"Now is the time to penetrate into the heart of Caledonia and to discover the utmost limits of Britain," cried the Romans, as they began their advance to the Highlands of Scotland. While a Roman fleet surveyed the coasts and harbours, Agricola led his men up the valley of the Tay to the edge of the Highlands, but he could not follow the savage Caledonians into their rugged and inaccessible mountains. To the north of Scotland they never penetrated, and no part of Ireland ever came under Roman sway, in that air "the Roman eagle never fluttered." The Roman account of Britain at this time is interesting. "Britain," says Tacitus, "the largest of all the islands which have come within the knowledge of the Romans, stretches on the east towards Germany, on the west towards Spain, and on the south it is even within sight of France.... The Roman fleet, at this period first sailing round this remotest coast, gave certain proof that Britain was an island, and at the same time discovered and subdued the Orkney Islands, till then unknown. Thule was also distinctly seen, which winter and eternal snow had hitherto concealed.... The sky in this country is deformed by clouds and frequent rains; but the cold is never extremely rigorous. The earth yields gold and silver and other metals--the ocean produces pearls." The account of Ireland is only from hearsay. "This island," continues Tacitus, "is less than Britain, but larger than those of Our Sea. Situated between Britain and Spain and lying commodiously to the Bay of Biscay, it would have formed a very beneficial connection between the most powerful parts of the Empire. Its soil, climate, and the manners and dispositions of its inhabitants are little different from those of Britain. Its ports and harbours are better known from the concourse of merchants for the purposes of commerce." Not only the British Isles, but a good deal of the wild North Sea and the low-lying coast on the opposite side were explored by Roman ships and Roman soldiers. Caesar had crossed the Rhine; he had heard of a great forest which took a man four months to cross, and in 16 A.D. a Roman general, Drusus, penetrated into the interior of Germany. Drusus crossed the Rhine near the coast, made his way across the river Weser, and reached the banks of the Elbe. But the fame of Drusus rests mainly on his navigation of the German Ocean or North Sea in a Roman fleet. Near the mouth of the Rhine a thousand ships were quickly built by expert Romans. "Some were short, with narrow stern and prow and broad in the middle, the easier to endure the shock of the waves; some had flat bottoms that without damage they might run aground; many were fitted for carrying horses and provisions, convenient for sails and swift with oars."