he city of Abakansk."

"Where's that?" asked Tom for he had forgotten most of his Russian
geography.

"The Altai Mountains are a range about the middle of Siberia,"
explained Mr. Petrofsky. "They begin at the Kirghiz Steppes, and run
west. It is a wild and desolate place. I hope we can find poor Peter
alive."

"And this city of Abakansk?" went on the young inventor.

"It is many miles from here, but I can give you a good map," said the
Nihilist. "Some of our friends are there," he added with a half-growl.
"I wish we could rescue all of them."

"We'd like to," spoke Tom. "But I fear it is impossible. But now that
we have a clew, come on! Let's start at once! It may be dangerous to
stay here. On to Siberia!"

CHAPTER XVIII

IN A RUSSIAN PRISON

The news they had waited for had come at last. It might be a false
clew, but it was something to work on, and Tom was tired of inaction.
Then, too, even after they had started, the prisoner might be moved and
they would have to trace him again.

"But that is the latest information we could get," said Mr. Androwsky.
"It came through some of our Anarchist friends, and I believe is
reliable. Can you soon make a thousand miles in your airship?"

"Yes," answered Tom, "if I push her to the limit."

"Then do so," advised the Nihilist, "for there is need of haste. In
making inquiries our friends might incur suspicions and Peter Petrofsky
may be exiled to some other place."

"Oh, we'll get there," cried Tom. "Ned, see to the gas machine. Mr.
Damon, you can help me in the pilot house."

"Here is a map of the best route," said the Nihilist, as he handed one
to Mr. Petrofsky. "It will take you there the shortest way. But how can
you steer when high in the air?"

"By compass," explained Tom. "We'll get there, never fear, and we're
grateful for your clew."

"I never can thank you enough!" exclaimed the exile, as he shook hands
with Mr. Androwsky.

The Nihilist left, after announcing that, in the event of the success
of Tom and his friends, and the rescue of the exile from the sulphur
mine, it would probably become known to them, as such news came through
the Revolutionary channels, slowly but surely.

"Here we go!" cried the young inventor gaily, as he turned the starting
lever in the pilot house, and silently, in the darkness of the night,
the Falcon shot upward. There was not a light on board, for, though
small signal lamps had been kept burning when the craft wa