The same circumstances which caused the seat of empire to be transferred to Baghdád brought about a corresponding change in the whole system of government. Whereas the Umayyads had been little more than heads of a turbulent Arabian aristocracy, their successors reverted to the old type of Oriental despotism with which the Persians had been familiar since the days of Darius and Xerxes. Surrounded by a strong bodyguard of troops from Khurásán, on whose devotion they could rely, the ‘Abbásids ruled with absolute authority over the lives and properties or their subjects, even as the Sásánian monarchs had ruled before them. Persian fashions were imitated at the court, which was thronged with the Caliph's relatives and freedmen (not to mention his womenfolk), besides a vast array of uniformed and decorated officials. Chief amongst these latter stood two personages who figure prominently in the _Arabian Nights_--the Vizier and the Executioner. The office of Vizier is probably of Persian origin, although in Professor De Goeje's opinion the word itself is Arabic.[481] The first who bore this title in ‘Abbásid times was Abú Salama, the minister of Saffáḥ: he was called _Wazíru Áli Muḥammadin_, 'the Vizier of Muḥammad's Family.' It was the duty of the Vizier to act as intermediary between the omnipotent sovereign and his people, to counsel him in affairs of State, and, above all, to keep His Majesty in good humour. He wielded enormous power, but was exposed to every sort of intrigue, and never knew when he might be interned in a dungeon or despatched in the twinkling of an eye by the grim functionary presiding over the _naṭ‘_, or circular carpet of leather, which lay beside the throne and served as a scaffold. [Sidenote: Two periods of ‘Abbásid history.] We can distinguish two periods in the history of the ‘Abbásid House: one of brilliant prosperity inaugurated by Manṣúr and including the reigns of Mahdí, Hárún al-Rashíd, Ma’mún, Mu‘tasim, and Wáthiq--that is to say, nearly a hundred years in all (754-847 A.D.); the other, more than four times as long, commencing with Mutawakkil (847-861 A.D.)--a period of decline rapidly sinking, after a brief interval which gave promise of better things, into irremediable decay.[482] [Sidenote: Reign of Manṣúr (754-775 A.D.).] [Sidenote: Outbreaks in Persia.]