The Clue of the Silver Key by Edgar Wallace

THE DETECTIVE LEFT the Yard a few minutes after eleven and, turning to his left, walked towards Blackfriars. To Surefoot Smith that long ribbon of pavement which runs without a break from Scotland Yard to Savoy Hill was a garden of thought. At headquarters somebody with a florid mind had christened it his 'Boulevard of Cogitation'. Summer or winter, rain or fine, Surefoot Smith found here the solution of all his problems. Men had been hanged, swindlers had be sent down to the shades, very commonplace happenings had assumed a sinister importance and, by contrast, seemingly guilty men and women had had their innocence established the course of Surefoot Smith's midnight recreation.

There were very few pedestrians at this hour of the night. The courting couples, for some strange reason, chose the better lighted river side of the road. Cars flashed past occasionally, and once an occasional night hawk shuffled along the kerbside in search of a stray cigarette end.