“The Scotland Yard man. He was to arrive at New Aidenn by motor early in the afternoon since the trains were slow. While the House was empty, they investigated, and found this machine. Mr. Salt expected something like it. This was the real weapon, of course; that stone half buried in the loam was a blind.”
“You’ve known this—long?”
“How could I? I had a hint of it when I kept finding in so many places how the old castle here was built on a mill-site: Cwm Melin, you know. It even happened that Mr. Bannerlee knew that name and that name only for this place. He had never heard of Aidenn Vale.”
“The devil with Bannerlee. What’s a mill got to do with it?”
“The mill-wheel, don’t you see, winds up the spring of the machine. It must be quite automatic, and I dare say at this moment the cat’s claw—I suppose that’s what it is—the long heavy arm of iron, is ready to leap out again.”
Doctor Aire’s face revealed a ferment within. “By jingo—I think I have it. That mocking roar—hideous—was the sound of water tumbling into a cistern, or a heavy cask. Then if the cistern discharged over the wheel, the gear actuating the arm would wind until—yes, by thunder, that’s it!”
“What’s what?”
“We heard the purr. That was the gear winding against the resistance of the spring—a sword-spring, perhaps. When the tension exceeded the strength of the spring, the accursed thing let fly. There must be a shaft. . . .” The Doctor lapsed into mumbling.
“Beneath the perfidious tree!” screamed Mrs. Bartholomew so suddenly that we all jumped. “What does that mean?”
Miss Lebetwood answered, “There was once a cross—see the traces—carved on the chimney.”