"I don't know how to put it any plainer," Dick admitted--and what followed caused the waiter to wheel around and suddenly commence dusting off a table that already was bright enough to see your face in. "There wasn't the slightest clue left after the Black Tom affair?" Mary asked, as she straightened her hat. "Not one. We did find two of the bombs that hadn't exploded--devilishly clever arrangements, with a new combination of chemicals. Something was evidently wrong with the mixture, though, for they wouldn't go off, even when our experts started to play with them. The man who made them evidently wasn't quite sure of his ground. But there wasn't a thing about the bombs themselves that would provide any indication of where they came from." "The man who made them must have had a pretty thorough knowledge of chemistry," Mary mused. "Mighty near perfect," admitted Walters. "At least six exploded on time, and, from what I understand, they were loaded to the muzzle with a mixture that no one but an expert would dare handle." "And," continued Mary, with just a hint of excitement in her voice, "the bomb-maker would continue to investigate the subject. He would want to get the latest information, the most recent books, the--" "What are you driving at?" Walters interrupted. "Just this," and Mary leaned across the table so that there was no possibility of being overheard. "We girls have a good deal of time on our hands, so we get into the habit of making conjectures and forming theories about the 'regulars'--the people who come into the Library often enough for us to know them by sight. "Up to a month ago there was a man who dropped into the reference room nearly every day to consult books from Shelf Forty-five. Naturally he came up to my desk, and, as he usually arrived during the slack periods, I had plenty of time to study him. Maybe it was because I had been reading Lombroso, or possibly it's because I am just naturally observant, but I noticed that, in addition to each of his ears being practically lobeless, one of them was quite pointed at the top--almost like a fox's. "For a week he didn't show up, and then one day another man came in and asked for a book from Shelf Forty-five. Just as he turned away I had a shock. Apparently he wasn't in the least like the other man in anything save height--but neither of his ears had any lobes to speak of and the top of them was pointed! When he returned the book I looked him over pretty thoroughly and came to the conclusion that, in spite of the fact that his general appearance differed entirely from the other man's, they were really one and the same!" "But what," grumbled Walters, "has that to do with the Black Tom explosion?" "The last time this man came to the Library," said Mary, "was two days before the night you failed to arrive--two days before the explosion. And--Do you know what books are kept on Shelf Forty-five?" "No. What?" "The latest works on the chemistry of explosives!"