VIII. CYCLES. 



Significance of cycles. It has already been stated that three charac- 

 teristics were observed in the curves of tree-growth: (1) correlation 

 with rainfall; (2) correlation with simspots; (3) general periodic 

 variation. In the first and second of these the trees are compared 

 directly with existing records, but in the third the tree record is avail- 

 able over hundreds and even thousands of years during which no 

 human observations were recorded. Thus, if previous inferences are 

 correct, the trees may reasonably be expected to give us some knowl- 

 edge of prehistoric conditions. In the first attempt to secure such 

 knowledge, the method which promises the most certain results is the 

 analysis of ring variations in terms of cycles. 



Correlatively, the study of cycles is of special value in climatic 

 investigations. Such studies are undertaken for the purpose of pre- 

 dicting the future. The basis of daily or short-distance prediction is 

 found in the conditions existing about the country at a given moment 

 and a knowledge of the usual movement of storm areas. A basis for 

 long-distance prediction is now generally sought in climatic cycles. 

 Such cycles may or may not be permanent. Perhaps they are nothing 

 more enduring than a series of wave systems on a water surface. Yet 

 for the navigator a knowledge of the existing system is important, and 

 so for the purpose of weather prediction we need to know the nature 

 of the pulsations actually operating, and each one should be studied 

 minutely. For this purpose the very long tree records and their pre- 

 sumably fair accuracy seem especially advantageous, since they give 

 us a range in centuries which the meteorological records, with few 

 exceptions, give only in decades. 



A special and rapid method of carrying on the study of cycles has 

 been developed in the periodograph which has been used in checking 

 fully all the results in the present chapter. But after its recent com- 

 pletion and trial the fact became clear to the writer that its real 

 service will be in a complete and thorough examination of all curves 

 obtained, in order to derive a quantitative statement of the extension 

 in time and space shown by each cycle. This in itself is a long process. 

 Moreover, preliminary analysis of many tree curves reveals a very 

 complex system of short-period variations in the trees, some of evident 

 significance and some of little-known value as yet. The study of this 

 complex of short periods together with other problems naturally sug- 

 gested in the course of the work is reserved for the future; we shall 

 now touch upon a few of the most important results reached in the 

 analyses already accomplished. 



Predominant cycles. With the understanding that the study of 

 cycles is not yet complete, it may be stated at once that the more 



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