his starting-line in reckoning north and south he used a meridian
passing through the First Cataract, Alexandria, Rhodes, and Byzantium.

The next two hundred years after Eratosthenes' 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 Ierne, 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.