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 littoral, he frightened Maximian and his brother emperor Constantius into a renewal of active hostility.

Maximian built a large fleet in the mouths of the Rhine, and undertook the naval, while Constantius made himself responsible for the military, conduct of operations. The Emperors besieged their rival in Boulogne. They could do little on the land side, and at first, the sea being open to Carausius, he was in no danger from failure of supplies. But after a time, the besiegers found means to block up the mouth of the harbour with earth and sand, supported by trees driven in as piles; and when Carausius realised his position, he made his way by night through the camp of the enemy, and, going on board one of his own vessels, escaped to Britain, where his strength was greatest. He must have been much annoyed when he learnt that on the day after his escape a storm had destroyed the elaborate works of his foes, and that Boulogne harbour was once more open.

It has been already noted that Carausius had entered into treaties with certain Mediterranean rovers. These people were the descendants of the Franks who, under the Emperor Probus, had been sent as colonists to the shores of the Euxine to keep down the Scythians and other barbarians of those districts. The Franks, instead of withstanding the Scythians, in time made common cause with them against Rome, and, entering the Mediterranean, harassed it from end to end, burnt Syracuse, devastated the coasts of Spain and Africa, and terrified the Empire. In them Carausius recognised congenial spirits. It was arranged that the Frank pirates should come into the Atlantic, effect a junction with the British fleet, and fall upon the armada which Maximian had collected in the Rhine. Had the project been successful, Carausius might have become the most powerful prince of his day, and the whole Empire might possibly have been his.

But the piratical alliance found in Constantius a worthy opponent. Maximian, a man of very inferior capacity, had not been ready in time to take part in the operations against Boulogne; and Constantius, perhaps apprehensive of further delay, assumed the command of the thousand ships which were at length in a condition to sail, assembled and hastily built yet others, and, having stationed squadrons to observe Carausius and keep him in check, took the main body of his fleet towards the Straits of Gibraltar. Somewhere near the mouth of the Mediterranean, he met the Franks,