When we turn to Chaldæa we find similar evidence as to the advance of science, and especially of astronomical science, in the earliest historical times. Every important city had its temple, and attached to its temple its ziggurat, which was a temple-observatory. The ziggurat is in some respects the counterpart of the pyramid, being a pyramidal structure built up in successive stages or platforms superimposed on one another and narrowing as they rose, so as to leave a small platform on the top, on which was a small shrine or temple, and from which observations could be made. These ziggurats being built entirely of bricks, mostly sun-burnt, have crumbled into shapeless mounds of rubbish, but a fair idea of their size and construction may be obtained from the descriptions and pictures of them preserved in contemporary tablets and slabs, especially from those of the great ziggurat of the seven spheres or planets at Borsippa, a suburb of Babylon, which was rebuilt by Nebuchadnezzar about 500 B.C., on the site of a much more ancient ruined construction. This, which was the largest and most famous of the ziggurats, became identified in after times with the tower of Babel and the legend of the confusion of tongues, but it was in fact an astronomical building in seven stages dedicated to the sun, moon, and five planets, taken in the order of magnitude of their respective orbits, and each distinguished by their respective colours. Thus the lowest or largest platform was dedicated to Saturn, and coloured black; the second to Jupiter was orange; the third to Mars red; the fourth to the Sun golden; the fifth to Venus pale yellow; the sixth to Mercury an azure blue, obtained by vitrifying the facing bricks; and the seventh to the Moon was probably coated with plates of silver. The height of this ziggurat was 150 feet, and standing as it did on a level alluvial plain, it must have been a very imposing object. [Illustration: ZIGGURAT RESTORED (Perrot and Chipiez), THE TOWER OF BABEL.]