Not satisfied with the possession of the forts Agricola had erected for the protection of the Britons against the Caledonians, the Emperor Hadrian, who visited the island in person, finding that they had extended their warlike and predatory excursions to the south of the Tyne, was compelled to drive them back to their mountain fastnesses by the superior force of fresh legions. Alarmed, however, by their daring and intrepidity, or not considering it worth while to follow them north of that river, he caused the wall to be built which still bears his name, from the Eden in Cumberland to the Tyne in Northumberland, to restrain them from again making incursions into the Roman province. Hadrian’s rampart, however, was soon breached, for seventy miles of wall required more soldiers to protect it than the Romans could spare, and though subsequent walls were built at different times, all of these proved equally ineffective in repressing the advances of the Picts: till, at length, Severus was compelled to purchase a peace by money which his arms had failed to secure.[459] [Sidenote: Piratical invasions of the Germans.] [Sidenote: Carausius seizes the fleet of Maximian, and declares himself Emperor of Britain.] But Rome had innumerable difficulties to contend against in the government of Britain beyond the inroads of the Caledonians, who had ever spurned and resisted her rule. The Franks and other German nations had for some time invaded the coasts adjacent to them with piratical incursions, and had reaped a rich harvest from the plunder of the sea-coast towns and of the vessels of the traders employed between Britain and Gaul. In order to repress these sea-robbers, the Emperor Maximian built a fleet of ships, the command of which he gave to Carausius, an officer of great experience in naval and military affairs, to whose history we have already alluded. Though a traitor to the government he professed to serve, and possessing himself no claims to the allegiance of the Britons, there is no doubt that the island prospered under his brief rule, and for the first time was able to claim the proud title of mistress of the northern seas, a title, however, she soon lost, and only regained after tremendous struggles, twelve centuries afterwards. Britain, during the seven years’ reign of Carausius, seems to have possessed more wealth and prosperity than during any similar period of Roman rule, while the flourishing state of the arts at that period of her history is shown by the number and elegance of his coins, three hundred varieties of which, in gold, silver, or copper, are now preserved in different numismatic collections.