Ill] THE EFFECT OF CLIMATE 121 



climate during past years from the record of varying rates of 

 growth which the tree, by the thickness of its annual rings, has 

 preserved for us. Mr A. E. Douglass, of the University of 

 Arizona, has made a careful study of this question*, and I have 

 received (through Professor H. H. Turner of Oxford) some measure- 

 ments of the average width of the successive annual rings in " yellow 

 pine," 500 years old, from Arizona, in which trees the annual 

 rings are very clearly distinguished. From the year 1391 to 1518, 

 the mean of two trees was used; from 1519 to 1912, the mean of 

 five; and the means of these, and sometimes of larger numbers, 

 were found to be very concordant. A correction was applied by 

 drawing a long, nearly straight line through the curve for the 

 whole period, which line was assumed to represent the slowly 

 diminishing mean width of ring accompanying the increase of 

 size, or age, of the tree ; and the actual growth as measured was 

 equated with this diminishing mean. The figures used give, 

 accordingly, the ratio of the actual growth in each year to the 

 mean growth corresponding to the age or magnitude of the tree 

 at that epoch. 



It was at once manifest that the rate of growth so determined 

 shewed a tendency to fluctuate in a long period of between 100 and 

 200 years. I then smoothed in groups of 100 (according to Gauss's 

 method) the yearly values, so that each number thus found 

 represented the mean annual increase during a century: that is 

 to say, the value ascribed to the year 1500 represented the average 

 annual growth during the whole period between 1450 and 1550, 

 and so on. These values give us a curve of beautiful and surprising 

 smoothness, from which we seem compelled to draw the direct 

 conclusion that the climate of Arizona, during the last 500 years, 

 has fluctuated .with a regular periodicity of almost precisely 150 

 years. Here again we should be left in doubt (so far as these 



* I had not received, when this was written, Mr Douglass's paper, On a method 

 of estimating Rainfall by the Growth of Trees, Bull. Arner. Geograph. Soc. xlvi. 

 pp. 321-335, 1914. Mr Douglass does not fail to notice the long period here 

 described; but he lays more stress on the occurrence of shorter cycles (of 11, 21 

 and 33 years), well known to meteorologists. Mr Douglass is inchned (and I think 

 rightly) to correlate the variations in growth directly with fluctuations in rainfall, 

 that is to say with alternate periods of moisture and aridity; but he points out 

 that the temperature curves (and also the sunspot curves) are markedly similar. 



