y scientist left the cave.

The weather was very cold, but, in the cavern it was hardly noticed.
The adventurers were warmly dressed, and when they did get chilly
from working over the airship, they had but to go into the well-heated
and cozy cabin to warm themselves.

It was on the third day of their habitation in the cave, and work on
putting the patches on the gas-bag was almost finished. Mr. Parker
had gone out to make further observations, his previous ones not
having satisfied him. Tom was on an improvised platform, putting a
patch on top of the bag, when he heard a sudden yell, and some one
dashed into the cavern.

"They're coming! They're coming!" cried a voice, and Tom, looking
down, saw Mr. Parker, apparently in a state of great fear.

"What's coming?" demanded the young inventor, "the icebergs?"

"No--the Indians!" yelled the scientist. "A whole tribe of them is
rushing this way!"

"I thought so!" cried Abe Abercrombie. "Where's my gun?" and he
dashed into the airship.

Tom slid down off the platform.

"Get ready for a fight!" he gasped. "Where are you, Ned?"

"Here I am. We'd better get to the mouth of the cave, and drive 'em
back from there."

"Yes. If I'd only thought, we could have blockaded it in some way.
It's as big as a barn now, and they can rush us if they have a mind
to. But we'll do our best!"

The adventurers were now all armed, even to Mr. Parker. The
scientist had recovered from his first fright, when he spied the
Indians coming over the snow, as he was "observing" some natural
phenomenon. Tom, even in his excitement, noticed that the professor
was curiously examining his gun, evidently more with a view to
seeing how it was made, and on which principle it was operated,
rather than to discover how to use it.

"If it comes to a fight, just point it at the Indians, pull the
trigger, and work that lever," explained the young inventor. "It's
an automatic gun."

"I see," answered Mr. Parker. "Very curious. I had no idea they
worked this way."

"Oh, if I only had my electric rifle in shape!" sighed Tom, as he
dashed forward at the side of Ned.

"Your electric rifle?"

"Yes, I've got a new kind of weapon--very effective. I have it
almost finished. It's in the airship, but I can't use it just yet.
However, maybe these repeaters will do the work."

By this time they were at the entrance of the cave, and, looking out
they saw about a hundred Indians, dressed in furs, striding acros