"I'm not so sure. Easy to hide in some out-of-the-way place. That machine worries me more than the loss of those blue prints. That's a sort of warning we ought to be thankful for, though it doesn't tell us much we didn't feel pretty sure of. We're working on the certainty that they'll try to interfere with this plant, and they've verified it for us.... But that aeroplane.... We can watch the employees, and we can do a pretty good job of guarding the plant on the ground, but how in thunder, Mr. Waite, are we going to stop anybody from flying over the top of it some night and dropping a ton or so of explosive on the roof? There's a German with something pretty close to genius running things hereabouts.... You've probably eaten dinner with him," Downs said, with a chuckle. "Eh? You mean you suspect some one?" "I mean that I don't, worse luck. What I mean is that the fellow who ought to be suspected is probably above suspicion. He's somebody you know well and meet every day. The kind of man who would have the enterprise to think of stealing an aeroplane is mighty apt to be the man you invite to dinner." "You'll have me suspecting everybody I know." "While you are making motors for the Signal Corps," said Downs, soberly, "that's exactly what you _must_ do. So far as your work is concerned, treat every living soul as if you knew he was a German spy." "It seems as if a thing as big as a hydro-aeroplane ought to be found," Potter said. "It's got to be found," said Downs. "Not only for the protection of the industrial plants in Detroit, but think of the danger it throws over the ship-canal at the Flats and the channel into Lake Erie. They could even work east as far as the Welland. An aeroplane and a few tons of explosive could come pretty close to bottling up the commerce of the Lakes, Mr. Waite.... And I've a notion that's the big game.... Our ore comes down the Lakes. Stop the ore and you stop the steel-mills. Then what?" "I guess," said Potter, slowly, "that we're really in the war at last." "The country will wake up some morning to a dazing realization of it," Downs said, as one states a fact which he dreads but knows to be inevitable. CHAPTER XIX