( 294) 



A clear and concise exposition of the variations of terrestrial 

 niapietism lias been recently published l)y Ciihee ^). The "mean 

 nionlhly ran,ue" of a magnetic rpiantity is according to his definition: 

 "the difference l>et\vcen the greatest and least of the twenty-fonr 

 hourly \alues in the mean diurnal inequality for the month in 

 question, based on the five quiet days selected for the month by the 

 Astronomer Royal". If (his range be repi-esented by 11 and if S 

 means the nund)er by which Wolfer expresses the sun-spot frequency, 

 Ave have, according to Chkee, the following relation : 



R = „ ^I, S. 



His investigation extends over the 1 J -year period 181)0 — 1900. 

 He divides the twelve months into tliree seasons: November to 

 Februarv : winler ; March, April, Sc[)tember and ()clol)er : spriiig 

 and autuniJi ; May to August: summer, and linds the following 

 values for a and A. 



a marks the variability according to the seasons, irrespective of 

 the spot-period. 



h shows in ho^v far the influence of the spot-period depends on 

 the seasons. .S' in the period under consideratioji oscillated between 

 0,3 and J 29,2, its mean value being 41,7. 



From the poiiit of view of our theory these figures })rove that : 



a in each element shows a mininmm in winter and a maximum in 

 summer; this we explain by the greater intensity of the Sun's irradi- 

 ation in summer increasing both the general circulation and the 

 generation of electricity. But the table shows also that the values of 

 a in spring and autumn are invariably greater than the mean \alue 

 for the whole year: this points to superposed mchcinui duriiKj flu' 

 e<julno,i'('s, and this we ascribe to the way in which the variability 

 of illumination is dependent on the positioti of the Earth's axis with 

 respect to the surfaces of disconlinuity, (periodicity A p. 289). 



1) Ghree, Preliininaiy Note on the Relationship.s behveen Sun-spot? and Ter- 

 restrial Magnetism. Proc. P.oy. Soc. 71, p. 221 224, 19U3. 



