﻿22S Mr. L. VegarJ on the Properties of 



that the mere existence of two night maxima, one before and 

 one after midnight, is a consequence of mathematical theory; 

 but entering into details we should meet with many difficulties 

 if we were to suppose that the predominant part of aurorse 

 observed belonged to the interval (Ij). 



Thus the two night maxima observed at the various stations 

 are not symmetrical with respect to magnetic midnight, 

 which should follow from theory provided the station w T as 

 situated near the meridian passing through the magnetic 

 axis (P f ). Such a position has Kingua Fjord, but still 

 the morning maximum occurs more than six hours after, 

 while the evening maximum occurs only two hours before 

 midnight. 



Another difficulty arises from the fact that aurorse occur 

 when B m has such large values that the line ijr = 8 m does not hit 

 the curve A at all. Thus at Kingua Fjord the average value 

 of 8 m at the time of the evening maximum is 24°*5, and at 

 midwinter even much greater, the largest (negative) value 

 of -v> in the interval (1^ is no more than 20°. 



Further, the diurnal maxima are not by any means so 

 sharp as should be expected from theory, provided most 

 aurorse were caused by the rays following orbits of the 

 interval (Ij). 



The broadening out of the maxima may have several 

 causes. In the first place, auroras are observed and may be 

 counted even when they do not appear with their main bulk 

 on the meridian of the place. Secondly, the curves are 

 deduced from observations covering a period of several 

 months, and during that time h m will undergo variations 

 which will produce corresponding changes of <£> or the time 

 of occurrence of the maxima. But these causes alone are 

 not competent to explain the broadening of the maxima ; 

 for the same form may be observed near the zenith repeatedly 

 for many hours during the very same evening ; neither can 

 it be due to differences of stiffness* of the solar rays, because, 

 according to mathematical theory, when yjr is the same, the 

 value of <I> is very little changed by considerable changes of 

 stiffness. 



The discrepancies, however, may to a certain extent be 

 ascribed to the incompleteness of the mathematical investi- 

 gations, because the path of the rays near the earth may be 

 greatly different from that of an elementary magnet, and 

 near the poles a small difference of path may considerably 

 alter the hour of occurrence. 



* " Stiff rays " means rays which are not easily deflected by electric or 

 magnetic forces. 



