thenes death was filled up by the spread of the Roman Empire, by the taking over by the Romans of the vast possessions previously held by Alexander and his successors and by the Carthaginians, and by their spread into Gaul, Britain, and Germany. Much of the increased knowledge thus obtained was summed up in the geographical work of Strabo, who wrote in Greek about 20 B. C. He introduced from the extra knowledge thus obtained many modifications of the system of Eratosthenes, but, on the whole, kept to his general conception of the world. He rejected, however, the existence of Thule, and thus made the world narrower; while he recognised the existence of lerne, or Ireland, which he regarded as the most northerly part of the habitable world, lying, as he thought, north of Britain.
Between the time of Strabo and that of Ptolemy, who sums up all the knowledge of the ancients about the habitable earth, there was only one considerable addition to men's acquaintance with their neighbours, contained in a seaman's manual for the navigation of the Indian Ocean, known as the Periplus of the Erythræan Sea. This gave very full and tolerably accurate accounts of the coasts from Aden to the mouth of the Ganges, though it regarded Ceylon as much greater, and more to the south, than it really is; but it also contains an account of the more easterly parts of Asia, Indo-China, and China itself, "where the silk comes from." This had an important influence on the views of Ptolemy, as we shall see, and indirectly helped long afterwards to the discovery of America.
It was left to Ptolemy of Alexandria to sum up for the ancient world all the knowledge that
