The fourth Mosque was erected in A.H. 191, by Al-Mahdi, third prince of the Benu Abbas or Baghdad Caliphs-celebrated in history only for spending enormous sums upon a pilgrimage. He enlarged the building by adding ten handsome pillars of carved marble, with gilt capitals, on the Northern side. In A.H. 202, Al-Ma'amun made further additions to this Mosque. It was from Al-Mahdi's Masjid that Al-Hakim bi'Amri 'llah, the third Fatimite Caliph of Egypt, and the deity of the Druze sect, determined to steal the bodies of the Prophet and his two companions. About A.H. 412, he sent emissaries to Al-Madinah: the attempt, however, failed, and the would-be violators of the tomb lost their lives. It is generally supposed that Al-Hakim's object was to transfer the Visitation to his own capital; but in one so manifestly insane it is difficult to discover the spring of action. Two Christians, habited like Maghrabi pilgrims, in A.H. 550, dug a mine from a neighbouring house into the temple. They were discovered, beheaded, and burned to ashes. In relating these events the Moslem historians mix up many foolish preternaturalisms with credible matter. At last, to prevent a recurrence of such sacrilegious attempts, Al-Malik al-Adil Nur al-Din of the Baharite Mamluk Sultans, or, according to others, Sultan Nur al-Din Shahid Mahmud bin Zangi, who, warned by a vision of the Apostle, had started for Al-Madinah only in time to discover the two Christians, surrounded the holy place with a deep trench filled with molten lead. By this means Abu Bakr and Omar, who had run considerable risks of their own, have ever since been enabled to occupy their last homes undisturbed. In A.H. 654, the fifth Mosque was erected in consequence of a fire, which some authors attribute to a [p.368]volcano that broke out close to the town in terrible eruption[FN#51]; others, with more fanaticism and less probability, to the schismatic Benu Husayn, then the guardians of the tomb. On this occasion the Hujrah was saved, together with the old and venerable copies of the Koran there deposited, especially the Cufic MSS., written by Osman, the third Caliph. The piety of three sovereigns, Al-Mustasim (last Caliph of Baghdad), Al-Muzaffar Shems al-Din Yusuf, chief of Al-Yaman, and Al-Zahir Beybars, Baharite Sultan of Egypt, completed the work in A.H. 688. This building was enlarged and beautified by the princes of Egypt, and lasted upwards of two hundred years.