“It’s all right.”

“Good.”  He took the book out of its shelf again.  “Now then, you can hold Ussher.  Hold him in the left hand so.  With the right or dexter hand, grasp this shelf firmly so.  Now, when I say ‘Pull,’ pull gradually.  Got that?”

Bill nodded, his face alight with excitement.

“Good.”  Antony put his hand into the space left by the stout Ussher, and fingered the hack of the shelf.  “Pull,” he said.

Bill pulled.

“Now just go on pulling like that.  I shall get it directly.  Not hard, you know, but just keeping up the strain.”

His fingers went at it again busily.

And then suddenly the whole row of shelves, from top to bottom, swung gently open towards them.

“Good Lord!” said Bill, letting go of the shelf in his amazement.

Antony pushed the shelves back, extracted Ussher from Bill’s fingers, replaced him, and then, taking Bill by the arm, led him to the sofa and deposited him in it.  Standing in front of him, he bowed gravely.

“Child’s play, Watson,” he said; “child’s play.”

“How on earth—­”

Antony laughed happily and sat down on the sofa beside him.

“You don’t really want it explained,” he said, smacking him on the knee; “you’re just being Watsonish.  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 for the dullest books I could find, the books nobody ever read.  Obviously the collection of sermon-books of a mid-Victorian clergyman was the shelf we wanted.”

“Yes, I see.  But why were you so certain of the particular place?”