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 sunspots; (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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