as sequoias are concerned, a distance of 50 miles between groups is likely to be no particular obstacle in cross-identification.
The difficult ring 1580.—The small ring 699 A. D. and several other difficult ones were absent in comparatively few trees and any uncertainty regarding them was removed in the early part of the work, but it was not so with the ring of the year 1580. The best of the tree records were from the uplands and usually omitted it, while many of the basin trees which showed it were at first very uncertain in identification. The ring was therefore provisionally called 1580a and held in doubt for several years. The question of its reality was finally settled in the affirmative by a special trip to the sequoias in 1919 and the collection of a dozen carefully selected radial samples. The final review of all the tree-records has resulted in satisfactory identification of some previously doubtful cases and in complete conviction regarding the ring for 1580 A. D. No other uncertain cases were discovered. Considering the 35 sequoia records now (1919) made use of, it seems possible that all errors of dating have been removed.
MEASURING.
Having prepared and identified the wood samples, the first method of measuring was to lay a steel rule on edge across the series of rings in a radial direction and to read off from the rule the position of the outside of every red ring. These were either recorded at once by the person measuring or were noted by a clerical assistant. This method applied to the Flagstaff and Prescott trees and to the European and Vermont groups. In nearly all of them the steel rule used was a meter in length. It was ascertained by tests that the errors in readings of this kind were less than 0.1 mm. on the average for a single reading. For the Oregon group a microscope slide was used with a vernier which gave at once readings to 0.01 mm. The readings obtained by either of these methods were recorded in two columns on a page, and the subtractions were performed afterwards, giving the actual width of the ring in millimeters and fractions. Thus any error in the original reading would affect two rings only. Very great numbers of readings have been done a second time and vast numbers have been checked over approximately; hence it is believed that errors of this kind are extremely rare; out of 20,000 measures, perhaps 4 or 5 have been discovered. Errors of subtraction may have occurred, but it is thought that these also are extremely rare indeed, since practically all of the work has been checked over a second time.
In the case of the sequoias, however, the method of measuring was much more highly developed. It required a cathetometer with a thread micrometer and adding machine. The cathetometer is placed horizontally on the table and the wood to be measured is also put
