212 Simultaneity of Magnetic Variations at different Places, fyc. 



It has been further shown that at Greenwich the sudden magnetic 

 impulses immediately preceding or commencing disturbance are con- 

 current in time with the sudden appearance in each case of an earth 

 current, in advance by a few seconds of the magnetic impulse, and 

 having always the same relation with the magnetic movements, 

 increase of magnetic declination, horizontal force, and vertical force, 

 being accompanied by a current in one and the same direction, and 

 decrease of these elements by a current in the opposite direction. A 

 like concurrence in time between such magnetic movements and 

 earth currents is presumably true also for other places. 



If the near time relation thus established between initial magnetic 

 and earth current movements at Greenwich applies generally during 

 the course of a magnetic storm, any difficulty as respects the assump- 

 tion that the irregular changes of magnetic declination and horizontal 

 force may be produced by the accompanying earth currents seems 

 removed. But though the changes of horizontal force during a mag- 

 netic disturbance closely follow the earth current changes, those of 

 declination do not show the same correspondence, and the variations 

 of vertical force have also to be explained, in addition to which it 

 would conclusively seem that the diurnal magnetic variation does 

 not depend on earth current, since the bold sweep of this curve 

 (when undisturbed) is accompanied by comparative earth current 

 calm. 



With reference to the comparison of times of magnetic impulses at 

 different places, it does not seem probable that with existing appa- 

 ratus any better result would be obtained by making comparison for 

 an increased number of days. But it would be interesting, even with 

 the present apparatus, to obtain corresponding times, if possible, for 

 a greater number of places, in order more conclusively to determine 

 whether the constant difference of time that appears to exist between 

 some stations (Table II) is really a physical fact, or whether it may 

 not be due to small systematic error in the individual registering 

 apparatus. Registers on an extended scale, on the system mentioned 

 in a preceding paragraph, would, however, be much more likely to 

 give definite information on this point. 



If this paper has added anything to the knowledge of a difficult 

 subject, it will be felt that some labour has not been expended in 

 vain. 



