indeed, here a chain of forts on the frontier, but they are of small size, with an average area of only 1½ acres. The bulk of the auxilia lay some way behind the frontier, in forts which held some two or three regiments apiece.[326] In Britain we have traces of a similar system at the same date. The ‘Agricolan’ fort at Barr Hill is a frontier post which would require some two centuries at most for its defence, while the early fort at Newstead, which was probably occupied from about 80 to 100 or later, could accommodate at least 1500 men.[327] The essentially temporary nature of such _hiberna_ is emphasized by the character of their defences, which usually consist simply of an earth wall or palisade, little more elaborate in construction than the _vallum_ which an army in the field was expected to throw up round its camp after a day’s march. In provinces whence archaeological evidence is not forthcoming, inscriptions indicate the same system. From Spain, for instance, we have an early inscription referring to an officer who held command over four cohorts,[328] and a similar brigade of three cohorts appears at Syene in Egypt in the reign of Trajan.[329] On the Danube frontier von Domaszewski has concluded from the epigraphical evidence that Aquincum and Arrabona each held two alae in the first century.[330] This period of expansion may be considered to end with Trajan’s annexation of Dacia and his failure a few years later to execute a similar forward move on the Eastern frontier. With the accession of Hadrian a new policy begins, which advertised by the elaborate character of its defensive measures that the imperial government was firmly determined to renounce all further schemes of aggression, a determination which was adhered to until the power of decision lay no longer in Roman hands. The outward signs of this new spirit were the abandonment of the old _hiberna_, and the removal of their garrisons to stone forts of a new type, each arranged to hold no more than a single unit, which were placed at more or less regular intervals along the frontier instead of behind it.[331] The auxilia, that is, were transformed from a potential field army into a frontier police. This policy of passive defence depended, of course, for its success upon the extent to which the frontier could be made defensible. Fortunately by this date it lay for the greater part of its length along positions of great natural strength. The Rhine, Danube, and Euphrates, when guarded by a continuous line of forts and watch-towers,[332] and patrolled by flotillas of guard-boats, formed a serious military obstacle to a raiding force, an obstacle even more dangerous to its retreat than its advance. The desert frontiers of Africa and Arabia were more easily defended, since the routes by which a hostile force could advance were limited in number and the defence could concentrate upon them, and the same of course holds true of the southern frontier of Egypt.