RELATION BETWEEN TERRESTRIAL AND SOLAR RECORDS 131 



type practically without an 11-year cycle and with about the same minor 

 cycles in each group. Figure 55 brings out the agreement found in modern 

 trees in the long chronologies, in buried trees, living perhaps 2000 years ago, 

 in Miocene sequoias from Yellowstone Park, with Schuster's periodogram of 

 the sunspot numbers — an evident similarity outlining an immense interval 

 of time. 



But there is evidence that this cycle complex was not continuous from 

 Permian times to the present, for studies of Pleistocene varves and logs of 

 swamp cypress, Taxodium distichum, probably of the same age, seem to 

 indicate that the Pleistocene cycles resemble the terrestrial and solar cycles 

 during the sunspot dearth near A.D. 1700, in the weakness of any 11-year 

 cycle. 



It is evident that the cycle complex in rings and in annual sediments offer 

 us two or three openings; first, an opportunity of extending to great length 

 our information upon certain climatic changes, both about the earth now 

 and back into past climates; and second, we can reverse our approach to it, 

 as we do in so many lines of knowledge when we have gone far enough to 

 make deductions, and find in our cycle complex in past time the probable 

 length of the sunspot cycle in geological ages. Thus we can form an opinion 

 as to the stability of the sun ; and third, it is even possible that with the long 

 chronologies in Arizona and California, covering really a large area, we shall 

 be able by proper comparisons to learn something about the actual dates of 

 sunspot maxima — if not for every one in the last 1900 years, at least for some 

 of them. 



PHYSICAL CAUSE OF CLIMATIC CYCLES 



The correspondences between solar and terrestrial cycles imply a physical 

 relationship even though it is not yet traced in details. The direct and evi- 

 dent channel of such relation is the heat radiation from the sun that supplies 

 the energy which evaporates water and causes the winds to move it over the 

 land and produce rain. Ultra-violet and other radiation may play an im- 

 portant part in climatic conditions. No one doubts that variations in the 

 heat of the sun can cause variations in the rainfall or other climatic elements. 

 The difficulties have been quantitative: First, that the sun's variations are 

 not great enough; second, that distribution of the effects about the earth 

 would be infinitely complex, and such simple expressions of solar changes 

 would not be isolated sufficiently to become apparent by our measuring in- 

 struments. 



Neither of these difficulties offers any evidence against a relationship be- 

 tween climate and sun; they are merely signposts with the word "slow" 

 painted on them. And it is well to be cautious, for there are more pitfalls 

 in meteorology than in most sciences (see Humphreys's Paradoxes of Meteor- 

 ology). 



Imagine an increase of heat from the sun: Water, land, and air grow 

 warmer, but the air is set in motion and clouds are formed that permit less 



