The cellar! There was the end of this tangled clue.

Then I made inquiries as to this mysterious assistant and found that I had to deal with one of the coolest and most daring criminals in London.

He was doing something in the cellar--something which took many hours a day for months on end.

What could it be, once more?
I could think of nothing save that he was running a tunnel to some other building.
"So far I had got when we went to visit the scene of action.

I surprised you by beating upon the pavement with my stick.