"We will assume that when he left her Lady Tavener was alive," Quarles went on. "At Hyde Park Corner she was dead, and the driver Wood was entirely ignorant that anything had happened. Yet, if murder was done, some one must have joined Lady Tavener during the journey. Wood says he was not held up by the fog, but on being pressed a little, speaks of coming nearly to a standstill in Hammersmith Broadway. There, or somewhere else, because we must remember Wood may have forgotten nearly coming to other stoppages, since driving in a fog must have required the whole of his attention--somewhere, somebody must have joined her. The driver, again under pressure, admits the bare possibility, but does not think it likely. However, we must assume that some one at some place did enter the taxi." Zena was leaning forward eagerly, and I waited quietly for Quarles to continue. "It follows that whoever it was must have been known to Lady Tavener," he said slowly. "Otherwise she would have called out to the driver or to people passing." "You mean that he left it at Hyde Park Corner after the murder," said Zena. "You think it was Lester Williams." "There is the possibility that he was getting out of the taxi instead of rushing to it, because he noticed the occupant looked peculiar," Quarles admitted. "In that case would he have called the driver's attention?" I asked. "Your theory seems to demand actions which no man would be fool enough to commit." "You can never tell upon what lines a criminal's brain will work, Wigan. I maintain that the same arguments I have used with regard to Sir John would apply in Lester Williams's case. Still, there are one or two points to consider. If you go to Hyde Park Corner you will find it difficult to pitch on any lamp which could throw sufficient light upon the face of the occupant leaning back in the corner as to cause alarm to any one on the pavement. I am taking into consideration the position of the taxi in the roadway and the angle at which the light would have to be thrown. And, since motor lights are in the front of cars, and Lady Tavener was facing the way her taxi was going, it is very improbable that the lights of another car would serve this purpose. Besides, it was a foggy night." "Then you believe Williams was getting out of the taxi?" I asked. "Let me talk about the contents of this first," said Quarles, separating an envelope from some papers on the table. "You will admit that I examined the taxi fairly thoroughly." "You certainly did." "And I came to one or two very definite conclusions, Wigan. The engine is practically new, very different from that of the taxi we took to Twickenham, which was of exactly the same make. I took some trouble in my choice of a taxi, you remember. I grant, of course, this may not be a very reliable proof, but the tires told the same story, I think." "The first taxi might just have had new tires," I suggested.