DOUGLASS PLATE 9 
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. Periodogram of the sunspot numbers, 1755-1911. Corrugations show periods. The 
numbers give length of period in years. The white line is the year 1830 and shows 
phase. 
B. Differential pattern used in making the periodogram, consisting of the sunspot curve 
mounted in multiple. 
C. Same pattern photographed out of focus to show discontinuities in the vertical lines. 
D. Sweep of sunspot numbers, 1755-1911. 
K. Differential pattern of sunspot numbers made by the periodograph process. 
