The power spectra of cosmic-ray variations, as recorded by neutron monitors at various locations, is computed and analyzed herein. The, frequency range of the variations covered (from 1/200 to 1/2 per day) is found to contain only one significant peak, contributed by the well known 27-day variation. By use of the power spectrum, a quantitative estimate of the latitude dependence of the 27-day variation may be made. From this, the average rigidity dependence of the modulation amplitude is deduced, and is found to be essentially the same as that of Forbush decreases. The average was taken over the period July 1957 to December 1958, during which the peak decreased significantly.
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