In this connection, however--that of estuaries--there is one case which is puzzling: the case of the Thames. An explanation can, I think, be found, though at first it looks anomalous. The Romans dealt with the estuary of the Severn and of the Humber by ferries; they dealt by long bridges with lesser obstacles. In the same fashion they carried the north road over the Trent by a direct line without deflection for a special crossing. They carried it across the Tyne at deep water approached steeply. They carried it across the Thames at Staines with a sole regard to the direction of their road and without considering special opportunities of crossing. They did the same at Dorchester; and instances could be multiplied all over the kingdom. But apparently they did not attempt to attack the Thames estuary. When one considers the nature of the early fighting during the first conquest of the island by the Romans this is astonishing. All the campaigns began in Kent, and the more serious of them were carried on into East Anglia. The great rising under Nero was an East Anglian rising, and the Roman armies beaten there had to be rapidly reinforced from Kent. For 400 years troops poured in, under any special emergency, from Dover, came up through Kent, and any immediate necessity of reaching a point east of London necessitated a détour by London Bridge: though time might be vital, the deflection was suffered. Why did the Romans not solve the difficulty and establish at least a ferry across the lower Thames? Of course, they may have done so. You can never argue from the absence of traces to-day that a Roman road did not exist, for it is astonishing how thoroughly time eliminates such things. There are whole great towns like Aquilea and Hippo of which not even the foundations remain to-day. Even in England, where Roman survival is most marked, two towns, Silchester and Uriconium, have gone save for a few ruins; and there are great stretches of Roman road in every country of Western Europe which have mysteriously and wholly disappeared without leaving a trace of the tremendous work undertaken to build them; for instance, the miles after Epsom racecourse. Still, it does look as though no direct Roman line connected Canterbury, for instance, with Colchester. And I say again, how are we to account for it?