210 Mr. W. Ellis. On the Simultaneity of 



Diurnal magnetic Superposed magnetic 



variation. disturbance. 



Characteristic. One bold sweep Irregular motions, 



progressing 

 uniformly. 

 Occurs at differ- Successively. Simultaneously. 



ent places. 



Earth current No corresponding Active earth cur- 



condition, earth current; rents, 



only weak inter- 

 mittent currents. 



However bold the diurnal variation may be, so long as it progresses 

 uniformly, the earth currents remain fluctuating and feeble, and 

 apparently cannot produce the long sweep of the diurnal curve. But 

 if any magnetic irregularity arises, a corresponding earth current at 

 once appears. It was pointed out with respect to the magnetic move- 

 ments on the selected days (Tables I and III), that the earth 

 currents observed at Greenwich in connexion therewith always had 

 a definite relation with the motions in declination and horizontal force. 

 This, taken in connexion with the coincidence in time (Table IV) r 

 certainly suggests mutual relation. But, in view of the circumstance 

 that the diurnal magnetic variation does not appear to depend on 

 earth current, can we suppose that it may be otherwise with the 

 irregular magnetic motions, and that they are produced by earth 

 currents P When a magnetic storm arises the earth would appear 

 to become as a whole instantaneously affected, necessarily, it would 

 seem, from without. Is it that both classes of phenomena, irregular 

 magnetic variations and earth currents, are produced by independent 

 action, and that any mutual relation, if existing, is of secondary cha- 

 racter. In the Greenwich registers the variations of horizontal force, 

 during periods of magnetic disturbance, follow with surprising close- 

 ness the accompanying earth current variations, the turning points 

 being in a very remarkable degree simultaneous. But, after the initial 

 movement, there is not a similar resemblance as regards declination 

 changes. And how are the variations of vertical force produced? 

 Can there be here any relation with earth currents ? 



The general direction of strongest earth current at Greenwich at 

 the present time is one not very different from that of the line 

 joining the Angerstein Wharf Lady Well earth plates. Does this 

 direction of strongest earth current depend on geological formation, 

 or is it slowly variable, as are the magnetic elements ? If not variable, 

 the earth current influence on the decimation and horizontal force 

 magnets, if such influence exists, would be different when the direc- 

 tion of the magnetic meridian has greatly changed. This is a point 

 the study of which might in the course of time throw considerable 

 light on the relation between magnetic and earth current irregulari- 



