36 



CLIMATIC CYCLES AND TREE GROWTH 



simple graphic transformations. Figure 18a gives an approximate sunspot 

 curve on a horizontal time scale. The first transformation consists in split- 

 ting up this curve into 10-year blocks and placing one under the other, as 

 in figure 18b, at definite and equal vertical intervals. Each block is ex- 

 tended beyond the ten years to 20 years or more. The dotted line A-B-C 

 represents the same instant of time in three different blocks. The second 

 transformation consists in swinging this line A-B-C so that it becomes hori- 

 zontal, as in figure 18c, without changing the horizontal scale of figure 18b; 

 that is, each plotted point in the diagram moves downward vertically and 



Fig. 17 — a, b. Two methods of plotting cycles. Above, rectangular plot; 

 below, combination of rectangular and polar plots. 



not on an arc. By this transition we secure a correct vertical time scale on 

 rectangular coordinates. For convenience in use, this diagram as a whole is 

 now rotated 90° to bring the main time scale horizontal, as in figure lSd. 1 

 This produces a complete time progression from left to right on rectangular 

 coordinates. The vertical scale, which was the horizontal scale in b, ex- 

 presses residuals from 1840, 1850 which are multiples of 10. It could be 

 shown by trial that the exact multiple of any other convenient number could 

 equally well be used. 



1 This change in representing cyclograms is apparent in the different position they 

 hold in Climatic Cycles and Tree Growth, vol. I, Plate 12 (1919), and vol. II, Plate 9 

 (1928). 



