The Secret of the Old Mill by Franklin W. Dixon

morning, and continue a search on my own.”
    When Joe asked if the eight-and-one pattern referred to the saboteurs’ schedule, his father nodded. “In the other plants, the sabotage took place eight weeks plus one day apart.
    â€œIn each of those plants,” the detective went on, “the damage occurred right after closing time. Figuring the schedule would be exactly right for
    an attempt on Elekton in a couple of days, I started a systematic check of the various buildings. I planned to check daily, until the saboteurs had been caught here or elsewhere. At my request, one company security guard was assigned to assist me. I felt that the fewer people who knew what I was doing, the better. That’s how I ruined the saboteurs’ plan in Detroit.
    â€œNothing suspicious occurred here until today when I took up a post in the section of the building where the experimental work is being conducted. After all the employees had left, and the dim night-lights were on, I went toward the east lab wing to investigate.”
    Mr. Hardy paused, took a deep breath, and continued, “Just as I reached the lab, I happened to glance back into the hall. Things started to happen—fast.”
    â€œWhat did you see, Dad?” asked Joe, and all the boys leaned forward expectantly.
    The detective went on, “Hurrying down the hall from the west lab were two men in work clothes, one carrying a leather bag. I knew there weren’t supposed to be any workmen in the building. I stepped out to question them, but the pair broke into a run and dashed past me down the stairs.”
    â€œDid you see what either of them looked like?” Frank asked.
    â€œI did catch a glimpse of one before they broke away. He had heavy features and thick eyebrows. But just as I was about to take off after them, I smelled something burning in the east lab and went to investigate. The first thing I saw was a long fuse sputtering toward a box of dynamite, set against the wall.
    â€œI didn’t know if it was the kind of fuse that would burn internally or not, so I took my pen-knife and cut it close to the dynamite. Professional saboteurs don’t usually rely on just one explosive, so I started for the west wing to check the lab there.”
    Mr. Hardy leaned back in his chair and rubbed the bruise on his temple. In a low voice he said, “But I didn’t make it. I was running toward the hall when there was a roar and a burst of flame. The explosion lifted me off my feet and threw me against the wall. Though I was stunned, I managed to get back to the east wing. I reached for the phone, then blacked out.
    â€œI must have been unconscious for some time because when the firemen found me and helped me out of the building, the fire had been put out.”
    â€œYou’re all right now?” asked Frank.
    â€œYes. It was a temporary blackout from shock. What bothers me is that I had the saboteurs’ pattern figured out—only they must have become panicky, and moved up their nefarious scheme two days.”
    Joe looked grim. “I wish we’d been there to help you capture those rats!”
    Chet asked Mr. Hardy if he would like a fruit drink. “I’ll make some lemonade,” he offered.
    â€œSounds good.” Mr. Hardy smiled.
    As they sipped the lemonade, Frank and Joe questioned their father about his theories.
    â€œI’m still convinced,” said Mr. Hardy, “that one of those men works in the plant. How else would he have known when the watchman makes his rounds and how to disconnect the electronic alarms? But I can’t figure how the outside accomplice got in—those gates are carefully guarded.”
    At this point, Frank told his father about the green truck. “We suspected at first it might be connected with the counterfeiters. Now we have a hunch the saboteurs may have used it.”
    Fenton Hardy seemed greatly encouraged by this possible lead. Joe gave him the