The astronomer was Dr. A. E. Douglass, who was engaged in the study of the effect of sun spots on climatic conditions. The available meteorological records, of course, went back only a relatively few years and it soon became apparent that a much longer record must be obtained to be of any real value. In searching for information about climatic conditions for past centuries, Dr. Douglass thought of pines, for they may reach a great age and the presence or absence of adequate rainfall, particularly in a climate like Arizona’s, will greatly affect the development of a tree. Every year a new layer of wood is added to the entire living surface of a pine. The size of these layers, which show up as rings when the tree is cut and viewed in cross-section, varies with the amount of food and moisture which the tree has obtained in the course of the year. A dry year will produce a thin ring and a wet year will produce a wide one. By cutting down old trees it was thus possible to learn what the climatic conditions had been during the years of their life. None of the pines which were still living, however, had existed for more than a few hundred years, and the giant sequoias of California which would have covered a longer span did not reflect climatic change in the same way. Fortunately, through the study of living trees, Dr. Douglass had learned that the tree-rings over a period of years formed a distinct pattern which could be recognized when found on most conifers. Next he began to search for trees which had been cut perhaps many years before, but which contained a pattern which fitted some early portion of that tree whose cutting date was known. This led him to beams made from whole logs which have been a characteristic feature of Southwestern architecture for many centuries. By finding old beams whose outer rings formed the same pattern as the inner rings of living trees the known chronology was increased. Through correlating the patterns of progressively older trees with younger ones the pattern was finally established for the period between 1280 and 1929. [Illustration: Fig. 1—Diagram to illustrate chronology-building with tree rings. Because of space limitation the number of rings in the overlapping specimens has been arbitrarily reduced. (After Stallings.[121] Courtesy Laboratory of Anthropology.)] THE RING PATTERNS MATCH AND OVERLAP BACK INTO TIME A THIS WAS A LIVING TREE WHEN CUT BY US DATE OF LAST RING IS THAT OF YEAR WHEN WE CUT TREE B THIS BEAM CAME FROM A HOUSE THIS DATE OBTAINED BY COUNTING BACK FROM BARK OF A C THIS BEAM CAME FROM AN OLD HOUSE THIS DATE OBTAINED BY COUNTING BACK FROM BARK OF A THROUGH B SPECIMENS TAKEN FROM RUINS WHEN MATCHED AND OVERLAPPED AS INDICATED PROGRESSIVELY EXTEND THE DATING BACK INTO PREHISTORIC TIMES