Page:Climatic Cycles and Tree-Growth - 1919.djvu/130

104 the curve a little but not enough to change the pronounced fluctuations amply shown in recent analysis. This 200-year record showed a very clear combination of the double and triple sun-spot periods. This was illustrated at the time in a drawing which is largely reproduced in figure 35. Curve No. 1 is a triple solar cycle 32.8 years in length;



No. 2 is a double cycle 21.2 years long, and the third curve is a simple combination of the two. The fourth curve is the tree-growth, showing fluctuations which admirably combine these two periods. All subsequent analysis of these trees has entirely supported this result, as shown in the periodograph work below. When the length of curve was extended from 200 years to 500 years, the double solar period was found to prevail through almost the entire length, but the triple period does not appear to have affected the tree-growth in the earlier 300 years.

Two other plain examples of the double solar type are illustrated in figures 36 and 37. The former gives the double cycle shown in a