We all stood still and listened. A low moaning sound was coming from somewhere over our heads. Holmes rushed to the door and out into the hall. The dismal noise came from upstairs. He dashed up, the inspector and I at his heels, while his brother Mycroft followed as quickly as his great bulk would permit.

Three doors faced up upon the second floor, and it was from the central of these that the sinister sounds were issuing, sinking sometimes into a dull mumble and rising again into a shrill whine. It was locked, but the key had been left on the outside. Holmes flung open the door and rushed in, but he was out again in an instant, with his hand to his throat.

“It’s charcoal,” he cried. “Give it time. It will clear.”