“But the under side is as stained as the upper.  It must have left a mark.”

Lestrade chuckled with delight at having puzzled the famous expert.

“Now, I’ll show you the explanation.  There is a second stain, but it does not correspond with the other.  See for yourself.”  As he spoke he turned over another portion of the carpet, and there, sure enough, was a great crimson spill upon the square white facing of the old-fashioned floor.  “What do you make of that, Mr. Holmes?”

“Why, it is simple enough.  The two stains did correspond, but the carpet has been turned round.  As it was square and unfastened it was easily done.”

“The official police don’t need you, Mr. Holmes, to tell them that the carpet must have been turned round.  That’s clear enough, for the stains lie above each other—­if you lay it over this way.  But what I want to know is, who shifted the carpet, and why?”

I could see from Holmes’s rigid face that he was vibrating with inward excitement.

“Look here, Lestrade,” said he, “has that constable in the passage been in charge of the place all the time?”

“Yes, he has.”

“Well, take my advice.  Examine him carefully.  Don’t do it before us.  Well wait here.  You take him into the back room.  You’ll be more likely to get a confession out of him alone.  Ask him how he dared to admit people and leave them alone in this room.  Don’t ask him if he has done it.  Take it for granted.  Tell him you know someone has been here.  Press him.  Tell him that a full confession is his only chance of forgiveness.  Do exactly what I tell you!”

“By George, if he knows I’ll have it out of him!” cried Lestrade.  He darted into the hall, and a few moments later his bullying voice sounded from the back room.

“Now, Watson, now!” cried Holmes with frenzied eagerness.  All the demoniacal force of the man masked behind that listless manner burst out in a paroxysm of energy.  He tore the drugget from the floor, and in an instant was down on his hands and knees clawing at each of the squares of wood beneath it.  One turned sideways as he dug his nails into the edge of it.  It hinged back like the lid of a box.  A small black cavity opened beneath it.  Holmes plunged his eager hand into it and drew it out with a bitter snarl of anger and disappointment.  It was empty.

“Quick, Watson, quick!  Get it back again!” The wooden lid was replaced, and the drugget had only just been drawn straight when Lestrade’s voice was heard in the passage.  He found Holmes leaning languidly against the mantelpiece, resigned and patient, endeavouring to conceal his irrepressible yawns.

“Sorry to keep you waiting, Mr. Holmes, I can see that you are bored to death with the whole affair.  Well, he has confessed, all right.  Come in here, MacPherson.  Let these gentlemen hear of your most inexcusable conduct.”

The big constable, very hot and penitent, sidled into the room.