Originally the canal seems to have been derived from the Zab itself, the water of which was drawn off, on its northern bank, through a short tunnel—­the modern Negoub—­and then conducted along a cutting, first by the side of the Zab, and afterwards in a tortuous course across the undulating plain, into the ravine formed by the Shor-Derreh torrent.  The Zab, when this part of the work was constructed, ran deep along its northern bank, and, sending a portion of its waters into the tunnel, maintained a constant stream in the canal.  But after awhile the river abandoned its north bank for the opposite shore; and, water ceasing to flow through the Negoub tunnel, it became necessary to obtain it in some other way.  Accordingly the canal was extended northwards, partly by cutting and partly by tunnelling to the Ghazr Su at about two miles above its mouth, and a permanent supply was thenceforth obtained from that stream.  The work may have been intended in part to supply Calah with mountain water; but the remains of dams and sluices along its course sufficiently show that it was a canal for irrigation also.  From it water was probably derived to fertilize the whole triangle lying south of Nimrud between the two streams, a tract containing nearly thirty square miles of territory, mostly very fertile, and with careful cultivation well capable of supporting the almost metropolitan city on which it abutted.

In Assyria it must have been seldom that the Babylonian system of irrigation could have been found applicable, and the water simply derived from the rivers by side-cuts, leading it off from the natural channel.  There is but little of Assyria which is flat and alluvial; the land generally undulates, and most of it stands at a considerable height above the various streams.  The water therefore requires to be raised from the level of the rivers to that of the lands before it can be spread over them, and for this purpose hydraulic machinery of one kind or another is requisite.  In cases where the subterranean conduit was employed, the Assyrians probably (like the ancient and the modern Persians) sank wells at intervals, and raised the water from them by means of a bucket and rope, the latter working over a pulley.  Where they could obtain a bank of a convenient height overhanging a river, they made use of the hand-swipe, and with its aid lifted the water into a tank or reservoir, whence they could distribute it over their fields.  In some instances, it would seem, they brought water to the tops of hills by means of aqueducts, and then, constructing a number of small channels, let the fluid trickle down them among their trees and crops.  They may have occasionally, like the modern Arabs, employed the labor of an animal to raise the fluid; but the monuments do not furnish us with any evidence of their use of this method.  Neither do we find any trace of water-wheels, such as are employed upon the Orontes and other swift rivers, whereby a stream can itself be made to raise water from the land along its bunks.