“But how could you guess what the motive was?”
“Had there been women in the house, I should have suspected a mere vulgar intrigue. That, however, was out of the question. The man’s business was a small one, and there was nothing in his house which could account for such elaborate preparations, and such an expenditure as they were at. It must, then, be something out of the house. What could it be? I thought of the assistant’s fondness for photography, and his trick of vanishing into the cellar. 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. I was ascertaining whether the cellar stretched out in front or behind. It was not in front. Then I rang the bell, and, as I hoped, the assistant answered it. We have had some skirmishes, but we had never set eyes upon each other before. I hardly looked at his face. His knees were what I wished to see. You must yourself have remarked how worn, wrinkled, and stained they were. They spoke of those hours of burrowing. The only remaining point was what they were burrowing for. I walked round the corner, saw the City and Suburban Bank abutted on our friend’s premises, and felt that I had solved my problem. When you drove home after the concert I called upon Scotland Yard and upon the chairman of the bank directors, with the result that you have seen.”