The dangerous secret of the wealth and weakness of the empire once clearly discerned, new swarms of barbarians, encouraged by the successes of those who had gone before them, sprang into existence, overran the northern districts, flooded Gaul and Spain, and carried desolation and terror almost to the very walls of Rome. Of these the most formidable were the Franks, or Freemen, who now occupied the Rhenish frontier, and the Allemanni, a name now generally held to imply a confederacy of many tribes of German origin. As in the case of the Goths, their first successes were of short duration, and, after three bloody battles, Aurelian was able to drive them out of Italy, securing, at the same time, by the overthrow of Zenobia and the destruction of Firmus, a short breathing time for the distracted empire. But the policy now, though not for the first time, adopted by the Roman generals, was not one likely to succeed against the hosts now arrayed against the empire. The principle of building walls of great size and length, which had failed in England and in Scotland, failed even more conspicuously when adopted by Probus,[331] who, at the head of his legions, still maintained vigorously the prowess and the name of ancient Rome. His wall, two hundred miles in length, from the Danube to the Rhine, could be of little real value against an active and experienced enemy; indeed, Gibbon stated the truth when he said, “The experience of the world has exposed the vain attempt of fortifying an extensive tract of country.” Nor did Probus’s second scheme of enlisting in the Roman armies large levies taken from tribes but partially conquered, prove, as might have been anticipated, a more effective source of security and strength. It must, indeed, have been clear to any persons of observation, that no permanent stability could be secured for a government which wasted its means by acts of wanton and revolting extravagance, such as those which Gibbon has so eloquently described in his “Decline and Fall” of the great empire. The barbarians, ever on the alert, now discovered that there were traitors within the Roman camps who gave fresh assistance to their inroads. Such a traitor was Carausius, not, as Stukeley imagined, a descendant of the blood royal of Britain, but a Dutch soldier of fortune, of the tribe of the Menapii. Tampering with the fleet he commanded in the name of Maximian, he crossed with it from Gessoriacum (Boulogne), and, seizing on Britain, proclaimed himself Augustus and struck money with his effigy. For seven years he maintained his ill-gotten power, but, though at length murdered by his lieutenant, Allectus, Carausius deserves some historical remembrance as the first creator of a British-manned navy. [Sidenote: A.D. 287.] [Sidenote: The Veneti on the coast of Gaul.]