(_f_) The Cistern of Bonus, the next landmark to be considered, was built by the Patrician Bonus, celebrated in Byzantine history for his brave defence of the capital in 627 against the Avars and the Persians, while the Emperor Heraclius was in Persia carrying war into the enemy’s country.[81] Where this cistern was situated is a matter of dispute which cannot be definitely settled in our present state of knowledge. Gyllius identified it with a large cistern, three hundred paces in length, which he found robbed of its roof and columns, and turned into a vegetable garden, near the ruins of the Church of St. John in Petra, on the Sixth Hill.[82] The cistern has disappeared since that traveller’s day, but as the Wall of Constantine never extended so far west, the identification cannot be correct. In Dr. Mordtmann’s opinion, the Cistern of Bonus was the large open reservoir to the south-west of the Mosque of Sultan Selim, on the Fifth Hill,[83] and there is much to be said in favour of this view. The Cistern of Bonus was, in the first place, situated in one of the coolest quarters of the city, and beside it, on that account, the Emperor Romanus I. erected a palace,[84] styled the New Palace of Bonus,[85] as a residence during the hot season. Nowhere in Constantinople could a cooler spot be found in summer than the terrace upon which the Mosque of Sultan Selim stands, not to speak of the attractions offered by the superb view of the Golden Horn from that point. Furthermore, the Cistern of Bonus was within a short distance from the Church of the Holy Apostles, seeing that on the eve of the annual service celebrated in that church in commemoration of Constantine the Great, the Imperial Court usually repaired to the Palace of Bonus, in order to be within easy riding distance of the sanctuary on the morning of the festival.[86] A palace near the reservoir beside the Mosque of Sultan Selim would be conveniently near the Church of the Holy Apostles, to suit the emperor on such an occasion. To these considerations can be added, first, the fact that on the way from the Palace of Bonus to the Church of the Apostles there was an old cistern converted into market gardens,[87] which may have been the reservoir near the Mosque of Sultan Selim; and, secondly, the fact that the Wall of Constantine, on its way from the Cistern of Aspar to the Golden Horn passed near the site now occupied by the Mosque of Sultan Selim, and, consequently, close to the old cistern adjoining that mosque. But to this identification there is a fatal objection: the Cistern of Bonus was roofed in,[88] whereas the reservoir beside the Mosque of Sultan Selim appears to have always been open.