( 292 ) 



position of the Earth with respect to tlie surfaces of 

 fl i scon t i 11 n i t_v an (1 in the f 1 iic t ii a t ion s i ii i li e co ml i t i o n s 

 of i 1 1 n m i n a t i o n resulting f r o m it. 



Not only the fhictnations in tlie total intensity of illumination, but 

 the changes in the composition of the solar light also, will have 

 their significance in this respect. 



3. Polar Uiikts. 



Polar lights hcioiig to that chiss of plienomena which seem hut 

 little influenced by the local conditions on the surface of the Earth. 

 The altitnde at wdiich they originate has been variously estimated; 

 it is generally snpposed to be very high, several kilometers. No one 

 doubts but that this phenomenon is closely connected with tlie solar 

 radiation, an opinion supported by the existence of a daily period 

 with its maximum at 2'^40'n p „i .^Y^^\ \x^ minimum at 7''4()'» a. in. 

 (Caulheim-CtYLLENskiöld). Most probably polar lights owe their origin 

 to the discharges of electricity generated during the day in the 

 higher layers of the atmosphere through the ionising action of the 

 Sun's irradiation. 



If this be so, local differences in the Sun's irradiation must favour 

 the appearance of i)olar lights and consequently we may expect in 

 their frequency the semi-annual and annual |)eri()ds described under 

 A as well as the less regular variations spoken of under B. 



The following table, taken from the Lehrbuch der kosmischen 

 Physik by Arrhemls, p. 913, gives a survey of the frequency of 

 polar lights from the enumerations made by Ekhol.m and Akrhemus 

 for various [)arts of the globe. 



