AURORA BOREALIS. 



245 



being an average diminution of 2.7' annually. Observations made in otlier parts 

 of Europe give similar results. In the northeastern part of the United States 

 the dip attained its minimum about 1843, and is now slowly increasing.* This 

 change of dip implies a change of distance fiomthe magnetic pole, and probably 

 a corresponding change in the frequency of auroral displays. We do not know 

 what is the cause of this motion of the magnetic poles, and whether the motion 

 is periodical. It has been conjectured to be the result of great geological changes 

 going on in the crust of the earth. But whatever may be its origin, this cause 

 must modify and complicate the influence of extra-terrestrial bodies upon the 

 earth's magnetism. 



21. Whi/ do great auroral exJdhitions take 'place simultaneously in both 

 hemispheres of the earth ? — During the years 1841-'8, as has been mentioned 

 before, there were recorded at Ilobarton, latitude 42° 52' south, 34 auroras. In 

 29 of these cases an aurora was recorded either in Ilurope or America, and in 

 the 5 remaining cases there was recorded an unusual disturbance of the mag- 

 netic needle, indicating the existence of an aurora at no very remote station- 

 The great auroral displays of August 29 and September 2, lb59, were among 

 the most remarkable ever recorded in the northern hemisphere. Both of them 

 were conspicuous at Cuba, where but^o^^r auroras had ever before been recorded. 

 The aurora of September 2 was seen in latitude 12° north, where there was no 

 tradition that such a phenomenon had ever been seen before. In the southern 

 hemisphere an aurora occurred simultaneously, and was almost equally re- 

 markable for its brilliancy as well as its geographical extent. 



We cannot explain the great auroral displays in the northern hemisphere by 

 supposing that the electricity of the atmosphere is temporarily diverted from one 

 hemisphere and concentrated in the other. Such an idea is entirely refuted by 

 observations of the mean range of the magnetic needle, which exhibits its maxima 

 simultaneously in both hemispheres. This is shown by the following table, 

 which exhibits the amount of the mean diurnal variation at Prague, latitude 

 50° 5' north ; Toronto, latitude 43° 40' north ; and Hobarton, latitude 42° 52' 

 south. 



The great disturbances of the earth's magnetism, therefore, take place sim- 

 ultaneously in both hemispheres, and they exhibit the same periods. Now, we 

 cannot suppose that the absolute amount of electricity for the entire globe, as 

 developed by evaporation from the water of the ocean, should undergo any such 

 periodical variation, for we know that the mean temperature of the earth's 

 surface does not change sensibly from one year to another, and hence we seem 

 compelled to ascribe these great auroral displays in no small degree to the direct 

 action of the sun, through the agency, perhaps, of its magnetism, or of the 



* U. S. Coast Survej Report 1856, p. 244. 



