"We sure will have our hands full," declared the young inventor. "Trying to solve the problem of carrying guns on an aerial warship, and finding out who set this fire." "Then you're not going to give up your aerial warship idea?" "No, indeed!" Tom cried. "What made you think that?" "Well, the way your father spoke--" "Oh, dear old dad!" exclaimed Tom affectionately. "I don't want to argue with him, but he's dead wrong!" "Then you are going to make a go of it?" "I sure am, Ned! All I have to solve is the recoil proposition, and, as soon as we get straightened out from this fire, we'll tackle that problem again--you and I. But I sure would like to know who put this in my red shed," and Tom looked in a puzzled manner at the empty fire bomb he still held. Tom paused, on his way to the house, to put the bomb in one of his offices. "No use letting dad know about this," he went on. "It would only be something else for him to worry about." "That's right," agreed Ned. By this time nearly all evidences of the fire, except for the blackened ruins of the shed, had been cleared away. High in the air hung a cloud of black smoke, caused by some chemicals that had burned harmlessly save for that pall. Tom Swift had indeed had a lucky escape. The young inventor, finding his father quieted down and conversing easily with Mr. Damon, who was blessing everything he could think of, motioned to Ned to follow him out of the house again. "We'll leave dad here," said Tom, "and do a little investigating on our own account. We'll look for clues while they're fresh." But, it must be confessed, after Tom and Ned had spent the rest of that day in and about the burned shed, they were little wiser than when they started. They found the place where the fire bomb had evidently been placed, right inside the main entrance to the shed. Tom knew it had been there because there were peculiar marks on the charred wood, and a certain queer smell of chemicals that confirmed his belief. "They put the bomb there to prevent anyone going in at the first alarm and saving anything," Tom said. "They didn't count on the roof burning through first, giving me a chance to use the sand. I made the roof of the red shed flimsy just on that account, so the force of the explosion if one ever came, would be mostly upward. You know the expanding gases, caused by an explosion or by rapid combustion, always do just as electricity does, seek the shortest and easiest route. In this case I made the roof the easiest route." "A lucky provision," observed Ned. That night Tom had to confess himself beaten, as far as finding clues was concerned. The empty fire bomb was the only one, and that seemed valueless. Close questioning of the workmen failed to disclose anything. Tom was particularly anxious to discover if any mysterious strangers had been seen about the works. There was a strict rule about admitting them to the plant, however, and it could not be learned that this had been violated.