On the eastern side of the river, on the other hand, there are several important tributaries descending from the Persian mountains: the Khabur, a little north of 37° N., navigable for rafts; the Great Zab, at 36° N., just below Nimrud, the ancient Calah; the Little Zab, about 35° 15' N.; the 'Adhem at 34° N.
South of Mosul, at which point navigation is blocked by two ancient dams, erected, apparently, to control the river for the Assyrian city of Calah, the ruins of which are called Nimrud by the natives after these dams, which they conceive to be the work of that mythical hero.
It remained the capital long after the Assyrians had become the dominant power in western Asia, but was finally supplanted by Calah (Nimrud), Nineveh (Nebi Yunus and Kuyunjik), and Dur-Sargina (Khorsabad), some 60 m.
At Nimrud (Calah) were also excavated, and hundreds of enamelled tiles were disinterred.
A crystal lens, turned on the lathe, was discovered by Layard at Nimrud along with glass vases bearing the name of Sargon; this will explain the excessive minuteness of some of the writing on the Assyrian tablets, and a lens may also have been used in the observation of the heavens.