Commenting upon one of my earlier novels, in respect of which I had claimed to have been careful to adhere to common probabilities and to have made use only of really practicable methods of investigation, a critic remarked that this was of no consequence whatever, so long as the story was amusing.  
Few people, I imagine, will agree with him. To most readers, and certainly to the kind of reader for whom an author is willing to take trouble, complete realism in respect of incidents and methods is an essential factor in maintaining the interest of a detective story. Hence it may be worth while to mention that Thorndyke's method of producing the track chart, described in Chapters II and III, has been actually used in practice. It is a modification of one devised by me many years ago when I was crossing Ashanti to the city of Bontuku, the whereabouts of which in the far interior was then only vaguely known. My instructions were to fix the positions of all towns, villages, rivers and mountains as accurately as possible; but finding ordinary methods of surveying impracticable in the dense forest which covers the whole region, I adopted this simple and apparently rude method, checking the distances whenever possible by astronomical observation.
 
The resulting route-map was surprisingly accurate, as shown by the agreement of the outward and homeward tracks, It was published by the Royal Geographical Society, and incorporated in the map of this region compiled by the Intelligence Branch of the War Office, and it formed the basis of the map which accompanied my volume of Travels in Ashanti and Jaman. So that Thorndyke's plan must be taken as quite a practicable one.
 
New Inn, the background of this story, and one of the last surviving inns of Chancery, has recently passed away after upwards of four centuries of newness. Even now, however, a few of the old, dismantled houses (including perhaps, the mysterious 31) may be seen from the Strand peeping over the iron roof of the skating rink which has displaced the picturesque hall, the pension-room and the garden. The postern gate, too, in Houghton Street still remains, though the arch is bricked up inside. Passing it lately, I made the rough sketch which appears on next page, and which shows all that is left of this pleasant old London backwater.
 
 
R. A. F.
GRAVESEND