Though the summer was almost spent when he arrived in Britain, knowing how much the vigor and success of the first stroke influences all subsequent measures, he entered immediately into action. After reducing some tribes, Mona became the principal object of his attention. The cruel ravages of Paulinus had not entirely effaced the idea of sanctity which the Britons by a long course of hereditary reverence had annexed to that island: it became once more a place of consideration by the return of the Druids. Here Agricola observed a conduct very different from that of his predecessor, Paulinus: the island, when he had reduced it, was treated with great lenity. Agricola was a man of humanity and virtue: he pitied the condition and respected the prejudices of the conquered. This behavior facilitated the progress of his arms, insomuch that in less than two campaigns all the British nations comprehended in what we now call England yielded themselves to the Roman government, as soon as they found that peace was no longer to be considered as a dubious blessing. Agricola carefully secured the obedience of the conquered people by building forts and stations in the most important and commanding places. Having taken these precautions for securing his rear, he advanced northwards, and, penetrating into Caledonia as far as the river Tay, he there built a _prætentura_, or line of forts, between the two friths, which are in that place no more than twenty miles asunder. The enemy, says Tacitus, was removed as it were into another island. And this line Agricola seems to have destined as the boundary of the Empire. For though in the following year he carried his arms further, and, as it is thought, to the foot of the Grampian Mountains, and there defeated a confederate army of the Caledonians, headed by Galgacus, one of their most famous chiefs, yet he built no fort to the northward of this line: a measure which he never omitted, when he intended to preserve his conquests. The expedition of that summer was probably designed only to disable the Caledonians from attempting anything against this barrier. But he left them their mountains, their arms, and their liberty: a policy, perhaps, not altogether worthy of so able a commander. He might the more easily have completed the conquest of the whole island by means of the fleet which he equipped to coöperate with his land forces in that expedition. This fleet sailed quite round Britain, which had not been before, by any certain proof, known to be an island: a circumnavigation, in that immature state of naval skill, of little less fame than a voyage round the globe in the present age.