2\. KU. BIRK ILAND. M.N. Kl. 



I see a conliniiation of this hypothesis in the fact that auroral rays 

 may drop down ahnost sinuiltaneously from an ahx'ady existing arc, as 

 mentioned above. The arc is often only a dehcate manifestation of the 

 mighty ray-precipitation giving rise to magnetic storms. ^Vhen this preci- 

 pitation already consists of numerous small co-ordinated groups of rays, it 

 is well conceivable that under given conditions some few rays of each 

 group ma^' drop in rapid succession and even simultaneously deeply down 

 into ihf alhmosphere and thus form an auroral drapery. 



The Error in Professor Størmer's Reasoning. 



}{. During the last couple of years, other attempts have been made 

 in different ways, ui)on the basis of the corpuscular rays, to obtain a 

 plausible explanation of the formation of auroral curtains. 



Vn.i.AUi) has put forward a bold and most ingenious idea '. 



I le tried after some beautiful experiments to conceive th.e auroral 

 drapery as formed by cathode rays emanating from Cirrus-clouds: this was 

 something like the manner in which Adam Paulsen viewed the matter. 



But V'n.LAKi) tal<es a remarkable step further, lie thinks the rays 

 from the clouds are drawn towards a terrestrial magnetic pole e. g. the 

 North Pole whence the rays return after having penetrated into the 

 atmosphei^e and formed an auroral my. I le supposes that the rays return 

 and go towards the South I'ole, where the same rays penetrate into the 

 atmosphere and form a southern auroral ray. The rays then return again 

 and go towards tlu' magnetic North Pole and there form a new auroral 

 ?-ay by the side of the first, and so on, a very great number of times. 



On account of the great absorbing power of the asmosphere, it does 

 not appear that this bold theory can be maintained; but Vili.akd has 

 certainly made important steps in the right direction. 



Further SroRMKu - has advanced an interesting attempt at a mathe- 

 matical theory on the creation of the auroral curtains, based upon the 

 supposition that the rays which in reality occur in the auroral curtains, 

 always keep close to the rays which were calculated to go through the 

 centre of an elementary magnet representing the earth. But Stormkr's 

 theory can only be considered as a beautiful mathematical experiment, for 

 the image given does not conform to the phenomena we observe; so I do 

 not think, therefore, that we can sa}' that his theory has anything to do 

 with nature as far as auroral curtains are concerned. Stormer calculates 



' Villard: Les rayons cathodiques et l'aurore boréale. Paris 1907. 



3 Stokmer: Archives des Sciences physiques et naturelles, Genève 1907. 



