Two rigid methods of disclosing periodicities, the periodogram analysis and self-correlation, are discussed, and several examples of their use in deriving the 28-month period in solar and meteorological data are given. The empirical or non-rigid method of deriving periods, illustrated by the determination of the epochs of sunspot maxima and minima, is contrasted with the rigid method, and it is shown that if the results by the two methods are properly interpreted, they are in substantial agreement.

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