Following the ridge of the ruined city wall from the excavated portion farther to the north-west, one reaches a gap in the wall where it was ruthlessly broken down by later canals, now themselves dried up (Fig. 3). They were forerunners of the present Nil Canal. The Arabic word _nil_ denotes the blue colour which is generally produced by indigo, and has given its name to various watercourses on Arab soil; the name of the Egyptian Nile is probably connected with it. The Nil Canal runs to-day a few hundred metres to the north-east along the city wall and roughly parallel with it. The embankments of these canals, which in places are of immense height, intersect the plain with a sharp line. The contrast with the plain is most striking when they are seen on the horizon, where the mirage comes to their aid and makes them look like hills of some importance. At first sight, also, they appear to be entirely out of proportion with the small amount of water that flows so slowly through the canal. That, however, is only the case where the canal has been in use for some long time. When the canal is first constructed each embankment, under normal circumstances, consists of no more than half of the earth which is dug out, as these irrigation works, wherever the lie of the ground permits, are so arranged that the surface of the water may be higher than the surrounding plain. Only in this way would it be possible with comparatively small expenditure, and without special machinery for raising water, to provide the field with a gentle supply of the fructifying moisture. But the Euphrates at the period of high water, when the irrigation takes place, bears a quantity of material in suspension that is specially valuable for agriculture. If the water stands quiet for long, as it does in a lake, it becomes clear as glass, and is no longer suitable for irrigation, it is “dead,” as the Arabs say. As the water flows slowly through these canals it deposits this precious material in the canal-beds, and especially sand and mud in great quantities. Thus it is necessary every year to clear out the canals, and the material thrown out on to the embankments continually raises them in height (Fig. 4). Obviously there must come a moment in the history of each canal when it is more expensive to clear it out than to construct a new one, and thus every canal bears within it the germ of its own destruction. The sanding up of the canal-bed is naturally more insistent in portions nearest the river, and hence it is that this canal displacement occurs so frequently in the neighbourhood of the river-course. On the way from Bagdad to Hilleh in the neighbourhood of the Euphrates, one crosses extraordinarily numerous groups of abandoned canals, most of which are nothing else than the older courses of the same irrigation system that is in use to-day. [Illustration: FIG. 5.—View of the mound “Babil.” ]