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Time variations in the magnetic field are generally a planetary, rather than a local, phenomenon. However it has been known for some years that variations with a time scale of tens of minutes differ remarkably at observatories not far apart. This is especially true of the vertical component. This difference is usually ascribed to gradients in underground conductivity. From what we know of the conductivity distribution it seems likely that these gradients are in the upper mantle. There are many ways of studying these regional differences in magnetic variations. The method we have adopted in Australia is to establish correlations in the simultaneous changes in the three elements during bays and similar variations. Usually there is a definite correlation at each station which often differs from that at a neighbouring station. This method has the advantage that a large network of stations operating simultaneously is not necessary. It gives a statistical rather than a synoptic picture of the phenomenon. We find that generally there is a tendency for the vertical component to increase when the horizontal components change in a particular horizontal direction, and to decrease when the horizontal components change in the opposite direction. Often this correlation is sufficiently good that the amount of vertical variation is proportional to the component of the horizontal variation in that particular direction. Both the direction which correlates with maximum vertical change and the ratio of vertical to horizontal change vary from station to station, even when the stations have similar latitudes and are not far apart. If you divide a magnetogram into intervals of, say, 20 minutes, then by measuring the

Conductivity Anomalies in Australia and the Ocean Effect