VII. METHODS OF PERIODIC ANALYSIS. 



Need for such analysis. During these modern times of rainfall and 

 sunspot records we may compare such records with tree-growth and 

 obtain the interesting correlations exhibited in the last two chapters; 

 but the tree records extend centuries and even thousands of years back of 

 the first systematic weather or sun records of any kind. Without being 

 over-precise or exhaustive, it is interesting to note that California 

 weather records began about 1851. Records on the Atlantic coast 

 began largely in the half-century before that date. London has a 

 rainfall record since 1726, Paris since 1690, and Padua since 1725. 

 Good sunspot records began about 1750, but the number of maxima 

 and minima is known between 1610 and 1750, although the exact dates 

 are uncertain. All this does not carry us very far back, but it 

 serves as an excellent basis for the correct interpretation of the record 

 in the trees. 



It would be possible to apply correlation formulas to the Arizona 

 tree records and perhaps to the sequoias and construct a probable 

 rainfall record for long periods of time, but apart from Huntington's 

 study of the " Climatic Factor in History," the chief use of such a 

 record would be in studying the laws which govern rainfall; and this 

 is best done through cycles. We shall find that the sunspot cycle plays 

 an important role in rainfall. But we find traces of the solar cycle in 

 nearly all of our tree groups, and evidently the way to read the trees 

 is to study first of all their alphabet of cycles. Hence the best methods 

 of identifying cycles must be used. 



Proportional dividers. If a short series of observations is to be 

 tested for a single period, it can be done by mathematics, but it will 

 take many hours and give a result in terms so precise as often to 

 deceive. This, for example, has been the difficulty with the mathe- 

 matical solution of the sunspot curve. It seems to the writer that the 

 safer way to solve such a curve is by a graphic process, plotting the 

 curve and applying equal intervals along it. An extremely good in- 

 strument for this purpose is the multiple-point proportional dividers. 

 By a system of pivots and bars, 16 or more points are maintained in 

 a straight line and at equal intervals, while the space between two 

 successive points may be drawn out from one-eighth inch to one inch. 

 The remarkable persistence of the half sunspot period in the early 

 Flagstaff trees was detected in this way. 



The projection of equal spacing on curves as long as 12 to 15 feet 

 has been done by a 10-foot india-rubber band with small metal clips 

 pinched on at regular intervals. As the band was stretched all the 

 intervals were enlarged by equal amounts, and periodic phenomena 

 were detected. Similar use could be made of the sharp shadows cast 

 by the glowing carbon of an arc-light. The shadow of a transparent 



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