During the first years of this century an astronomer, Dr A. E. Douglass, started a series of examinations of the annual increment in trees just south of Flagstaff, Arizona, to see if the cyclical nature of sun-spot appearances was reflected in tree growth, through their influence on climate. He found that there was a rather high correlation between tree growth and sun-spots in the living trees he examined. In order to extend his studies into the past—beyond the 500 years recorded by living trees—he collected material taken from beams in the old Spanish Missions that dot the southwestern United States. He had found that it was possible to identify certain characteristic sequences of tree-ring widths with certain years, and thus project into the past his chart of tree growth from timbers cut at an unknown past date. In doing so he discovered a technique that has founded a new branch of science—dendrochronology.