The number of the stages varied, eight being the largest. At the summit of the tower there was a chapel or an altar, and the ascent was by steps or an inclined plane which wound around the sides of the tower. The Babylonians were famous astronomers, and it is believed that these towers were used as observatories as well as for places of worship. At the base of the tower there was a chapel for the use of those who could not ascend the height, and near by, in the open air, different altars were placed, for the worship of the Babylonians included the offering of sacrifices. Very ancient writers describe the riches of the shrines at Babylon as being of a value beyond our belief. They tell of colossal images of the gods of solid gold; of enormous lions in the same precious metal; of serpents of silver, each of thirty talents' weight (a talent equalled about two thousand dollars of our money), and of golden tables, bowls, and drinking-cups, besides magnificent offerings of many kinds which faithful worshippers had devoted to the gods. These great treasures fell into the hands of the Persians when they conquered Babylon. The Birs-i-Nimrud has been more fully examined than any other Babylonish ruin, and a description of it can be given with a good degree of correctness. As it now stands, every brick in it bears the name of Nebuchadnezzar; it is believed that he repaired or rebuilt it, but there is no reason to think that he changed its plan. Be this as it may, it is a very interesting ruin (Fig. 23). It was a temple raised on a platform and built in seven stages; these stages represented the seven spheres in which the seven planets moved (according to the ancient astronomy), and a particular color was assigned to each planet, and the stages colored according to this idea. That of the sun was golden; the moon, silver; Saturn, black; Jupiter, orange; Mars, red; Venus, pale yellow, and Mercury, deep blue. [Illustration: FIG. 23.--BIRS-I-NIMRUD, NEAR BABYLON.] It is curious to know how the various colors were obtained. The lower stage, representing Saturn, was covered with bitumen; that of Jupiter was faced with bricks burned to an orange color; that of Mars was made of bricks from a bright red clay and half burned, so that they had a blood-red tint; the stage dedicated to the sun was probably covered with thin plates of gold; that of Venus had pale yellow bricks; that of Mercury was subjected to intense heat after it was erected, and this produced vitrification and gave it a blue color; and the stage of the moon was coated in shining white metals.