Whenever the water is rough, they raise the bulwarks with additional planks, according to the increasing height of the waves, till the vessel is covered in like a house. Thus they roll about amid the billows, and as they have a prow at both extremities alike, and a convertible arrangement of oars, they may be paddled in one direction or another, indifferently and without risk.”[328] It was in such frail craft as these that the Goths committed themselves to a sea with which they were wholly unacquainted, yet, under the guidance of local and impressed mariners, they succeeded in surprising the city of Trebizond, wherein immense treasures had been collected for safety, its capture, with many of its merchants, being the reward of their daring. Seven years after obtaining possession of Dacia they were, however, again driven from it by Probus; but the skill and courage of these barbarous tribes were always conspicuous at sea; and in no instance more so than in the case of a party of Franks, who had been settled near the sea coast, and who, being suddenly seized with the desire of returning to their homes, surprised a small fleet then on the Euxine, and were thus enabled to carry out their plan. Unskilled in the art of navigation, and unacquainted with any other except the Baltic and the Black Seas, these adventurers made their way through the Dardanelles into the Mediterranean, and, after sacking Syracuse and other towns, boldly steered through the Straits of Gibraltar, entered the Atlantic with a degree of hardihood almost unparalleled, and made a triumphant passage to the Batavian or Frisian coasts.[329] [Sidenote: Rebellion of Egypt, A.D. 273.] Though repeatedly defeated, their successes, together with the steady advance of other barbarian hordes from the north, were sufficient to destroy the _prestige_ which had for centuries made Rome invincible. A general spirit of revolt was now awakened in all the provinces. Even Egypt, so long the most docile of the slaves of Rome, was aroused into active rebellion by Firmus, a wealthy merchant of Alexandria, who, after plundering that city at the head of a furious mob, maintained, though for only sixty days, the imperial purple, published edicts, and raised an army, supported, as he boasted vaingloriously, from the profits of his paper trade.[330] [Sidenote: Franks and Allemanni.]