Soon the queer actions of the stranger were quite forgotten in the deep interest of the three young men in their work. With the prospect of a world tour before them if the Sky-Bird turned out well, they now had more incentive than at the beginning to build the machine with the utmost skill and attention to every detail. Some changes, calculated to make the craft better adapted to the peculiar conditions she would be likely to meet in such a varied temperature were put into effect, but on the whole they found their original plans so well laid that no important features seemed to require modification or abandonment. But if the man who had followed Bob dropped out of their minds the rest of that day, he was soon to occupy a prominent place in their thoughts. For the very next morning, when Paul and John arrived at the hangar, they were met at the door by a very agitated Bob Giddings. "Fellows, what do you think has happened?" cried Bob, clearly very much excited. Without giving his friends time to answer the question he blurted out: "Somebody got in here last night and stole our plans!" "Stole our plans!" reiterated Paul and John in the same gasp. "That's it," said Bob,--"stole the set of blue-prints we have been working from. What's more, they must have seen the airplane before they got out. I went to take the plans out of the bench drawer here where we have kept them locked up, and there was the drawer wide open, the lock picked, and the drawings gone. I'll bet a herring we can thank my dark-skinned shadow of yesterday for this little visit!" "It does look as if he might have had something to do with this," agreed John soberly. "I wonder how the rascal, whoever he is, could have gotten in the building. There's a heavy Yale lock on the doors." "The doors were locked all right when I came this morning," vouched Bob. "I don't see myself how--" "Here you are, gentlemen!" called Paul, who had stepped to a good-sized window near the head of the workbench. "Here's the fellow's private entrance!" And he pointed to where a heavy nail locking the lower sash had been forced aside, also to a series of indentations in the outer sill, where some prying tool had obviously been recently at work. "It's a clear case of theft, that's sure," observed John; "and since its only our plans that have been taken, it goes to show that this chap is very much concerned about this new airplane." "Perhaps he wishes to beat us out in getting the patent rights," Bob hinted darkly. "No, I don't think it's that," differed Paul; "our application was sent in to Washington some weeks ago, and you know the first one to apply for a certain patent gets the attention." "Well, then, he could use our plans and make and sell airplanes of their pattern, couldn't he?" asked Bob, whose ideas of patent laws were still a little vague.