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FROM an examination of the solar-diurnal variation in the horizontal force of the terrestrial magnetic field at Godhavn, Greenland, it has been established that the amplitude of the daily variation due to disturbance, SD, varies in a regular way over a period of 26.875 days, so that the mean amplitude of SD on the ninth day of the period is four times as great as that on the nineteenth day. The investigation was made in the following way: the days in the period (seven sequences of 27 days followed by one of 26 days) were numbered 0-26, and the material used consisted of 185 sequences of 26.875 days covering the epoch July 1, 1926-September 11, 1940 (eight periods in 1931 in which the material had not yet been rally prepared being omitted). Only the sequences for the summer months, May-August, were used at first. The daily variation due to disturbance, SD = S - Sq, was determined for the 1st day of the period (mean of the 0th, the 1st and the 2nd day), the 4th day (mean of 3rd, 4th, 5th day), 7th, 10th, 13th, 16th, 19th, 21st, 25th day for the total epoch (66 series), SD was analysed harmonically, the non-cyclic variation being eliminated at first, and the two first Fourier coefficients were determined (the 24- and the 12-hour term). Fig. 1 a gives the amplitudes c1, and c2, as functions of the number of the day. Next the material was divided in three groups: the disturbed summers 1926-31, the quiet summers 1932-36, and the disturbed summers 1937-40. The resulting values for c1 are given in Fig. 1, b, c and d, where the marked points represent the observed values and the full curve is the curve from Fig. 1 a multiplied by a convenient factor. Figs. 1 b-d confirm that the period is persistent throughout the fifteen summers. Comparison of 1 b, 1 c and 1 d makes it probable that the length of the period has an error of ± 0.025 day.

Persistent Solar Rotation Period of 26.875 Days and Solar-Diurnal Variation in Terrestrial Magnetism