"It isn't new," he remarked. "No, guv'nor." "How long has it been running?" "I couldn't say. I'm not buying this on the hire system." "You fellows do that sometimes, eh?" "Yes, guv'nor, there are several of us chaps own their own taxi." "That's good. Now for Richmond, and go slowly from Hyde Park Corner." I never remember a more tedious journey. Quarles hardly spoke a word the whole way, but sat leaning forward, looking keenly from one side of the road to the other, as if he were bent on obtaining a mental picture of every yard of the way. Arriving at Richmond Green he did no more than just glance at the house where Sir John had dined that night, and then told the man to drive to Twickenham as fast as he liked to go. "Stop him when we reach Tavener's house, Wigan. You know it, I suppose?" I did, and stopped the driver when we got there. Quarles had the car turned round, then he got out and examined the tires with his lenses. The driver winked at me, and I nodded to assure him that I knew the eccentric gentleman I had to deal with, and that he was quite harmless. We then drove back to Richmond rapidly, and from there went toward town, but more slowly. By Kew Gardens along to Kew Bridge Quarles did not seem particularly interested in the journey, but as we drew near Hammersmith he became alert again. We were going slowly past St. Paul's school when he told the driver to take the second turning to the left. It was a narrow street, a big warehouse, which was being enlarged, on one side, and a coal yard on the other. About fifty yards down this street, the driver was instructed to stop. "We will get out for a minute and look at the view," said Quarles facetiously. I confess I found nothing whatever to interest me, but Quarles seemed to find the blank walls of the warehouse and coal yard attractive. "Now, driver, you can turn round and get us back to Hyde Park Corner as quickly as you like," said the professor as we got into the taxi again. Arriving at our destination he told the driver to go into the park, and there stopped him. Again he examined the tires and the texture of them, picking some soil from the rubber, and he scraped up some dust from the floor of the taxi with a penknife and put it in an envelope. "Thank you, my man," he said, paying a substantial fare. "You're welcome, guv'nor," said the driver with a grin. "He is fully persuaded that he has been driving a lunatic and his keeper," Quarles said as he walked away. "I suppose you can find the driver of the other taxi, Wigan." "We might have found him this morning. He lives at Twickenham." "I want you to see him and ask him two questions. First, was the fog in Hammersmith, or elsewhere on the journey, thick enough to bring him to a standstill before he reached Hyde Park Corner? Secondly, is he quite sure that the man who opened the door and called to him had not just got out of the taxi?" "But--"