"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.
The man who was interested in the chemistry of explosives apparently wasn't in the least afraid of being followed, for he took a bus uptown, alighted at Eighty-third Street, and vanished into one of the innumerable small apartment houses in that section of the city. Walters kept close behind him, and he entered the lobby of the apartment house in time to hear his quarry ascending to the fourth floor. Then he signaled to the four men who had followed him up the Avenue in a government-owned machine—men who had been stationed outside the Library in the event of just such an occurrence—and instructed two of them to guard the rear of the house, while the other two remained in front.
"I'm going to make this haul myself," Walters stated, "but I want you boys to cover up in case anything happens to me. No matter what occurs, don't let him get away. Shoot first and ask questions afterward!" and he had re-entered the house almost before he finished speaking.
On the landing at the third floor he paused long enough to give the men at the rear a chance to get located. Then—a quick ring at the bell on the fourth floor and he waited for action.