It is not unlikely that the later Sumerians had already evolved these primitive forms of irrigation-machine, and that the Babylonians of the First Dynasty merely inherited them and passed them on to their successors. When once invented they were incapable of very great improvement. In the one the skin must always remain a skin; in the other the wheel must always be lightly constructed of boughs, or the strength of the current would not suffice to turn it. We have seen reason to believe that, in the palace of Nebuchadnezzar II. at Babylon, the triple well in the north-west corner may be best explained as having formed the water-supply for a hydraulic machine, consisting of an endless chain of buckets passing over a great wheel. Such is a very common form of raising water in Babylonia at the present day. It is true that in some of these machines the wheel for the buckets is still geared by means of rough wooden cogs to the long pole or winch, turned by beasts, who move round in a circle. But it is very unlikely that the early Babylonians had evolved the principle of the cogged wheel, and it was probably not till the period of the later Assyrian empire that bronze was so plentiful that it could have been used in sufficient quantity for buckets on an endless chain. There seems reason to believe that Sennacherib himself introduced an innovation when he employed metal in the construction of the machines that supplied water to his palace;[15] and we may infer that even in the Neo-Babylonian period a contrivance of that sort was still a royal luxury, and that the farmer continued to use the more primitive machines, sanctioned by immemorial usage, which he could make with his own hands. [Illustration: FIG. 40. THE OLD BABYLONIAN FORM OF PLOUGH IN USE. The drawing is taken from seal-impressions on a tablet of the Kassite period. (After Clay.)]