"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.