Moreover, the proportion of the known to the unknown is considerable: 60 per cent. of the total distance of 120 miles is known to 40 per cent. unknown, and it must be understood that throughout this book I speak of those parts as 'known' or 'recognised,' which have been universally admitted since the study of the subject was approached by archæologists. [Illustration] These two facts, the considerable proportion of the known to the unknown, and the absence of any very long stretch in which the Road is lost, facilitate the task in a manner that can be put best graphically by some such little sketch as the preceding, where the dark line is the known portion of the Road. It is evident that the filling up of the gaps is indicated by the general tendency of the rest. There is a mass of other indications besides the mere direction to guide one in one's research; and a congeries of these together make up what I have called the method by which we approached the problem. That method was to collate all the characteristics which could be discovered in the known portions of the Road, and to apply these to the search for traces of the lost portion. Supporting such a method there are the _a priori_ arguments drawn from geographical and geological conditions. There are place-names which point out, though only faintly, the history of a village site. There is the analogy of trails as they exist in savage countries at the present day. There is the analogy of other portions of prehistoric tracks which still exist in Britain. All these confirm or weaken a conclusion, but still the most important arguments are found in the characteristics which can be discovered in the known portions of the Road, and which may be presumed, in the absence of contradictory evidence, to attach to the lost portions also. When a gap was reached, it was necessary to form an hypothesis to guide one in one's next step, and such an hypothesis could best be formed upon a comparison of all these various kinds of knowledge. The indication afforded by any one of them would, as a rule, be slight, but the convergence of a number of such indications would commonly convey a very strong presumption in favour of some particular track. It was then our business to seek for some remaining evidences, apparent to the eye, whereby the track could be recovered. Such evidences were the well-known fact that a line of very old yews will often mark such a road where it lies upon the chalk; the alignment of some short path with a known portion behind, and a known portion before one; while, of course, the presence of a ridge or platform upon such an alignment we regarded (in the absence of any other lane) as the best guide for our search.