Geophysics and cosmical physics owe to Sir Arthur Schuster the fundamental discovery that research on periodicities, cycles, and related phenomena requires, besides purely analytical calculations, such as harmonic analysis or Fourier series, essentially statistical considerations based on the theory of probability. His first paper on this subject, which appeared in this Journal, gives the outline of a theory which was later applied to various meteorological, terrestrial-magnetic, and cosmical periodicities. These classical papers, the study of which should be part of the training of every geophysicist, are: (1) On the investigation of hidden periodicities with application to a supposed 26-day period of meteorological phenomena (Terr. Mag., 3, 13-41, 1898); (2) The periodogram of magnetic declination as obtained from the records of the Greenwich Observatory during the years 1871-1895 (Cambridge, Trans. Phil. Soc., 18, 107-135, 1899); (3) On the spectrum of an irregular disturbance (Phil. Mag., 5, 344-346, 1903); (4) The periodogram and its optical analogy (Proc. R. Soc., A, 77, 136-140, 1906); (5) On the periodicity of sunspots (Phil. Trans. R. Soc., A, 206, 69-100, 1906); and (6) On the periodicity of sunspots (Proc. R. Soc., A, 85, 50-53, 1911).