Sea is a Roman lake, and there is not a spot upon its shores which is
not under Roman rule. In round numbers the empire is three thousand
miles in length and two thousand in breadth. Its population, which, at
least in the western parts, was much thinner then than it is over the
same area at present, cannot be calculated with any accuracy, but an
estimate of one hundred millions would perhaps be not very far from
the mark.

Beyond its borders--sometimes too dangerously near to them and apt to
overstep them--lay various peoples concerning whom Roman knowledge was
for the most part incomplete and indefinite. Within its own boundaries
the Roman government carefully collected every kind of information.
Such precision was indispensable for the carrying out of those Roman
principles of administration which will be described later. But of the
nations or tribes beyond the frontiers only so much was known as had
been gathered from a number of more or less futile campaigns, from
occasional embassies sent to Rome by such peoples, from the writings
of a few venturous travellers bent on exploration, from slaves who had
been acquired by war or purchase, or from traders such as those who
made their way to the Baltic in quest of amber, or to Arabia,
Ethiopia, and India in quest of precious metals, jewels, ivory,
perfumes, and fabrics.

There had indeed been sundry attempts to annex still more of the
world. Roman armies had crossed the Rhine and had twice fought their
way to the Elbe; but it became apparent to the shrewd Augustus and
Tiberius that the country could not be held, and the Rhine was for the
present accepted as the most natural and practical frontier. In the
East the attempts permanently to annex Armenia, or a portion of
Parthia, had so far proved but nominal or almost entirely vain.