"I don't know about that," answered the Russian. "It is almost
impossible to rescue a person from Siberia. Of course it is not going
to be easy to locate the lost mine, but as for that we can keep on
searching, that is if the air glider works, but there are so many
forces to fight against in rescuing a prisoner."

They had a long journey ahead of them, and not an easy route to follow,
but as the days passed, and they came nearer and nearer to their goal,
they became more and more eager.

They were passing over a desolate country, for they avoided the
vicinity of large towns and cities.

"I wonder when we'll strike Siberia?" mused Tom one afternoon, as they
sat on the outer deck, enjoying the air.

"At this rate of progress, very soon," answered the exile, after
glancing at the map. "We should be at the foot of the Ural mountains in
a few hours, and across them in the night. Then we will be in Siberia."

And he was right, for just as supper was being served, Ned, who had
been making observations with a telescope, exclaimed:

"These must be the Urals!"

Mr. Petrofsky seized the glass.

"They are," he announced. "We will cross between Orsk and Iroitsk. A
safe place. In the morning we will be in Siberia--the land of the
exiles."

And they were, morning seeing them flying over a most desolate stretch
of landscape. Onward they flew, covering verst after verst of
loneliness.

"I'm going to put on a little more speed," announced Tom, after a visit
to the storeroom, where were kept the reserve tanks of gasolene. "I've
got more fluid than I thought I had, and as we're on the ground now I
want to hurry things. I'm going to make better time," and he yanked
over the lever of the accelerator, sending the Falcon ahead at a rapid
rate.

All day this was kept up, and they were just making an observation to
determine their position, along toward supper time, when there came the
sound of another explosion from the motor room.

"Bless my safety valve!" cried Mr. Damon. "Something has gone wrong
again."

Tom ran to the motor, and, at the same time the Falcon which was being
used as an aeroplane and not as a dirigible, began to sink.

"We're going down!" cried Ned.

"Well, you know what to do!" shouted his chum. "The gas bag! Turn on
the generator!"

Ned ran to it, but, in spite of his quick action, the craft continued
to slide downward.

"She won't work!" he cried.

"Then the intake pipe must be stopped!" answere