During ten successive days, the capital was assaulted by the Avars, who had made some progress in the science of attack; they advanced to sap or batter the wall, under the cover of the impenetrable tortoise; their engines discharged a perpetual volley of stones and darts; and twelve lofty towers of wood exalted the combatants to the height of the neighbouring ramparts. But the senate and people were animated by the spirit of Heraclius, who had detached to their relief a body of twelve thousand cuirassiers; the powers of fire and mechanics were used with superior art and success in the defence of Constantinople; and the galleys, with two and three ranks of oars, commanded the Bosporus, and rendered the Persians the idle spectators of the defeat of their allies. The Avars were repulsed; a fleet of Slavonian canoes was destroyed in the harbour; the vassals of the chagan threatened to desert, his provisions were exhausted, and after burning his engines, he gave the signal of a slow and formidable retreat. The devotion of the Romans ascribed this signal deliverance to the Virgin Mary; but the mother of Christ would surely have condemned their inhuman murder of the Persian envoys, who were entitled to the rights of humanity, if they were not protected by the laws of nations. THIRD EXPEDITION OF HERACLIUS [Sidenote: [626-627 A.D.]] After the division of his army, Heraclius prudently retired to the banks of the Phasis, from whence he maintained a defensive war against the fifty thousand gold spears of Persia. His anxiety was relieved by the deliverance of Constantinople; his hopes were confirmed by a victory of his brother Theodorus; and to the hostile league of Chosroes with the Avars, the Roman emperor opposed the useful and honourable alliance of the Turks. At his liberal invitation, the horde of Khazars transported their tents from the plains of the Volga to the mountains of Georgia; Heraclius received them in the neighbourhood of Tiflis, and the khan with his nobles dismounted from their horses, if we may credit the Greeks, and fell prostrate on the ground, to adore the purple of the cæsar. Such voluntary homage and important aid were entitled to the warmest acknowledgments; and the emperor, taking off his own diadem, placed it on the head of the Turkish prince, whom he saluted with a tender embrace and the appellation of son. After a sumptuous banquet he presented Ziebel with the plate and ornaments, the gold, the gems, and the silk, which had been used at the imperial table, and, with his own hand, distributed rich jewels and earrings to his new allies.