Walters sat up with a jerk that threatened to overthrow the table. "Mary," he said, in a whisper, "I've a hunch that you've succeeded where all the rest of us fell down! The disguises and the constant reference to books on explosives are certainly worth looking into. What name did this man give?" "Names," she corrected. "I don't recall what they were or the addresses, either. But it would be easy to find them on the cards. We don't have very many calls for books from Shelf Forty-five." "It doesn't matter, though," and Walters slipped back into his disconsolate mood. "He wouldn't leave a lead as open as that, of course." "No, certainly not," agreed Mary. "But the last time he was there he asked for Professor Stevens's new book. It hadn't come in then, but I told him we expected it shortly. So, unless you men have scared him off, he'll be back in a day or two--possibly in a new disguise. Why don't you see the librarian, get a place as attendant in the reference room, and I'll tip you off the instant I spot that pointed ear. That's one thing he can't hide!" The next morning there was a new employee in the reference room. No one knew where he came from and no one--save the librarian and Mary McNilless--knew what he was there for, because his principal occupation appeared to be lounging around inconspicuously in the neighborhood of the information desk. There he stayed for three days, wondering whether this clue, like all the rest, would dissolve into thin air. About five o'clock on the afternoon of the third day a man strolled up to Mary's desk and asked if Professor Stevens's book had come in yet. It was reposing at that moment on Shelf Forty-five, as Mary well knew, but she said she'd see, and left the room, carefully arranging her hair at the back of her neck with her left hand--a signal which she and Dick had agreed upon the preceding evening. Before she returned the new attendant had vanished, but Dick Walters, in his usual garb, was loitering around the only entrance to the reference room, watching the suspect out of the corner of his eye. "I'm sorry," Mary reported, "but the Stevens book won't be in until to-morrow," and she was barely able to keep the anxiety out of her voice as she spoke. Had Dick gotten her signal? Would he be able to trail his man? Could he capture him without being injured? These and a score of other questions rushed through her mind as she saw the German leave the room. Once outside--well, she'd have to wait for Dick to tell her what happened then.