Estimated time-fluctuations in the rate of westward drift of the geomagnetic field, 1885-1945, re shown to correspond reasonably well with the unexplMned systematic fluctuations in the earth's rate of rotation derived by Brouwer. In view of other geophysical evidence on the rigidity of the earth, it appears reasonable to conclude (1) that the source of the geomagnetic field lies within a large-scale fluid circulation inside the central core of the earth; and (2) that this fluid circulation in the core must be considered established as real, since no other