8oo 



r.IKKFI AM). Till! XOK\VK(,IAN AL'KORA POLARIS KXPKIHTIO.N, I QO2 1903. 



In fig. 287 \vr have also drawn a chart of the course of the current-lines at the surface. 



Then.- is to he noticed here a striking agreement with the main features of Schuster's chart of 

 tin potential lines for the diurnal variation (cf. his admirable memoir in Phil. Trans., 1889, p. 508). 



In addition to this equatorial system, there are polar systems also at work, and we ought therefore 

 bv rights to bring such together il we want to represent the earth-current system that is induced bv 



IMC;. 288. 



Fig. 289. 



rotation in relation to the entire external system. In this wav it would be easy, bv a suitable choice, 

 to find current-fields that in their details too, exhibited a more perfect agreement with Schuster's chart. 



A composition such as this, however, will in the first place always be rather arbitrary, and in 

 the second place it will only answer to a part of the earth-current system that characterises the diurnal 

 variation of these currents; while in the third place a chart of the earth-current lines and the magnetic 

 potential lines are only comparable in certain respects. 



As there is, moreover, no chart of the diurnal variation of the earth-currents all over the earth, 

 we will not here undertake any such composition as regards the entire earth. All that exist are the 

 determinations of the diurnal variation in Germany and England. 



