58 CLIMATIC CYCLES AND TREE-GROWTH. 
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 uncer- 
tainty 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 identifica- 
tion. The ring was therefore provisionally called 15804 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 
