"Murdoch did something to his that makes it different," explained Benj. "What, no one has ever known until that brilliant brother of mine unraveled the code. But if the Hoard had been a standard resonator, people would have uncovered it long years ago. There's nothing tricky about getting a response from a resonant cavity." Benj set the flier on the autopilot and went forward into the nose of the craft with tools. He emerged a moment later with a crooked smile. "All I had to do was to hitch up Cal's original junk. The detector is running as it always was, but now I can shoot forth a signal from Cal's equipment, stop it, and receive on my own detector. We had a fistful of duplicate Keys around the lab. We can't follow Murdoch's Hoard, but we can follow Cal--who is on the trail of Murdoch's Hoard." He snapped a switch, and a thrumming whine came immediately. "That will be Cal's response," said Benj cheerfully. "No matter how he tries, he'll lead us to the spot." Cal sped along in the thick white blanket of fog, not knowing that his own Key was furnishing a lead-spot for another. Had he known, it is possible that he would have stopped and had his argument when the other arrived, or perhaps he could have damped the resonator enough so that its decrement was short enough to prevent any practical detection of the response. But Cal was admittedly no technician. He did not realize that his own resonator would become a marker. So he sped along through the white at a killing pace. He snapped the switch after some time and listened to the response from Murdoch's Hoard--as well as another signal that blended with his. The latter did not bother him as it might have bothered an engineer. Cal had no way of knowing what the results would be, and so he accepted the dual response as a matter of fact. It was in the third hour of travel that the inevitable came. By rights, it should have come easily and quietly, but it came with all of the suddenness of two fliers running together at better than five hundred miles per hour. Out of the whiteness that had blocked his vision all day, Cal saw his brother's black flier. It came through the sky silently, skirling the fog behind it into a spiral whirl. It came at a narrow angle from slightly behind him, and both pilots slammed their wheels over by sheer instinct. The fliers heeled and cut sweeping arcs in the fog. Inches separated their wingtips and they were gone on divergent courses. Cal mopped his brow. In the other ship, Benj swore roundly at Cal, and mopped his brow, too. And Tinker sat on the divan, letting her breath out slowly. But Benj whipped the wheel around, describing a full, sharp loop in the sky. He crammed a bit of power on, and the tail of Cal's ship came into sight through the fog. Cal saw him coming and whipped his plane aside. Benj anticipated the maneuver and followed Cal around, crowding him close. "What are you trying to do?" screamed Tinker, white-faced. "Run him down," gritted Benj.