Going from the brightly lighted shop into the darkness of the night, illuminated as it was only by the stars, neither Tom, Ned, nor Eradicate, could see anything at first. They had to stand still for a moment to accustom their eyes to the gloom.

"Can you see them?" cried Tom to his chum.

"No, but I can hear them! Over this way!" yelled Ned, and then, being able to dimly make out objects, so he would not run into them, he started off, followed by the young inventor.

Tom could hear several persons running away now, but he could see no one, and from the sound he judged that the spies, if such they were, were hurrying across the fields that surrounded the shop.

It was almost a hopeless task to pursue them, but the two lads were not the kind that give up. They rushed forward, hoping to be able to grapple with those who had looked in the shop window, but it was not to be.

The sound of the retreating footsteps became more and more faint, until finally they gave no clew to follow.

"Better stop," advised Tom. "No telling where we'll end up if we keep on running. Besides it might be dangerous."

"Dangerous; how?" panted Ned.

"They might dodge around, and wait for us behind some tree or bush."

"An' ef dat Foger feller am around he jest as soon as not fetch one ob us a whack in de head," commented Eradicate grimly.

"Guess you're about right," admitted Ned. "There isn't much use keeping on. We'll go back."

"What sort of fellows were they?" asked Tom, when, after a little further search, the hunt was given up. "Could you see them well, Ned?"

"Not very good. Just as I went to get you that wrench I noticed two faces looking in the window. I must have taken them by surprise, for they dodged down in an instant. Then I yelled, and they ran off."

"Did you see Andy Foger?"

"No, I didn't notice him."

"Was either of them one of the spies who had Mr. Petrofsky in the hut?"

"I didn't see those fellows very well, you remember, so I couldn't say."

"That's so, but I'll bet that's who they were."

"What do you think they're after, Tom?"

"One of two things. They either want to get our Russian friend into their clutches again, or they're after me--to try to stop me from going to Siberia."

"Do you think they'd go to such length as that?"

"I'm almost sure they would. Those Russian police are wrong, of course, but they think Mr. Petrofsky is an Anarchist or something like that, and they think they're justified in doing anything to get him back to the Siberian mines. And once the Russian government sets out to do a thing it generally does it--I'll give 'em credit for that."

"But how do you suppose they know you're going to Russia?"

"Say, those fellows have ways of getting information you and I would never dream of. Why, didn't you read the other day how some fellow who was supposed to be one of the worst Anarchists ever, high up in making bombs, plotting, and all that sort of thing--turned out to be a police spy? They get their information that way. I shouldn't be surprised but what some of the very people whom Mr. Petrofsky thinks are his friends are spies, and they send word to headquarters of every move he makes."

"Why don't you warn him?"

"He knows it as well as I do. The trouble is you can't tell who the spies are until it's too late. I'm glad I'm not mixed up in that sort of thing. If I can get to Siberia, help Mr. Petrofsky rescue his brother, and get hold of some of that platinum I'll be satisfied. Then I won't go back to the land of the Czar, once I get away from there."

"That's right. Well, let's go back and work on the glider."

"And we'll have Eradicate patrolling about the shop to make sure we're not spied on again."

"By golly! Ef I sees any oh 'em, I suah will pinch 'em!" cried the colored man, as he clicked the pliers.

But there was no further disturbance that night, and, when Tom and Ned ceased work, they had made good progress toward finishing the air glider.

The big airship was almost ready to be given a trial flight, with her motors tuned up to give more power, and as soon as the Russian exile had a little more definite information as to the possible whereabouts of his brother, they could start.

In the days that followed Tom and his friends worked hard. The air glider was made as nearly perfect as any machine is, and in a fairly stiff gale, that blew up about a week later, Tom did some things in it that made his friends open their eyes. The young inventor had it under nearly as good control as he had his dirigible balloons or aeroplanes.

The big airship, too, was made ready for the long voyage, extra large storage tanks for gasolene being built in, as it was doubtful if they could get a supply in Siberia without arranging for it in advance, and this they did not want to do. Besides there was the long ocean flight to provide for.

"But if worst comes to worst I can burn kerosene in my motor," Tom explained, for he had perfected an attachment to this end. "You can get kerosene almost anywhere in Russia."

At last word was received from Russia, from some Revolutionist friends of the exile, stating that his brother was supposed to be working in a certain sulphur mine north of the Iablonnoi mountains, and half way between that range and the city of Iakutsk.

"But it might be a salt mine, just as well," said Mr. Petrofsky, when he told the boys the news. "Information about the poor exiles is hard to get"

"Well, we'll take a chance!" cried Tom determinedly.

The preparations went on, and by strict watchfulness none of the spies secured admission to the shop where the air glider was being finished. The big airship was gotten in shape for the voyage, and then, after a final trial of the glider, it was taken apart and put aboard the Falcon, ready for use on the gale-swept plains of Siberia.

The last of the stores, provisions and supplies were put in the big car of the airship, a route had been carefully mapped out, and Tom, after saying good-bye to Mary Nestor, his father, the housekeeper, and Eradicate, took his place in the pilot house of the airship one pleasant morning at the beginning of Summer.

"Don't you wish you were going, Rad?" the young inventor asked, for the colored man had decided to stay at home.

"No indeedy, Massa Tom," was the answer. "Dat's a mighty cold country in Shebeara, an' I laik warm wedder."

"Well, take care of yourself and Boomerang," answered Tom with a laugh. Then he pulled the lever that sent a supply of gas into the big bag, and the ship began to rise.

"I guess we've given those spies the slip," remarked Ned, as they rose from the ground calling good-byes to the friends they left behind.

"I hope so," agreed Tom, but could he have seen two men, of sinister looks, peering at the slowly-moving airship from the shelter of a glove of trees, not far off, he might have changed his opinion, and so would Ned.

Then, as the airship gathered momentum, it fairly sprang into the air, and a moment later, the big propellers began revolving. They were off on their long voyage to find the lost platinum mine, and rescue the exile of Siberia.