8 RENAISSANCES nean.r Though Roman roads and Roman legions left a deeper im- •ession on the mind of Posterity than Roman shipping-lanes and naval itrols, the Roman Empire was in truth a pool of water surrounded by hollow ring of land,2 in contrast to the structure of the Han Empire, hich was a plain of ploughland flanked on one side by a moated ghland citadel.3 When we compare this maritime structure of the Roman Empire ith the continental structure of the Han Empire, we can see that the an-Hellenic 'thalassocracy' had two politico-geographical weaknesses om which the Pan-Sinic terrene empire was exempt. In the first place a Power that ruled the shores of the Mediterranean j virtue of ruling its waves could not extend its rule inland in any irection very far beyond the range of action of naval landing-parties, ad therefore, in most directions, was constrained to draw the line of s limes along an alinement that fell far short of the nearest 'natural rentier'. The halo of impregnability with which the semi-official pane- yrists of the Roman Empire sought to crown its landward defences uring its illusory 'Indian Summer'4 was rudely dissipated by the istorical sequel; and the Emperor Hadrian had already seen through : as early as the year A.D. 117, when the death of a frustrated Roman dexander had given this Roman Antipater his chance of liquidating Trajanic adventure by reverting to a sober Augustan policy of terri- orial retrenchment. The Romans failed to find either a natural frontier >r a satisfactory artificial substitute, not only in the South-West Asian linterland of the Mediterranean,s but in its European hinterland as veil.-6 The only two natural frontiers that they did succeed in reaching vere the First Cataract in the Nile Valley—where they had merely to ake over a line that an Augustan-minded Pharaoh, Psammetichus I, lad laid down for them7 more than six hundred years before Augustus's >wn occupation of Psammetichan Egypt after the Battle of Actium— md the Atlantic coast of Continental Europe between the Straits of 3-ibraltar and the Delta of the Rhine; but the Roman hold on this Continental European natural frontier was never confirmed by an ntegral occupation of its natural outworks in the British Isles. Though Claudius and his successors realized that the occupation of Britain was i necessary complement to that of Gaul, Lower Germany, and the north-west corner of the Iberian Peninsula, they were never willing to face the truth that, so long as their occupation of Britain stopped short at the line of the Solway and the Tyne, or even at the line of the Clyde and the Forth, without going on to embrace Caledonia and Ireland, they were condemning themselves to an increase in their military liabilities instead of securing a diminution of them. The Romans' failure to round off their conquest of Britain by pushing i See VI. vii. 216-17. a See VI. vii. 217. 3 The plain—consisting of the Wei Basin '-within the Passes' and the Lower Yellow River Basin to the east of that mountain barrier—Corresponded physiographically to the western and eastern basins of the Mediterranean in the structure of the Roman Empire. The 'Festung Sudland', perched on the southern watershed of the Yangtse Basin behind a network of waterways, corresponded to the Roman dominions in North-West Africa. * See the panegyrics cited in VI, vit, 43-44 and 45-46. s See IX. viii. 411-13. * See V. v. 591-5. 7 See II. ii. 116.