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The study of the long-period variations of the earth's magnetic field requires a great number of observatories having long records of continuous operation. Observation at Hong Kong was commenced by the Royal Observatory in 1884 and continued without a break until 1939 (NEIGHBOUR et al., 1974). The data from several dozen observatories had been used in analyses of the secular variations by many authors, e.g. YUKUTAKE (1965), BHARGAVA and YACOB (1969), CURRIE (1973), RIVIN (1974), ALLDREDGE (1976,1977), COURILLOT and MODEL (1976), SRIVASTAVA and ABBAS (1977). However they did not use the data of Hong Kong, except RIVIN's article (1974). Rivin used the horizontal component of Au-Tau (Hong Kong) only, but his result was different from this paper. The details will be discussed later. The coordinate of Au-Tau (Hong Kong) Royal Observatory is 114.2E (Geographic Longitude) and 22.2N (Geographic Latitude). This study utilizes the annual means of midnight values of the horizontal component (H), the vertical component (Z), and the declination (D). In order to remove the major trend, a suitable filtering technique is required. In this paper, the data were first preconditioned by subtracting values of a fifth degree polynomial curve, which is the best-fitting curve to the annual mean values. The residual data were then fitted by a sum of trigonometric sine terms, and also were analysed by the maximum entropy method (MEM) (ULRYCH, 1972; CHEN and STEGEN, 1974; ANDERSEN, 1974). Figures 1 and 2 show the regression curves-fifth degree polynomials for H and Z respectively. The plots are the annual means of midnight values. The following equations are obtained for the best fitting curve to the annual values by the method of least squares.

Geomagnetic variations with periods from 5 to 30 years at Hong Kong.