﻿Properties of the Rays producing Aurora Borealis. 211 



earlier— certainly several millions of years ago. Prestwich 

 gave a somewhat different turn to StapfFs theory in ascribing 

 the heat to dynamical movements in the granite; movements 

 which might have been of somewh it later date. But this 

 source of heat was almost certainly inadequate. It appears 

 difficult to urge a better hypothesis than that of the radio- 

 active origin of the heat in a granitic mass extending 

 deeply beneath the tunnel. 



Unfortunately no other similar instance can be cited. 

 The temperature observations in the case of the Simplon 

 tunnel suggest themselves. Prof. Lees has calculated that 

 a small amount of radium in those rocks would produce 

 little effect*. A complete estimate of the radioactivity of the 

 Simplon rocks has yet to be made. In view of the recent 

 work which has been done in my laboratory, I must conclude 

 that the general means for the Simplon rocks, which I 

 formerly arrived at, may suffer from the same unknown 

 source of error which apparently affected some other con- 

 temporaneous experiments. 

 Dec. 15,1911. 



XVIII. On the Properties of the Rays producing Aurora 

 Borealis. By L. Vegard, Universitetsstipendiat, University 

 of Climstiania f. 



^Introductory. 



1. "\lt7E are at the present time quite familiar with the 

 T T idea that the sun is sending out electric rays of 

 some sort, and that such a radiation is the primary cause of 

 Aurora Borealis and most magnetic disturbances. 



Professor Kr. Birkeland % has worked on this hypothesis 

 for about fifteen years, and through a great number of 

 observations treated by Birkeland and his collaborators, as 

 well as through a number of laboratory experiments, a new 

 light has been thrown upon these phenomena, and the 

 results obtained are most strongly in favour of the radiation 

 theory. 



The theoretical work of Professor 0. Stormer § on the 



* C. H. Lees, Roy. Soc. Proc. A. vol. lxxxiii. p. 344. 



f Communicated by the Author. Read before the British Association. 

 Portsmouth, September 1, 1911. 



| Kr. Birkeland, Archives des Sciences phvs. et nat. Geneve. 1896; 

 Reeherchcs sur les inches dn Soteil, &c, 1899 ; Expedition Novvec/ienne de 

 1899-1900, &c, 1U01 ; The Norwegian Aurora Polaris Expedition, 

 1902-1903. 



§ C. Stormer, ChristianiaVidenskabsselskabs Skr. t Math.-Nat. B7.No. ' : 

 1904 ; Archives des Sciences phys. et nat. Geneve [-1] vol. xxiv. 1907. 



