The systematic knowledge of the topography of Baghdad is derived from two nearly contemporary sources, namely Ya'kubi, who wrote near the end of the third century of the Hijrah, and Ibn Serapion, whose work dates from the beginning of the fourth, in other words, respectively a short time before and after the year. As is well known, the Arabs had inherited from the Persians, their predecessors in Mesopotamia, the system of canalization which connected the lower course of the Euphrates with the Tigris, making the Sawad–as the alluvial plain to the west and south of Baghdad was named–one of the most fruitful countries of the East. This canal, turning sharp off to the north-east, almost atKasright angle, flowed round the outer side of the northern suburb of Baghdad, and beyond this its waters joined the Tigris about a mile above the Round City, at a place which, like the mouth of the Kasr 'isa, is known as 'the Harbour'.