
he did not appear.

After sitting in the dark cave until afternoon, Tom said:

"I think we might as well go and take another observation of the big
cave. We can tell what the men are doing, then, for they don't seem to
have been near us. Maybe they have given up the search for us, and we
can see them at work, and Mr. Jenks can gain what further knowledge he
needs."

"That will be a good plan," agreed the diamond man. "It's maddening to
sit here, doing nothing."

"And it will be comparatively safe to go from here to our former post of
observation," added Tom, "for there doesn't seem to be any opening along
the tunnel, into the larger cave, except the place where we were."

Accordingly they started off. Cautiously they looked through the opening
into the apartment where they had seen the diamonds made.

"There's not a soul here!" exclaimed Tom, in a whisper. The others
looked. The place was deserted--the machinery silent. Mr. Jenks peered
in for a moment, and then exclaimed:

"I'm going in! Now's my chance to find out all that I wish to know! It
may never come again, and then we can soon leave Phantom Mountain!"

It was a daring plan, but it seemed to be the best one to follow. They
were all tired of inactivity. Mr. Jenks managed to get through the
opening, and dropped into the big cave. The others followed. Mr. Jenks
hurried over to the steel box, and began an examination of it. Tom Swift
was looking at the electrical switch. He saw how it was constructed. Mr.
Damon and Mr. Parker were peering interestedly about.

Suddenly the sound of voices was heard, and the echo