"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!"
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."