Besides its troubles from the threats of these barbarians on its north-west borders, the Empire, as we have seen, had its troubles through most of this century from Persians and others on the south-east; and I now want to ask you to notice an effect of these troubles and threats of trouble on the Empire itself, for it was an effect which made a very great difference to the story. This effect was the dividing up of the one Empire into two, with a Western Empire, as of old, having its seat of government at Rome; but also with an Eastern Empire having its centre of government at Byzantium, as Constantinople was then called. {23} CHAPTER IV THE DIVISION OF THE EMPIRE The causes that led to the dividing up of the Empire are easily understood. What is far less easy to understand is how Rome ruled the world, as the world then was known, so long as she did. Remember this: at that time you could only travel, and you could only send a message, as fast as a horse could gallop, if it was by land that you went or sent; and only as fast as a ship--a ship with a very simple and primitive way of setting the sails--could be urged through the water by sailing or by rowing, if your going was by sea. For practical purposes of getting news or of moving troops, the world of the Romans of that date, say from Egypt to Britain at its furthest points, was a very great deal larger than the whole of the globe is to us to-day. If you can understand it in that sense, their Empire was very much larger, much less under the eye and the direction of the centre of government, than the whole British Empire to-day. And we find that large enough. The Romans had the further trouble, which we have not, that the leaders of the legions in the provinces, when they had repelled the barbarians, sometimes claimed to be independent of the central authority, as we saw both in Gaul and in Asia Minor.