THE northern frontier of the Roman Empire is shown in atlases of ancient geography, and that of the Achaemenian Empire of the Persians and of the dynasties which succeeded in the Middle East. The,frontier of the ancient Chinese Empire has not been made similarly familiar, and in place of it there is the representation of the Empire of China as it has been in mediaeval and modern times. From this most of Manchuria, all Mongolia, and the Ili valley must be shorn off in order to get the Chinese northern frontier as it was under the Han dynasty in the beginning of the second century after Christ, the age of the Antonines in Europe. At this time, when the ancient civilisation of Eurasia was at the height of its culture and apparently at the maximum of its power, the northern frontier once controlled by the Persian Achaemenidae was divided between the Parthians, capitalled at Ctesiphon, and the Kushan dynasty of the Yue-chi, capitalled at Peshawar. These four northern frontiers, Roman, Parthian, Kushan, and Chinese, were consecutive, forming an unbroken line from the mouth of the Rhine near the modern Katwyk in Holland, 52° N., to the east coast of Korea in about 41° N. South of the line a vast array of established cities stretched for seven thousand miles across Eurasia, in some parts protected by natural barriers, in others defended by lines of masonry fortification. North of the line were the tents of nomads, huts of forest dwellers, and stockaded defences of earth and wood. In the northern part of modern Germany there were territories north of the line which the Romans had abandoned as untenable or unprofitable. South of the line in Eastern Europe was the district of Dacia which Augustus preferred not to touch, but Trajan was compelled to occupy. In this country the native people had in the interval begun to construct masonry fortifications.