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 was in the forest, to guide the Nihilists to her, now that she was up in the air, and in motion, it was feared that her presence would become known to the authorities of the town, so even these had been extinguished.

"After we get well away we can turn on the electrics," remarked Tom, "and if they see us at a distance they may take us for a meteor. But, so close as this, they'd get wise in a minute."

Mr. Damon, who had done all that Tom needed in the starting of the craft, went to the forward port rail, and idly looked down on the black forest they were leaving. He could just make out the clearing where they had rested for over a week, and he was startled to see lights bobbing in it.

"I say, Mr. Petrofsky!" he called. "Did we leave any of our lanterns behind us?"

"I don't believe so," answered the exile. "I'll ask Tom."

"Lanterns? No," answered the young inventor. "Before we started I took down the only one we had out. I'll take a look."

Setting the automatic steering apparatus, he joined Mr. Damon and the Russian. The lights were now dimly visible, moving about in the forest clearing.

"It's just as if they were looking for something," said Tom. "Can it be that any of your Nihilist friends, Mr. Petrofsky are--"

"Friends--no friends--enemies!" cried the Russian. "I understand now! We got away just in time. Those are police agents who are looking for us! They must have received word about our being there. Androwsky and the others never carry lights when they go about. They know the country too well, and then, too, it leads to detection. No, those are police spies. A few minutes later, and we would have been discovered."

"As it is we're right over their heads, and they don't know it," chuckled Tom. The airship was moving silently along before a good breeze, the propellers not having been started, and Tom let her drift for several miles, as he did not want to give the police spies a clew by the noise of the motor.

The twinkling lights in the forest clearing disappeared from sight, and the seekers went on in the darkness.

"Well, we've got the hardest part of our work yet ahead of us," remarked Tom several hours later when, the lights having been set aglow, they were gathered in the main cabin. There was no danger of being seen now, for they were quite high.

"We've done pretty well, so far," commented Ned. "I think we will have easier work rescuing Mr. Petrofsky's brother than in locating the mine.

"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!" answered the young inventor. "Never mind, I'll volplane to earth and we can make repairs. That magneto has gone out of business again."

"Don't land here!" cried Ivan Petrofsky.

"Why not?"

"Because we are approaching a large town--Owbinsk I think it is-the police there will be there to get us. Keep on to the forest again!"

"I can't!" cried Tom. "We've got to go down, police or no police."

Running to the pilot house, he guided the craft so that it would safely volplane to earth. They could all see that now they were approaching a fairly large town, and would probably land on its outskirts. Through the glass Ned could make out people staring up at the strange sight.

"They'll be ready to receive us," he announced grimly.

"I hope they have no dynamite bombs for us," murmured Mr. Damon. "Bless my watch chain! I must get rid of that Nihilist literature I have about me, or they'll take me for one," and he tore up the tracts, and scattered them in the air.

Meanwhile the Falcon continued to descend.

"Maybe I can make quick repairs, and get away before they realize who we are," said Tom, as he got ready for the landing.

They came down in a big field, and, almost before the bicycle wheels had ceased revolving, under the application of the brakes, several men came running toward them.

"Here they come!" cried Mr. Damon.

"They are only farmers," said the exile. He had donned his dark glasses again, and looked like anything but a Russian.

"Lively, Ned!" cried Tom. "Let's see if we can't make repairs and get off again."

The two lads frantically began work, and they soon had the magneto in running order. They could have gone up as an aeroplane, leaving the repairs to the gas bag to be made later but, just as they were ready to start, there came galloping out a troop of Cossack soldiers. Their commander called something to them.

"What is he saying?" cried Tom to Mr. Petrofsky.

"He is telling them to surround us so that we can not get a running start, such as we need to go up. Evidently he understands aeroplanes."

"Well, I'm going to have a try," declared the young inventor.

He jumped to the pilot house, yelling to Ned to start the motor, but it was too late. They were hemmed in by a cordon of cavalry, and it would have been madness to have rushed the Falcon into them, for she would have been wrecked, even if Tom could have succeeded in sending her through the lines.

"I guess it's all up with us," groaned Ned.

And it seemed to; for, a moment later, an officer and several aides galloped forward, calling out something in Russian.

"What is it?" asked Tom.

"He says we are under arrest," translated the exile.

"What for?" demanded the young inventor.

Ivan Petrofsky shrugged his shoulders.

"It is of little use to ask--now," he answered. "It may be we have violated some local law, and can pay a fine and go, or we may be taken for just what we are, or foreign spies, which we are not. It is best to keep quiet, and go with them."

"Go where?" cried Tom.

"To prison, I suppose," answered the exile. "Keep quiet, and leave it to me. I will do all I can. I don't believe they will recognize me.

"Bless my search warrant!" cried Mr. Damon. "In a Russian prison! That is terrible!"

A few minutes later, expostulations having been useless, our friends were led away between guards who carried ugly looking rifles, and who looked more ugly and menacing themselves. Then the doors of the Russian prison of Owbinsk closed on Tom and his friends, while their airship was left at the mercy of their enemies.