( 293 ) 



The last two ('(tliimns (lisliiicllv show Hie cxpeeted period icih : 

 niaxinui in Alai-cli or April and in S('|»k'inl)ei' or Oetober, iiiiniina 

 ill .June or .Inly and Deceinhcr or .lanuary, wliilst in each case liie 

 wiiiler ininiiniiin is lowest, altlioii.uli the loiiii' winler iii,i>lits favonr 

 the ohserxalion of poh\r liuiils. In the other ihi'cc cohinins, dcalinii- 

 with lii^i»her laliludes, the snmnier inaxinniin, as Akriikmus observes, 

 is only apparently so low, because in I hese regions no lime is left 

 for the observation of polar liuhls oxving lo (he length of the davs. 



The data collected by Fkit/ and AuRiiKAirs (Leiirb. d. kosni. I*livs. 

 }). 015), moreover afford snflicient evidence that the snn-spol period 

 too is reflected in the freqnency of polar lights. 



4. The (iiimiiil i-dfKilioii In the (Uni'iinl uicinKildii of 

 tci'i'i'.sti'Kil iiKniiK'tlsni. 



It is an acknowledged tact that the magnetism of the Earth is 

 under the intliience of the Snn's irradiation. In recent years this view 

 has l)een strengthened by the aj)pearaiice of magnetic disturbances 

 within the belt of totality during total eclipses of the Sun. 



Let ns suppose the mean magnetic force at every [»oint of the Earth 

 to be re]n-eseiited l)y a vector. If \Ne now represent the daily varia- 

 tion by an additional, vai-iable \ector, the whole of all these additional 

 vectors will form the "\ariation field". Schustkr and v. Bkzold have 

 computed and constructed this field and shown that by its move- 

 ment from east to west with a velocity of 15' an hour, a fairly 

 accurate idea may be obtained of the diurnal deflections of the mag- 

 netic needle on all parts of the Earth. 



This "variation field", according to S( iiustkr, is formed for about 

 7, [)arts by electrical currents in the atmosphere and for V, part by 

 Earth currents. By the electrical current in the atmosphei-e we under- 

 stand a convection current formed by electrificMl particles, which are 

 carried along by the cyclonic and anticyclonic nio\(Mueiits of the 

 general circulation. 



This theory of Sciustkr and \. Hkzoi,!) therefore implies that the 

 diurnal magnetic xariatiou'^ \\ill iiici*ease both with the inleiisity of 

 the general circulation and with the amount of ionisalion in the 

 higher layers of the atmosphere. If one or both of these processes 

 are in a great measure inlluenced by the \ai-iability of solar irradia- 

 tion (which is not improbal»le, \ide .Vrkiiknus, Lehrbuch p. 886, 

 888, 890, 898), then all the periods which according lo our theorv 

 occur in the xariability of the Sun's irradiation ninsl find theii' counter- 

 part ill the diiinial inequality ol" the elements ol" terrestrial magnetism. 



20 



Pioceeclings Royal Acad. Amslordam. Vol. V(. 



