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.