"Indeed it is--a great success! We must give the Newton shock absorber due credit." Ned blushed with pleasure. "It was only my suggestion," he said. "Tom worked it all out." "But I needed the suggestion to start with," the young inventor replied. "Of course something may develop when you take your craft high in the air, and discharge the guns there," said the lieutenant. "In a rarefied atmosphere the recoil check may not be as effective as at the earth's surface. But, in such case doubtless, you can increase the strength of the springs and the hydrostatic valves." "Yes, I counted on that," Tom explained. "I shall have to work out that formula, though, and be ready for it. But, on the whole, I am pretty well satisfied." "And indeed you may well feel that way," commented the government official. The Mars was hauled back into the shed, and the roof slid shut over the craft. Much yet remained to do on it, but now that Tom was sure the important item of armament was taken care of, he could devote his entire time to the finishing touches. As his plant was working on several other pieces of machinery, some of it for the United States Government, and some designed for his own use, Tom found himself obliged to hire several new hands. An advertisement in a New York newspaper brought a large number of replies, and for a day or two Tom was kept busy sifting out the least desirable, and arranging to see those whose answers showed they knew something of the business requirements. Meanwhile Lieutenant Marbury remained as Tom's guest, and was helpful in making suggestions that would enable the young inventor to meet the government's requirements. "I'd like, also, to get on the track of those spies who, I am sure, wish to do you harm," said the lieutenant, "but clues seem to be scarce around here." "They are, indeed," agreed Tom. "I guess the way in which we handled that fire in the red shed sort of discouraged them." Lieutenant Marbury shook his head. "They're not so easily discouraged as that," he remarked. "And, with the situation in Europe growing more acute every day, I am afraid some of those foreigners will take desperate measures to gain their ends." "What particular ends do you mean?" "Well, I think they will either try to so injure you that you will not be able to finish this aerial warship, or they will damage the craft itself, steal your plans, or damage some of your other inventions." "But what object would they have in doing such a thing?" Tom wanted to know. "How would that help France, Germany or Russia, to do me an injury?" "They are seeking to strike at the United States through you," was the answer. "They don't want Uncle Sam to have such formidable weapons as your great searchlight, the giant cannon, or this new warship of the clouds." "But why not, as long as the United States does not intend to go to war with any of the foreign nations?" Tom inquired.