Science: Tree Clocks

Leonardo da Vinci seems first to have noticed that widths of tree rings vary in wet and dry years, but there is a big gap between his discovery and a history of tree rings that chronicles Midwest weather as far back as 1300 A.D.—a study released this week by Anthropologist Florence May Hawley of the University of Chicago.

Systematic study of rings, so that old wood can be dated by its growth patterns, was begun in 1904 by Astronomer Andrew Ellicott Douglass of the University of Arizona, who was interested in solar activity. Douglass became able to look at a pine ladder...

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