On the investigation of hidden periodicities with application to a supposed 26 day period of meteorological phenomena

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Abstract
1. Obvious and hidden periodicities. A variable quantity may show periodic changes which become obvious as soon as a sufficient record has been obtained; such are the semi‐diurnal changes of the tides, or the eleven years recurrence of sunspot maxima. We may call these obvious periodicities. Most often, however, small periodic variations are hidden behind irregular fluctuations, and their investigation then becomes a matter of considerable difficulty.