=3. Roman Occupation.=--The _Romans_, who had already made themselves masters of South Britain, were led into the northern part of the island by _Julius Agricola_, A.D. 80. But the _Celts_ whom they found there, and whom they called _Caledonians_, were so well able to defend themselves among their mountains that the Romans, though they defeated them in a great battle on the Highland border, gave up the idea of conquering the country, and retreated again south of the Firths of _Forth_ and _Clyde_. Across the isthmus between the two, which is about thirty miles wide, they built a line of forts, joined by a rampart of earth. This rampart was intended to serve as a defence to their colonists, and as a boundary to mark the limit of their empire; though, as many Roman remains have been found north of the isthmus, they must have had settlements without as well as within the fortifications. But the Caledonians, who were too high-spirited to look on quietly and see their country thus taken possession of, harassed the colonists by getting over the wall and seizing or destroying everything they could lay their hands on. At length (A.D. 120) the Roman Emperor _Hadrian_ built a second rampart across the lower isthmus, between the rivers _Tyne_ and _Solway_, leaving the district between the two pretty much at the mercy of the fierce Picts, as the Romans now began to call the Caledonians. Twenty years later, in the reign of the Emperor _Antoninus Pius_, one of his generals, _Lollius Urbicus_, again drove them back beyond the first wall, and repaired and strengthened the defences of _Agricola_. But, before half a century had passed, the Picts again burst the barrier, and killed the Roman commander. In 208 the Emperor _Severus_ cut his way through Caledonia with a large army. He reached the northern coast, but had no chance of fighting a battle, and lost many of his men. He repaired and strengthened the rampart of Hadrian. In time the Picts got over the second rampart too, and came south as far as _Kent_, where, in the latter part of the fourth century, _Theodosius_ the Roman general, father of the famous Emperor of the same name, had to fight his way to London through their plundering hordes. Theodosius drove them back with great vigour, restored the Empire to its former boundary, and made the district between the walls into a Roman province, which he called _Valentia_, in honour of _Valentinian_, who was then Emperor. It was probably about this time that the great stone wall was built across the lower isthmus. The dangers which threatened the capital of the Empire in the beginning of the next century forced the Romans to forsake this as well as all their other provinces in Britain, and the withdrawal of their troops left the Romanized Britons of Valentia a helpless prey to their merciless enemies the Picts. At the end of the three centuries of Roman occupation, the _Britons_ south of the Firths had so little in common