"I don't see how he got them," observed Mark. "There doesn't seem to be anything broken, to indicate how the thief got in, and he certainly didn't touch Professor Henderson's live wire." Not a window or a door had been forced, and the two machinists, who slept in the shop, declared they had heard no suspicious sounds during the night. It was a mysterious theft, and there seemed to be no means of solving it. At Mr. Roumann's suggestion they all increased their hours of work on the Annihilator. They wanted to have it finished ahead of the time set, and it seemed that this would be done. Day after day, and far into the night, they labored. Bit by bit the machinery was installed, the supplies were gathered together, the great water tanks were built, to provide a supply of the fluid in case of any accident to the distilling apparatus. The Etherium motor was almost finished, and the other, motor, which was to drive the Annihilator through the earth's atmosphere, was nearly ready to install. The steering apparatus necessitated considerable labor, and when it was finished Amos Henderson declared they had made a mistake, and would have to build it all over again. This lost them a week, and time was precious, as there was no telling what the thief would do with the stolen plates. "I tell you what, but we're going to have a better ship than any of the others we built," remarked Jack one day, as he and Mark were putting the finishing touches to the living-room. "This isn't a ship," said Mark. "It's a projectile." "I guess I can call it a ship if I want to," was the retort. "It's going to sail through the air, and it's an airship, of course. Wait until you see the one I'm going to build when I get that new gas invented." "I'll not go with you," said Mark. "There's too much danger of being blown up." "There won't be, after I have it perfected. But say, won't it be fine when we're shooting through space to sit here in an easy chair and read a book and eat sandwiches?" "I guess you think as much of eating as you do of reading, Jack." "Well, almost, that's a fact. I must cut out some of my eating, too. I've gained five pounds this week, because of not doing any studying. But wait until I get to Mars. Then I'll weigh less." "I hope Mr. Roumann lets us help run the machinery," went on Mark. "I guess he'll have to. He'll need help, and I understand that he and the professor, you and I, and Washington and Andy are the only ones going along. He and the professor can't run the affair all alone, and they'll have to have our help. Wash and Andy won't be much good at machinery." "That's so. My! Think of steering a two hundred-foot projectile through space, when we're moving at the rate of one hundred miles a second!" "Great, isn't it?" commented Jack. "It would be a bad thing if it ever got away from us," said Mark. "Yes; or if we steered into a comet." "That's so. We may run into one of those things--or a shooting star."