Roman empire—The cause of its decline—First invasion of Goths, A.D. 217—Their habits—Defeat the Emperor Decius, A.D. 257—Rebellion of Egypt, A.D. 273—Franks and Allemanni—The Veneti on the coast of Gaul—Constantinople founded, A.D. 323—Its commercial advantages and harbour—The extent of its ancient trade—Black Sea and Sea of Azov—The trading vessels on—Oppressive taxation—The laws affecting shipping—Constans and Julian—Produce of certain lands applied to the sea service—Neglect and decline of commerce, and sufferings of the people—Siege of Rome by Alaric and the Goths, A.D. 408—Genseric—His capture of Rome—Rise of Constantinople—Customs’ duties—Silk trade—Naval expedition of Justinian against the Vandals, A.D. 533, and conquest of Carthage—Rise of the Muhammedan power, A.D. 622—Rapid conquests; of Jerusalem, A.D. 636; of Alexandria, A.D. 638; and of Africa, A.D. 647—Sieges of Constantinople, A.D. 668-675. [Sidenote: Roman empire.] [Sidenote: The cause of its decline.] [Sidenote: A.D. 212-235.] From the close of the last Punic war to the middle of the third century after Christ, the Romans were the greatest if not the only enemies of Rome. The extravagance and heedless folly of her citizens were in themselves enough to ruin a less powerful nation. Without competitors, she maintained a position for centuries which, under other circumstances, the lavish expenditure of her people would have destroyed in as many years. The mistress of the world could do as she pleased; and the industry of the country districts was heavily burdened to support, in indolence and luxury, its rulers at Rome. Under such circumstances as these a fall must have come sooner or later; and so we find that even under the excellent government of the Antonines the decay of the imperial system had fully commenced. On their removal, and on the accession of such princes as Commodus, Caracalla and Elagabalus, the decay was much more rapid, till at length commerce, the true index of the real state of a nation, failed to recover its former position, even with the willing and active support of Alexander Severus. Nor was it possible that the corruption of Rome throughout its whole administration could be long kept a secret. On her distant and outlying provinces she had jealous enemies, burning to avenge past injuries and insults, and eagerly awaiting the opportunity of finding or making an entrance through her armour. [Sidenote: First invasion of Goths, A.D. 217.] [Sidenote: A.D. 249.]