( 296 ) 



the horizontal intensity, collected from the various readings in different 

 parts of the globe between 1841 and J 81)9, will be found in Prof. 

 Fkank Bigelow's "Stndies on the Statics and Kinematics of the atmos- 

 phere in the U. S. of America", p. 56 — 57. 



The figures tally in every respect with the above results. 



5. M(i<iii('tic tlishirlxinri's. 



If now we apply the preceding arguments to the irregular disturb- 

 ances or magnetic storms, their explanation will offer no difticulties. 



We attribute these phenomena to extraordinary differences in 

 density, which may at times be found on each side of the planes 

 of discontinuity in the line connecting the Earth with the Sun. The 

 system of the surfaces of discontinuity moves so rapidly with respect 

 to the Earth, that almost all parts of the illuminated hemisphere are 

 iulluenccd simultaneously by the extraordinary local condition of the 

 radiation field; whereas it is evident that the abnormal illuuiination 

 at the same time may be more intense in some regions of the 

 Earth than 'u\ others'). Consequently magnetic storms will be noticed 

 everywhere almost simultaneously, and their effects will be almost 

 identical in places lying rather close together, whilst, in regions 

 farther distant from each other, they may be entirely different, 

 perhaps (piite oi>posite. 



Ellis has made a study of the annual variation in the frequency 

 of magnetic disturbances and classed them into certain groups. 

 "Strong disturbances", (over I'' in declination and 300 units of the 

 fifth decimal in horizontal force) haye two maxima, one in Api-il and 

 one in September; "weak disturbances" (10' and 50 units) show a 

 maximum in summer and a minimum in winter. The characteristics 

 of periodicity ^4 (p. 289) at once strike us here, and it seems to ns 

 also to afford a satisfactory explanation in this case. Besides we find 

 especially in the curve of the disturbance frequency an argument in 

 favour of the opinion expressed on p. 291 viz. that along planes of 

 discontinuity at 6° or 7^ from the Sun's ecpiator, greater irregularities 

 in the distribution of density will be met with than in the equatorial 

 zones. And in the diurnal jieriod of the disturbances, which in the 

 tropics shows a maximum at midday, we see another argument in 

 support of this assumption. 



After the explanations given under 11, it will cause no surprise 

 to recognize in the magnetic disturbances the periodicity of solar 

 phenomena. There is but an indirect connexion between magnetic 



1) The rapid changes noticed in Hale's abnormal spectra support this view. 



