23. =Agricola's Conquests in the North.=--Agricola thought that there would be no real peace unless the whole island was subdued. For seven years he carried on warfare with this object before him. He had comparatively little difficulty in reducing to obedience the country south of the narrow isthmus which separates the estuary of the Clyde from the estuary of the Forth. Before proceeding further he drew a line of forts across that isthmus to guard the conquered country from attack during his absence. He then made his way to the Tay, but he had not marched far up the valley of that river before he reached the edge of the Highlands. The Caledonians, as the Romans then called the inhabitants of those northern regions, were a savage race, and the mountains in the recesses of which they dwelt were rugged and inaccessible, offering but little means of support to a Roman army. In =84= the Caledonians, who, like all barbarians when they first come in contact with a civilised people, were ignorant of the strength of a disciplined army, came down from their fortresses in the mountains into the lower ground. A battle was fought near the Graupian Hill, which seems to have been situated at the junction of the Isla and the Tay. Agricola gained a complete victory, but he was unable to follow the fugitives into their narrow glens, and he contented himself with sending his fleet to circumnavigate the northern shores of the island, so as to mark out the limits of the land which he still hoped to conquer. Before the fleet returned, however, he was recalled by the Emperor Domitian. It has often been said that Domitian was jealous of his success; but it is possible that the Emperor really thought that the advantage to be gained by the conquest of rugged mountains would be more than counterbalanced by the losses which would certainly be incurred in consequence of the enormous difficulty of the task.
[Illustration: Commemorative tablet of the Second Legion found at Halton Chesters on the Roman Wall.]
24. =The Roman Walls.=--Agricola, in addition to his line of forts between the Forth and the Clyde, had erected detached forts at the mouth of the valleys which issue from the Highlands, in order to hinder the Caledonians from plundering the lower country. In =119= the Emperor Hadrian visited Britain. He was more disposed to defend the Empire than to extend it, and though he did not abandon Agricola's forts, he also built further south a continuous stone wall between the Solway and the Tyne. This wall, which, together with an earthwork of earlier date, formed a far stronger line of defence than the more northern forts, was intended to serve as a second barrier to keep out the wild Caledonians if they succeeded in breaking through the first. At a later time a lieutenant of the Emperor, Antoninus Pius, who afterwards became Emperor himself, connected Agricola's forts between the Forth and Clyde by a continuous earthwork. In =208= the Emperor Severus arrived in Britain, and after strengthening still further the earthwork between the Forth and Clyde, he attempted to carry out the plans of Agricola by conquering the land of the Caledonians. Severus, however, failed as completely as Agricola had failed before him, and he died soon after his return to Eboracum.