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.