Magnetic Variations at different Places, $c. 209 



magnetic disturbances the variations of vertical force, which are some- 

 times very great, have also to be considered, as well as those of 

 declination and horizontal force. Bat even admitting the supposition 

 to be correct, there remains for consideration the question as to how 

 the diurnal magnetic variations are produced. 



Magnetic disturbances and irregular magnetic motions of every 

 kind are always accompanied at Greenwich by earth currents more 

 or less powerful, the correspondence being most complete, as may be 

 seen in the engraved copies of magnetic and earth current movements 

 given since the year 1882 in the annual Greenwich volume. The 

 diurnal magnetic variation of declination or horizontal force, on the 

 other hand, which (when there is no accidental magnetic irregularity) 

 consists principally of one bold daily sweep, has no corresponding 

 earth current, being accompanied principally by alternating currents, 

 weak in character, and generally of short period. That is to say, a 

 sudden magnetic movement of an amplitude no greater than that of 

 the undisturbed diurnal curve (comparing one part of the day with 

 another) will be accompanied by active earth current, whilst the bold 

 sweep of the undisturbed diurnal curve has no marked earth current 

 counterpart, but only fluctuations of feeble character. The ampli- 

 tude of the diurnal magnetic variation may be greater than the 

 motion in many of the cases of movement included in Table I, but 

 the earth current in the former case would be insignificant, whilst in 

 the latter it is most marked. Irregularity in magnetic action and 

 activity of earth current are correlative phenomena, rising and falling 

 together in intensity, earth current activity indicating irregularity of 

 magnetic movement or magnetic storm, superposed on, and, indeed, 

 frequently masking, the ordinary diurnal curve. 



The assumption that the magnetic movements in a magnetic storm 

 are due to action of the accompanying earth currents thus appears 

 not to be one that will explain the ordinary diurnal magnetic varia- 

 tion, as, indeed, Sir George Airy had previously found ('Phil. 

 Trans.,' vol. 160, p. 226) from discussion of the earth currents 

 observed in the old Dartford and Croydon earth current lines. The 

 phenomena have, indeed, different characteristics. The period of the 

 diurnal magnetic variation is the solar day, and the principal sweep 

 in the curve occurs whilst the sun is above the horizon. Magnetic 

 irregularities, on the other hand, and their accompanying earth 

 currents, appear at any moment. Although both probably solar in 

 origin, they are not produced in the same way. The diurnal variation 

 progresses gradually, the principal phase occurring successively at 

 different places, as they become turned towards the sun, but magnetic 

 irregularities arise suddenly and simultaneously at all places. The 

 special characteristics of the two classes of phenomena may be stated 

 as follows : 



p 2 



