Holmes raised his eyebrows.
"And yet you have sent for me?"
"Ah, yes, that's another matter -- a mere trifle, but the sort of thing you take an interest in -- queer, you know, and what you might call freakish. It has nothing to do with the main fact -- can't have, on the face of it."
"What is it, then?"
"Well, you know, after a crime of this sort we are very careful to keep things in their position. Nothing has been moved. Officer in charge here day and night. This morning, as the man was buried and the investigation over -- so far as this room is concerned -- we thought we could tidy up a bit. This carpet. You see, it is not fastened down; only just laid there. We had occasion to raise it. We found ----"
"Yes? You found ----"
Holmes's face grew tense with anxiety.
"Well, I'm sure you would never guess in a hundred years what we did find. You see that stain on the carpet? Well, a great deal must have soaked through, must it not?"
"Undoubtedly it must."
"Well, you will be surprised to hear that there is no stain on the white woodwork to correspond."
"No stain! But there must ----"
"Yes; so you would say. But the fact remains that there isn't."
He took the corner of the carpet in his hand and, turning it over, he showed that it was indeed as he said.
"But the underside 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. We'll 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.