The formation of the Babylonian triad, and the differentiation of the divine persons who composed it, must have been the work of a theological school. It is an artificial scheme elaborated after the union of the northern and southern parts of the country. The universe is divided between the three divine representatives of Northern, Central, and Southern Chaldæa, whose sanctuaries were the oldest in the land, and whose cult had been handed down from time immemorial. The triad once formed became a model after which others could be created. The other great gods followed the example that had been set them, and were similarly resolved into triads. As the orthodox theological system of Egypt rested on the Ennead, the corresponding Babylonian system rested on the triad. The principle in each case was much the same. The Ennead was but a multiple of the triad, and presupposed the sacred number. Perhaps we may see in it the result of a contact between Sumerian modes of thought and the Semitic conception of the divine family. Where the god had a wife and a son, the godhead would naturally be regarded as a trinity.(245) Under the first and supreme triad came the second triad of Sin, Samas, and Hadad. Sin, the moon-god, was adored under many names and in many forms. But his two chiefest temples were at Ur and at Harran. Ur, the modern Muqayyar, on the western bank of the Euphrates, had been dedicated to his service from the earliest times. The ruins of his temple still rise in huge mounds from the ground. The city stood outside the limits of the Babylonian plain, in the Semitic territory of the Arabian desert, and its Semitic population was therefore probably large. Harran, the other seat of the moon-god, was equally beyond the limits of Babylonia, and guarded the high road of commerce and war that led from the East to the West. But the name Harran, “the road,” is Babylonian, and, like its temple and god, the city doubtless owed its origin to Babylonian colonists. They probably came from Ur. The moon-god of Ur is called the son of En-lil of Nippur, and it may be therefore that Nippur was the mother-city of Ur. But it must be remembered that whereas Ur was built on the desert plateau of Arabia, Nippur stood among the marshes of the Babylonian plain. Its sanctuary could not have been founded before the marshes had been, at all events, partially drained, and the inundating rivers been regulated by dykes and canals. A settlement on the higher and drier ground would seem more naturally to precede one in the pestiferous swamps below it, and the fact that Ur was the neighbour of Eridu seems to point to its early foundation and connection with the old seaport of the country. At the same time the worship of the moon-god is associated with the Semites of Arabia and the west rather than with Eridu, whose god revealed himself to mankind by day and not in the shades of night.(246)