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 Rh extent occupied the ground between it and the second or northern wall, and even beyond the latter. Similarly the Roman limes in what is now Germany and Austria advanced or receded, not so much as indicating a fluctuating movement of the real boundaries of the Empire, as following the best line of military defence that was suggested by the exigencies of the time. Trajan's conquest of Dacia was of course a positive, though only a temporary, extension of the Empire.

Rudimentary in conception though these structural barriers may be thought to have been, they were effective in the age and against the foes for whom they were devised. There can be no greater mistake than to ridicule them as monuments of misdirected effort or of human vanity. The Great Wall of China, commenced before the Christian Era and continued at intervals for 1,700 years, was a genuine palladium to the heart of the Chinese Empire. Though occasionally circumvented and more than once pierced by the nomad hordes, for centuries it held back the Mongolian Tartars from Peking, acting as a fiscal barrier for the prevention of smuggling and the levying of dues, as a police barrier for the examination of passports and the arrest of criminals or suspects, and as a military barrier against hostile invasions or raids. It was even more a line of trespass than a Frontier. Much the same might be said of the Roman walls, whether directed against the Picts and Scots, or against the Marcomanni and Teutonic tribes. Guarded by fortified posts or forts at intervals, with watch-towers between, garrisoned in the early days of the Roman Empire by veterans of the army, in later times by native auxiliaries, with the great legionary camps in the rear, they kept the front of the