6o6 



HIKKKLANl). THE .NORWEGIAN' AL'RORA POLARIS EXPEDITION, IQO2 1903. 



qurntly to pass through the equator once more, viz. u -\- i times in all, and is afterwards precipitated 

 in the southern "auroral zone", while a corresponding bundle of rays, which had nearly been preci- 

 pitated in the southern "auroral zone", passes through the equator and is precipitated beside the 

 bundle of rays which had passed ;/ times through the equator. The next contiguous group of rays has 

 passed through the equator 11 + 2 times before being precipitated in the "auroral zone". As the rays 

 now fall symmetrically above and below the magnetic equator, the corresponding process of selection 

 will have taken place in the southern "auroral zone", so that in the northern and southern zones 

 auroral rays will be produced successively one after another, each one having passed through the 

 equator mice oftciii'r than the nearest preceding auroral ray. 



Aurora boreaiis observed at Bossekop on the 6th January, 1839, ai/cordin^; to Bravais. 



Although it is not our intention to deal with the auroral phenomena until we come to Volume II 

 of the present work, where we shall see how the different forms of auroral light are to be explained, 

 ue shall, however, now show, as an illustration connected with the terrella experiment just described, 

 how the formation of auroral draperies is to be understood. As a characteristic feature of this perhaps 

 the most peculiar form of auroral light, we would remind the reader that the aurora borealis frequently 

 appears as a vertically hanging curtain consisting of densely co-ordinated parallel rays. The curtain 

 has most frequently its longitudinal direction in the magnetic east and west. 



As further characteristics we would mention that the auroral curtain is frequently formed from east 

 to west, or vice versa, in such a way that the rays, one after another, seem to be precipitated from the 

 sky, and this so rapidly that the curtain can be completely formed and extend right across the heavens 

 in a few seconds. 



Another phenomenon, which is most closely related to the above, is that of the so-called luminous 

 riWiV.v which may rush through the auroral drapery. The rays blaze up and go out, and the phenomenon is 

 repeated successively on every ray from one end of the curtain to the other, the wave appearing to 

 pass through the entire length of the drapery. The waves move most frequently from west to east, but 

 also \ ery otten in the opposite direction. 



