“This is all very well,” said I, “but the thing becomes more unintelligible than ever. How about this mysterious ally? How came he into the room?”

“Yes, the ally!” repeated Holmes, pensively. “There are features of interest about this ally. He lifts the case from the regions of the commonplace. I fancy that this ally breaks fresh ground in the annals of crime in this country,—though parallel cases suggest themselves from India, and, if my memory serves me, from Senegambia.”

“How came he, then?” I reiterated. “The door is locked, the window is inaccessible. Was it through the chimney?”

“The grate is much too small,” he answered. “I had already considered that possibility.”

“How then?” I persisted.

“You will not apply my precept,” he said, shaking his head. “How often have I said to you that when you have eliminated the impossible whatever remains, however improbable, must be the truth? We know that he did not come through the door, the window, or the chimney. We also know that he could not have been concealed in the room, as there is no concealment possible. Whence, then, did he come?”

“He came through the hole in the roof,” I cried.

“Of course he did. He must have done so. If you will have the kindness to hold the lamp for me, we shall now extend our researches to the room above,—the secret room in which the treasure was found.”

He mounted the steps, and, seizing a rafter with either hand, he swung himself up into the garret. Then, lying on his face, he reached down for the lamp and held it while I followed him.

The chamber in which we found ourselves was about ten feet one way and six the other. The floor was formed by the rafters, with thin lath-and-plaster between, so that in walking one had to step from beam to beam. The roof ran up to an apex, and was evidently the inner shell of the true roof of the house. There was no furniture of any sort, and the accumulated dust of years lay thick upon the floor.

“Here you are, you see,” said Sherlock Holmes, putting his hand against the sloping wall. “This is a trap-door which leads out on to the roof. I can press it back, and here is the roof itself, sloping at a gentle angle. This, then, is the way by which Number One entered. Let us see if we can find any other traces of his individuality.”

He held down the lamp to the floor, and as he did so I saw for the second time that night a startled, surprised look come over his face. For myself, as I followed his gaze my skin was cold under my clothes. The floor was covered thickly with the prints of a naked foot,—clear, well defined, perfectly formed, but scarce half the size of those of an ordinary man.

“Holmes,” I said, in a whisper, “a child has done the horrid thing.”