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Abstract
The 27-day recurrence tendency in the cosmic ray intensity has been studied by the Chree method of analysis for the period 1937–55, using the ionisation chamber records at Huancayo. The amplitude of the 27-day variation changes from year to year. The variation in the amplitude is in general agreement with the changes in solar activity as revealed by the sun-spot number only for the years 1937–46 and 1952–55. This is in agreement with the results of Meyer and Simpson for the same periods, obtained from a slightly different method of analysis. The correlation breaks down for the period 1946–52 and this is discussed in terms of possible changes in the electromagnetic conditions in the interplanetary space during the solar cycle.