VI. CORRELATION WITH SUNSPOTS. 
Dry-climate tests.—In the work of 1907 (published 1909) upon the 
first group of 25 yellow pines from 1700 to 1900 A. D., several long 
sequences of variation ina 5 to 6 year period were noted. These were 
compared with rainfall records at Prescott and in southern California 
and the crests of rainfall and growth appeared to coincide in date. It 
was then seen that the temperature curve of southern California had 
a period and phase corresponding to the rainfall curve, but with the 
second minimum almost entirely suppressed, and that finally this tem- 
perature curve resembled in form and phase the inverted curve of sun- 
spot numbers. In connection with the publication referred to (1909), 
a set of curves was prepared to show these relationships. This set is 
partly reproduced in figure 34, page 104. In the original drawing the 
tree-curve was the least satisfactory, which was to be expected, as no 
real certainty in the dating of rings existed at that time. After cross- 
identification the tree-curve was again integrated for the 11-year period 
and far better results were obtained. This new curve is given in the 
figure referred to. 
This type of integrated curve gives many facts in a very condensed 
form. A differential or detailed form of presentation should accompany 
it, as in figure 25, showing the full series of individual observations and 
beside it the curve with which it is to be compared. The differential 
study of the Arizona trees will be taken up in connection with cycles, 
but can be summarized in the statement that in the last 160 years 10 
of the 14 sunspot maxima and minima have been followed about four 
years later by pronounced maxima and minima in the tree-growth. 
Also, during some 250 years of the early growth of these trees, they 
show a strongly marked double-crested 11-year variation. 
Wet-climate reaction—In the very first group of continental trees 
studied, those obtained at Eberswalde near Berlin, the remarkable 
fact was recognized at once that 13 trees from one of those carefully 
tended German forests show the 11-year sunspot curve since 1830 with 
accuracy. The variation in the trees is shown in plate 8. The arrows 
on the photographs are not to call attention to the larger growth, but 
to mark the’ years of maximum sunspots. The other trees of that 
group do not show quite so perfect rhythm as do the marked radii 
shown, but are like the other parts of these sections, showing strongly a 
majority of themaxima. Taking the group as a whole, the agreement is 
highly conspicuous, and the maximum growth comes within 0.6 year of 
the sunspot maximum. The Eberswalde curves arranged in two groups 
and compared with the sunspot curve were shown in figure 9, page 38. 
In the group of six sections from south Sweden, which were measured 
subsequently in Stockholm, a spruce (Picea excelsa) was discovered 
which shows the sunspot rhythm with the same striking clarity as the 
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