Don’s plan, quietly communicated to his two friends and to Mr. McLeod, was to be tried.
They would leave the disabled helicopter as it was, go back to their daily tasks in the designing department of the aircraft plant and the airport, and keep a watch on the control chief, on Doc Morgan, and on the swamp, from the control tower balcony, with strong binoculars.
No garage, accessory store or hardware shop could replace the special carburetor parts for some time; after instructing their own shop foreman to report any application for the parts that might be made to him, Don rode in to Port Washington with his uncle and visited every shop, garage and other place where such things were available, told enough to enlist attention, without disclosing any of their suspicions about the tracing and its possible meaning, and secured a promise to have a report made of any request for carburetor floats and valves of the unusual type they had looked up in the catalogues.
“Now,” remarked Don, as their sedan returned to the airport, “if any spook, or man who calls himself a thing that never was, comes around to put his helicopter into commission, we can grab him, for we will know that no one else would be after floats like the one we removed.”
His uncle nodded, morose and uncommunicative.
Rejoining his chums, Don explained his recent activities.
“I found that the blue-print files had the lock picked,” Chick told him, “and the blue-print I made of the sketch was gone.”
“I’ve just come down from the tower balcony,” added Garry. “The helicopter hasn’t moved. I just barely made out the blades above the grass. The way they are kept when it isn’t being used, the blades are sideways to the line of sight from the control tower. That’s why none of us, especially since we weren’t looking for it, ever saw the thing.”
“Probably Mister Spectre-Man had it there all during the haunting time,” Chick remarked.
“But what did he use it for—and how did he use it?” objected Don. “The spooky airplanes were biplanes, old-timers, and there never was a helicopter in sight.”