PROFESSOR A. SCHUSTER ON THE PERIODICITIES OF SUNSPOTS. 
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as the intensities at these periods are sufficiently well known. There is, moreover, 
the direct evidence of the periodograph curve that the absence of the 11-year period 
is not likely to be due to accidental causes. Uncertainty in the observations would 
act in the direction of giving a broad band extending over a wide range of periods, 
but would not, except through a freak, give the two pronounced maxima which 
curve B shows. 
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A closer inspection of the sunspot curves, as plotted down by Wolfer, shows that 
the two periods for which curve B shows its principal maxima were active successively 
rather than simultaneously. It appears that the period of 9 years is the important 
one as regards the observations previous to the maximum of 1788, while that of 
13 and 14 years is brought in through the variations observed between 1788 and 
1829. It would seem that more or less irregular variations, showing maxima of 
intervals of rather more than 9 years, were succeeded about 1788 by three unequal 
