"I do not fancy the whole four would have been renewed at the same time," he returned. "It is not usual. My conclusion was that the taxi had not been used very much." "I must confess I do not see where this is leading us," I said. "It led us to Twickenham, Wigan. In our down journey we covered the road taken by the taxi that night if it came direct to Hyde Park Corner. At Twickenham I examined the tires, and they satisfied me that so far there was nothing to negative a theory I had formed. On the return journey we turned into that side street--I had noted it on the way down--and at the end of our journey I examined the tires again and the floor of the taxi. I preserved what I found then in this envelope, and it is perfectly clear that our taxi had been driven over a road strewn with brick dust and coal dust, and that persons treading on such a road had entered the taxi." "Of course, we both got out," I remarked. "To admire the view," said Quarles. "And you may have noticed that there were few windows from which an inquisitive person could have told what we were doing. At night the place would be quite lonely unless the bricklayers and coal porters were working overtime. Now, Wigan, on the tires of the first taxi, and on its carpet, was dust exactly corresponding to that which I found on the tires and floor of our taxi. That is significant. Brick dust and coal dust together, remember. They are not a usual combination on a main road out of London." I did not answer, I had no comment to make. "If we have no very definite facts," Quarles went on, "we have many peculiar circumstances, and I will try and reconstruct the tragedy for you. Sir John and his wife have quarreled at times we know, and to some extent at any rate have gone each their own way recently. The fact that Sir John was the cause of her divorce, and married her, may be taken as proof that he was fond of his wife. A reformed rake constantly is, and often develops a strong vein of jealousy besides. That Lady Tavener was supposed by her husband to be dining with the Folliotts, who, as a fact, had no appointment with her that night, shows that she did not always explain her going and coming to her husband. I suggest that Sir John had begun to suspect his wife, and that his reason for leaving Richmond early was to ascertain whether she was going to the theater with the Folliotts as she had told him." "It is an ingenious theory," I admitted.