It's very nice of you, of course, and I appreciate it."
"No, but really, Tony."
"Oh, my dear Bill!" He smoked silently for a little, and then went on, "It's what I was saying just now—a secret is a secret until you have discovered it, and as soon as you have discovered it, you wonder why everybody else isn't discovering it, and how it could ever have been a secret at all. This passage has been here for years, with an opening at one end into the library, and at the other end into the shed. Then Mark discovered it, and immediately he felt that everybody else must discover it. So he made the shed end more difficult by putting the croquet-box there, and this end more difficult by—" he stopped and looked at the other "by what, Bill?"
But Bill was being Watsonish.
"What?"
"Obviously by re-arranging his books. He happened to take out 'The Life of Nelson' or 'Three Men in a Boat,' or whatever it was, and by the merest chance discovered the secret. Naturally he felt that everybody else would be taking down 'The Life of Nelson' or 'Three Men in a Boat.' Naturally he felt that the secret would be safer if nobody ever interfered with that shelf at all. When you said that the books had been re-arranged a year ago just about the time the croquet-box came into existence; of course, I guessed why. So I looked about
