Penny had not forgotten her resolution to visit the Hamilton Plant by daylight. As soon as she had helped Mrs. Gallup with the dishes, she left the house, walking directly to the scene of the previous evening's adventure. The street was deserted. No one questioned her actions as she made a careful inspection of the old building which had housed the Hamilton Manufacturing Company until its failure. She examined the walls inch by inch, but although she was convinced it was there, she could find no hidden entrance. Regardless of her failure to find evidence, Penny was unwilling to give up her original theory. She remained unshaken in her belief that the mysterious automobile had disappeared into the Hamilton building. "There's no other place it could have gone," she reasoned. "I'll talk it over with Dad and see what he thinks." When she stopped at his office, Mr. Nichols was not there nor could Miss Arrow tell her when he might return. The detective did not come home for luncheon and late in the afternoon telephoned to say that he would take dinner downtown. Rather than spend an evening alone Penny called Susan, arranging that they should go to the library together. The girls spent an hour in the reading room, but for some reason Penny could not interest herself in the magazines. She kept turning through them and laying them aside. She felt unusually restless. Presently an electrical magazine attracted her attention. She glanced over it carelessly until she came to a particular article which dealt with photo-electric cells and the clever purposes for which they were used. "Why, these 'magic-eyes' are almost human," she commented in an undertone to Susan. "They turn lights on and off, cook meals, and open doors, when a beam of light strikes the cell----" "I've heard of them before," Susan interrupted in a tone which clearly implied that she was not in the least interested. Penny took the hint and dropped the subject. But she became absorbed in the article. When she closed the magazine a half hour later, her face was flushed with excitement. "Susan, let's get away from here," she proposed in a whisper. "I've just had an inspiration!" Grumbling a little at being forced to leave a fascinating story before she had finished it, Susan followed her friend from the building. "What about this inspiration of yours?" she demanded as they walked to Penny's parked roadster. "It's this way, Susan. I knew there was a logical explanation for the mysterious disappearance of that car Rap Molberg was driving. Let's go over to the Hamilton Factory this minute and test out my new theory." "You may know what you're talking about, but I'm sure I don't, Penny Nichols." "That's because you wouldn't let me tell you about that article I was reading," Penny laughed. "But I'll explain everything as we go along."