To begin at the beginning.

I approached the house, as you know, on foot, and with my mind entirely free from all impressions.

I naturally began by examining the roadway, and there, as I have already explained to you, I saw clearly the marks of a cab, which, I ascertained by inquiry, must have been there during the night.

I satisfied myself that it was a cab and not a private carriage by the narrow gauge of the wheels.

The ordinary London growler is considerably less wide than a gentleman's brougham.
"This was the first point gained.