In working out the chronological scheme it only remains therefore to fix accurately the period of the First Dynasty, in order to arrive at a detailed chronology for both the earlier and the later periods. Hitherto, in default of any other method, it has been necessary to rely on the traditions which have come down to us from the history of Berossus or on chronological references to early rulers which occur in the later historical texts. A new method of arriving at the date of the First Dynasty, in complete independence of such sources of information, was hit upon three years ago by Dr. Kugler, the Dutch astronomer, in the course of his work on published texts that had any bearing on the history and achievements of Babylonian astronomy.[47] Two such tablets had been found by Sir Henry Layard at Nineveh and were preserved in the Kouyunjik Collection of the British Museum. Of these one had long of Sippar, Bunutakhtun-ila, occupied the throne in Sumu-la-ilum's re astronomical omens derived from observations of the planet Venus.[48] It was certain that this Assyrian text was a copy of an earlier Babylonian one, since that was definitely stated in its colophon. The second of the two inscriptions proved to be in part a duplicate,[49] and by using them in combination Dr. Kugler was able to restore the original text with a considerable degree of certainty.[50] But a more important discovery was that he succeeded in identifying precisely the period at which the text was originally drawn up, and the astronomical observations recorded. For he noted that in the eighth section of his restored text there was a chronological note, dating that section by the old Babylonian date-formula for the eighth year of Ammi-zaduga, the tenth king of the First Babylonian Dynasty. As his text contained twenty-one sections, he drew the legitimate inference that it gave him a series of observations of the planet Venus for each of the twenty-one years of Ammi-zaduga's reign.[51]