784 Mr. Charles Ghree [March 4, 



on the 24th, while the storm commenced shortly before noon on the 

 25th and was at its maximum about 4h. oOm. p.m. Thus the terres- 

 trial phenomenon followed the solar after an interval of at least 25f 

 hours, if not of 30 J hours. 



On the other hand, Mr. C. Michie Smith,* Director of Kodai- 

 kanal Observatory, India, saw no outstanding solar activity until 

 September 28, when there occurred " a sudden and very violent 

 outburst of bright gases" on and near the sunspot then visible. 

 Simultaneously " there was a sudden and large rise in the horizontal 

 force record " at Kodaikanal, and Mr. Michie Smith's opinion is 

 that " there is no reasonal)le doubt that the solar outburst and the 

 magnetic disturbance were directly connected with each other." 



Here, apparently, are two very similar solar phenomena, but the 

 terrestrial phenomena which are supposed to be associated with them 

 by Dr. Lockyer and Mr. Michie Smith respectively follow, the one 

 after more than a day, the other immediately. 



Mr. Michie Smith's observation seems exactly parallel to the 

 celebrated one made by Carrington in 1851), which has frequently 

 been quoted as evidence of the solar origin of magnetic storms. 



§ 18. The most elaborate investigation hitherto made into the 

 supposed solar origin of magnetic storms is due to Prof. Kr. Birke- 

 land of Christiania, who beheves cathode or analogous rays to be 

 the vehicle by which the solar disturbance is propagated to the earth. 

 He has made numerous experiments with cathode rays in a vacuum 

 tube, which contains a miniature earth or " terella." By means of 

 ■electric currents in wires wound on the terella, a magnetic field is 

 produced similar in type to the earth's field. The slide shows one 

 of Prof. Birkeland's experiments, in which a luminous discharge 

 encii'cles the terella at its magnetic equator. It was apparently 

 this experiment that suggested Prof. Birkeland's explanation of a 

 certain type of magnetic storm which he terms the " equatorial." 

 These " equatorial " disturbances, according to Birkeland, are 

 normally largest in the earth's equatorial regions, where they consist 

 mainly of a change in the horizontal force, but they are also well 

 marked in temperate latitudes. The cause postulated l)y Birkeland 

 is a circular electric current in the plane of the earth's magnetic 

 equator, at a height of several thousand miles in the atmosphere. 

 One criticism on the experiment that does not seem to have been 

 met, is that to make up for the small size of his " terella " compared 

 to the earth Birkeland ought to provide it with a magnetic field 

 10,000 times stronger than he has yet done. This criticism, unfor- 

 tunately, according to the mathematical calculations of Prof. Stormer,t 

 a Norwegian mathematician, seems to be rather a fundamental one. 

 It is not possible, according to Stormer's analysis, for cathode rays 



* Indian Monthly Weather Review, September 1909, p. 90. 



t Archives des Sciences Physiques et Naturelles. Geneva, 1907. 



