CLIMATE OF HISTOKIC TIMES 531 



a similar number of years before or after. Where a long series of years 

 is used, it is necessary to make corrections to eliminate the effects of age, 

 but this can be done by mathematical methods of considerable accuracy. 

 It is difficult to deterniine whether the climate at the beginning and end 

 of a tree's life was the same, but it is easily possible to determine whether 

 there have been pulsations while the tree was making its growth. If a 

 large number of trees from various parts of a given district all formed 

 thick rings at a certain period and then formed thin ones for a hundred 

 years, after which the rings again become thick, we seem to be safe, in 

 concluding that the trees have lived through a long dry period. The full 

 reasons for this belief and details as to the methods of estimating climate 

 from tree growth are given in "The Climatic Factor.^' 



A brief summary of the results set forth in that volume is as follows : 

 During the years 1911 and 1912, under the auspices of the Carnegie In- 

 stitution of Washington, I measured the thickness of the rings of growth 

 on the stumps of about 450 Sequoia trees in California. These trees 

 varied in age from 250 to nearly 3,250 years. The great majority were 

 over 1,000 years of age, 79 were over 2,000, and 3 over 3,000. Even 

 where only a few trees are available the record is surprisingly reliable 

 except where occasional accidents occur. Where the number approxi- 

 mates 100, accidental variations are largely eliminated and we may accept 

 the record with considerable confidence. Accordingly, we may say that 

 in California we have a fairly accurate record of the climate for 2,000 

 years and an approximate record for 1,000 years more. The final results 

 of the measurements of the California trees are shown in figure 13, where 

 the climatic variations for 3,000 years in California are indicated by the 

 solid lines. The high parts of the line indicate rainy conditions, the low 

 parts dry. An examination of this curve shows that during 3,000 years 

 there have apparently been climatic variations more important than any 

 which have taken place during the past century. In order to bring out 

 the details more clearly, the more reliable part of the California curve, 

 from 100 B. C. to the present time, has been reproduced in figure 14. 

 This is identical with the corresponding part of figure 13 except that the 

 vertical scale is three times as great. 



In addition to the solid line of figure 13 there is a dotted line. This 

 indicates the approximate climatic fluctuations of central and western 

 Asia as I had inferred them before doing any work on this subject in 

 America.^^ It is avowedly imperfect, especially in the earlier portions. 

 Before the time of Christ information is so scanty that in some cases 

 there are gaps of two or three centuries where no data are yet available. 



25 The curve is taken from "Palestine and its Transformation," pp. 327 and 403, 



