Abstract
Summary Mean values of observatory geomagnetic data for the intervals 1960.0 to 1965.0 and 1965.0 to 1970.0 are subjected to spherical harmonic analysis to obtain coefficients of the main magnetic field. Since the same set of 118 observatories was used for each interval, the difference between the two sets of coefficients gives a reliable measure of secular change. Westward drift and movements of the centred and eccentric dipole are discussed, and the value of secular acceleration as an aid to prediction is assessed. The correlation between changes in westward drift and in the length of the day is re-examined and found to be of doubtful significance.
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