Abstract
Summary The results are given of the harmonic analysis of the Admiralty magnetic charts of declination, horizontal intensity and inclination for the epoch 1942.5. Within the limits of observational error, the Earth's magnetic field appears to be entirely of internal origin. There is no evidence of a dipole field of external origin greater than 0-1 per cent of the field of internal origin. The intensity of the dipole field is at present decreasing at a rate of about 5 per cent per century. The geomagnetic poles have a westerly drift at a rate of 4°-5 per century; the north magnetic dip pole is moving in a direction a little to the west of north, but the south magnetic dip pole appears to be practically stationary. In consequence of the dearth of magnetic data over the oceans since 1929, magnetic charts are becoming less accurate and there is a great need for airborne magnetic surveys of ocean areas.
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