﻿the Rays producing Aurora Borealis. 223 



cannot be accounted for by any disturbing influence of day- 

 light, because this light effect would be symmetrical with 

 respect to midnight. The values of r show that at the three 

 eastern stations, during the time of sufficient darkness, more 

 aurorae of the forms considered appear on the evening side. 



Kingua Fjord, however, occupies a most singular position 

 in this respect ; also here the aurorse show two maxima, but 

 they occur with a decidedly greater frequency around the 

 morning maximum than around that of the evening. This 

 result is the more remarkable when it is compared with that 

 of Godthaab. This station is situated very nearly on the 

 same latitude as Kingua Fjord, and is not very far apart 

 from it, and further, the observations at the two places cover 

 the very same period. 



Passing on to the most westerly station, Fort Rae, we find 

 that the value of r increases again. For the curtain form it 

 is greater than one, and for aurora? through the zenith some- 

 what less than one ; but this fact does not show, as in the 

 case of Kingua Fjord, that more aurorae are found near the 

 morning maximum ; it is rather a consequence of the fact 

 that the evening maximum occurs much later at this station 

 than at the more easterly ones. 



The evening maximum is seen to occur at very different 

 local hours. At Cap Thordsen it occurs as early as 8 o'clock, 

 while at Fort Rae it occurs at 12. The occurrence of the 

 maxima at the various stations will become more regular 

 when regarded in relation to the magnetic axis instead of 

 the axis of rotation of the earth. Let on a geocentric sphere 

 be the position of the station, S that of the sun at the 

 moment of the evening maximum, and P and P / the points 

 where the axis of rotation and the magnetic axis cut the 

 sphere respectively. 



The hour angle of the sun SPO is given by the local time 

 of the occurrence of the maximum. The corresponding angle 

 with reference to the magnetic axis is SP'O, and has been 

 calculated for the various stations. The angle a given in 

 Table III. is supplementary to SP'O or Z SP'O^ISO -*. 



In Table III., 90° + S m is the angle between the north 

 direction of the magnetic axis and the direction to the sun 

 at the time of the evening maximum. 



In spite of the great differences of the hour-angle OPS 

 the angle SP'O comes out nearly constant. This will also 

 be evident from fig. 4 (p. 220), where a line is drawn giving 

 the time when the station cuts the plane through the sun 

 and the magnetic axis, or, as we might call it, the time of 

 magnetic midnight. 



