( 273 ) 



iietic observations ofBitGUN, Hornstein, Müli.kh und Liznak, 

 until 188(5) 25,92 ') 



Ad. Schmidt (magnetic observations at Batavia) 25,87 ^) 



VAN DEU Stok (baronielrical observations at IJatavia and 

 St. Petersburg and magnetic obser\'ations at Prague and 

 St. Petersburg) 25,80 '') 



VON Hezold (thunderstorms in S. Germany) 25,84 •■•) 



Ekholm and Arrhenius (polar lights) 25,929') 



BiGELOw (meteorological and magnetic oltservations in the 



United States of America) 26,68 ') 



The justitication of the choice we make between these different 

 numbers must for the greater part be found in the value of the con- 

 sequences we derive from it. However, there are some good reasons 

 why we should prefer a priori the value oi)tained from investigations 

 on the frequency of polar lights by Ekholm and Arrhenius. For 

 altliough tiie results of otliers (especially those of Ad. .Schmidt and 

 of VAN DER Stok), from a point of \iew of careful and critical reaso- 

 ning, are of no less value than those of Ekholm and Arrhenius, 

 the variations of the barometer and the oscillations of terrestrial 

 magnetism are phenomena of a more complicated nature than polar 

 lights. They are intluenced by local conditions, the distribution of 

 land and w^ater, etc.; because they partly depend on the circulation 

 in the lower layers of the atmosphere. On the other hand it woidd 

 appear that the polar lights take their origin principally in the higher 

 layers and thus, by revealing to us more directly the action of the 

 Sun's radiation, they will propably lead to a sharper determination 

 of the period. 



Whilst the periods of rotation necessarily differ in the various 

 parts of the Sun's mass, there must be somewhere in the plane of 

 the equator a series of points, where the synodical period of rotation 

 is 25,929 days. Through these points we imagine a sphere B, the 

 centre of which is laid in the centre of the Sun, and we make 

 the sphere rotate around the Sun's axis with a constant angular 

 velocity, so as to bring its synodical pei-iod of rotation at circa 

 25,929 days. This sphere represents to us "the rotating Sun", but 

 we must kee[» in mind that with res[)ect to B the various i»arts 

 of the gaseous mass may alter theii- i)Ositi()n. 



1) Ad. Schmidt, Silz. Ber. l<:ais. Akad. d. \V. Wien, Bil. DO, p. 'MO and 1005. 

 ~) Van dek Stok, Verli. Kon. Akad. v. W. Amslerdani, 1890. 

 ■^) Arrhenius, Lehib. d. kosinisclien Pliysik, p. 1 iS. 



^) BiGELOw, Un. States Weather Bmoau Bulletin No. !21, Wasliinglon, l^OS. See 

 also Schuster's criticism, Terrestrial Magnetism III, p. 179. 



