68 



the question: Where shall we fmd Ihese electric currents 

 of such a strength that they may keep the large body in 

 this magnetic state? Leaving the explanation of the cau- 

 ses of these currents to a låter occasion, we will only re- 

 mark that they can be found in the inducing power, which 

 the ether in the space exercises on the earth by its rotation 

 on its axis and its revolution around the sun. 



The principal, for our present aim, is that the earth 

 must be regarded as an electromagnet susceptible to sub- 

 mit changes in its magnetic state by electrical currents. 



We leave now the periodic variations out of sight and 

 will direct our attention to the magnetic disturbances and 

 their explanation. 



We have characterized them above as vehement chan- 

 ges in the earths magnetic force, both in its direction and 

 intensity. 



Every explorer in that dominion, especially in the po- 

 lar-regions, has been surprised by this singular phenomenon. 

 Watching a needle, provided with scale and mirror, and 

 having just succeeded to put it in the equilibrium position, 

 he often observes that the needle receives a vehement shock. 

 is flung to the side and begins, as an observer expressed 

 it, a dance, in which at least the regularity is not amongst 

 the characterizing features. When such a dance is perfor- 

 med simultaneously by needles in America, Asia and Eu- 

 ropé, not to speak of the south polar-regions, the pheno- 

 menon obtains a mysterious character, which will puzzle 

 the observer and seems difficult to explain. These changes 

 are howerer contained within certain limits; they will sel- 

 dom surpass 12 per cent of the whole force. 



The causes of that seemingly whimsical phenomenon 

 are to be found in the electrical earth- currents and the elec- 

 tric currents from the atmosphere to which we are now 

 passing. 



