( 2Ï>7 ) 



storms and siiii-s|)o(s, faculae, |)r()niiiioiicos. llolli kinds of pheno- 

 mciia depend on tlie presence of sharply defined sni-faees of disoon- 

 linnily, Imt llie appearance of the solar plieiiomena is more parlicularlv 

 determined by the direction and dix'erjiency of the rays of liuiit in 

 the \icinity of the ;S'/^//, whereas the terrestrial (hstni-Itances rather 

 (le|»en(l upoii the divergency of these lays neai-er the AWy7//. Tlierefore 

 it may often happen that magnetic storms coincide with tlic 

 appearance of large snn-si)ots, or facnlae, or prominences, hut this 

 is not an indispensable condition. 



Accoi'ding to Lockykk "gi-eat" magnetic storms ai'C synchronons 

 with maxima of prominence fretpiency near the poles of the Snn, 

 whilst the cnrve for the mean variabilil^' of terrestrial magnetism 

 is almost an exact reprotbiction of the cur\e for the ])i'ominence 

 freqnency in e([natorial regions ^). 



This fact may be explained as follows. The a})i)eai'ance of promin- 

 ences near tlie poles is the resnlt of the optical elfects of ])arts of 

 snrfaces of discontinuity ^^hich are strongly inclined on the i)lane 

 of the eqnator. We may assume that similar parts will also produce 

 irregnlarities in the radiation field, at the site where the Earth is situated, 

 and that the structure of these irregularities will not be parallel to the 

 principal structure, i. e. to the solar equator. In mo\'ing along the 

 Earth they must give rise to stronger and more evanescent distur- 

 bances in the terrestrial magnetism than the irregularities corre- 

 si)on(ling to the normal lamellar structure, which makes but very 

 small angles with the ecliptic. 



G. The aiinudl variation in the daihj oseiUatiinis 

 of atmosplteric pressure. 



The polar lights and the variations of terrestrial magnetism are 

 prijicipally governed by conditions in the higher layers of the atmos- 

 phere and l)ut little by those on the surface of the Earth. The 

 barometric pressure, the tem])ei-ature, the rainfall, the direction of 

 the wind and all meteorological phenomena which accomi)any them, are 

 to a considerable extent influenced by the distribution of laud and 

 water. Among local influences, cosmic action therefore, will not be 

 prominent in these latter phenomena. 



In the higher layers of the atmosphere the matter becomes much 

 simpler. A short time ago Bigelow ") has called attention to the fact that 



1) LocKYER, C. R. 135, p. 361—365, (1902); Proc. Roy. Soc. 71, (1903). 

 ~) Bigelow, Studies on the Meteorological Effects in the Un. States ol the Solar 

 and Teriesliial Physical Processes. Wliealher Bureau No. 200, Waahiugtou 11)03. 



