The Earth's magnetic field in Southern Africa at the epoch, 1 July 1930

Abstract

This paper is an attempt to determine the earth’s magnetic field in that part of Africa lying to the south of the Zambesi and Kunene Rivers, at the epoch 1930-50. The data used are: (1) Measurements made at about seven hundred stations by a number of previous workers, during the period 1900 to 1925, and already published. (2) Observations at about fifty of these stations made by the present writer between 1928 and 1930. These have been used to determine the secular variation, and thus to deduce the 1930-50 values of the magnetic field at all the other stations. The magnetic inclination is found to have changed in an almost linear manner, the maximum rate occurring in South-West Africa. The horizontal intensity has diminished at a gradually increasing rate, the maximum change being near Cape Town. The declination appears to have varied at a high rate until about 1928, and much more slowly since then. The greatest total changes are found near Durban. The results are presented in the form suggested by Ljungdahl. Maps with highly smoothed isomagnetic lines are used to show the probable ‘normal’ values of three magnetic elements (declination, inclination, and horizontal intensity), i.e. the component of the field not due to local magnetic disturbance. At each point of actual observation is placed a symbol indicating to what extent the observed value differs from that obtained by interpolation between the isomagnetic lines.