ANALYSIS OF TERRESTRIAL RECORDS 89 



of this two-year zigzag. Three degrees of mean sensitivity have been shown 

 in previous publications, 11 per cent in complacent sequoias, 35 per cent 

 approximately in the sensitive sequoias, and about 65 per cent in the sensitive 

 yellow pines of Arizona. It is a matter of investigation to find which of these 

 most nearly represents the percentage variation of rainfall from year to 

 year and what are the topographic features that bring the best climatic reac- 

 tion. Of course, we know that the more sensitive reactions occur near the 

 forest border when the trees are growing under the stress of dryness. 



But mean sensitivity is also a measure of cross-identification, and we see 

 that the two-year zigzag is strongly responsible for cross-identification and 

 all the important resulting operations in climatic history and chronology 

 building. Viewed from the opposite direction we see that cross-identifica- 

 tion, which we have used widely since 1911, indicates the presence and fair 

 uniformity in action of a two-year cycle over a large area. Since the two- 

 year zigzag, or scatter cycle, covers considerable areas with uniform results, 

 it is not a mere random local effect. 



Our present method of re-observing these facts is the same method used 

 in 1913; namely, taking the increments with signs plus or minus, from year 

 to year, and reversing alternate signs and making a mass plot of the con- 

 tinued algebraic sum of the resulting values. And we discover, as could have 

 been expected, that the two-year cycle is very uniform over each of the three 

 western zones taken by itself. For instance, figure 36 shows the mass curves 

 from several locations in the Arizona zone. Here we get a fairly uniform 

 general effect of the rising curve in the last 125 years to a maximum in 1880 

 to 1900 and a descent thence to the present time, and a series of minor crests 

 in this general curve at average intervals of 5.2 years. The latter are grouped 

 for comparison in figure 37. This 5.2 years becomes a beat or interference 

 effect between an exact 2.0-year cycle artificially produced by reversing 

 alternate signs and a cycle of either 3.23 years or 1.44 years. Hence such 

 an interval may be due to a cycle in winter precipitation of either length 

 given. 1 



On examining in the same way the Coast Zone and the Rocky Mountain 

 Zone we find for each a similar effect. These three results have good agree- 

 ment between 1850 and 1900, but before and after that the agreement is 

 poorer. This zone summary is shown in figure 37b. Figure 38 shows the 

 same process applied to random data. 



One naturally inquires whether this two-year cycle could be due to some 

 solar source. The sunspot cycle has been very stable since 1833, and some 

 harmonic might produce an effect. Clements has attributed this short cycle 

 to short cycles in monthly sunspot numbers. On the other hand, it may be 

 terrestrial in origin and result from some oscillation in ocean or atmosphere. 

 In any case its explanation is greatly to be desired. 



1 An attempt is being made to locate one or the other of these cycle lengths in 

 monthly rainfall values, but the interruptions produced by the short winter rainy season 

 necessitate careful compensations which are not yet finished. 



