Uniform sets of graphs are given expressing the average daily magnetic variations at seven observatories as influenced by season, sunspot‐cycle, and magnetic activity. The effect of disturbance on the daily mean values of the magnetic elements is discussed. International disturbed days are shown to be more disturbed around Greenwich noon than during the rest of the day. This second‐order effect must be considered in studies of the disturbance‐field.
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Bartels et al. (1939) studied this question.