"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 clew. 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."

"And how could you tell that they would make their attempt to-night?" I asked.

"Well, when they closed their League offices that was a sign that they cared no longer about Mr. Jabez Wilson's presence--in other words, that they had completed their tunnel. But it was essential that they should use it soon, as it might be discovered, or the bullion might be removed. Saturday would suit them better than any other day, as it would give them two days for their escape. For all these reasons I expected them to come to-night."

"You reasoned it out beautifully," I exclaimed in unfeigned admiration "It is so long a chain, and yet every link rings true."

"It saved me from ennui," he answered, yawning. "Alas! I already feel it closing in upon me. My life is spent in one long effort to escape from the commonplaces of existence. These little problems help me to do so."

"And you are a benefactor of the race," said I.

He shrugged his shoulders. "Well, perhaps, after all, it is of some little use," he remarked. " 'L'homme c'est rien--l'oeuvre c'est tout,' as Gustave Flaubert wrote to George Sand."

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