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

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.