CHAPTER XIII. 

 THE ELECTRICITY OF THE EARTH AND ATMOSPHERE. 



THE AURORA. 



THE relations of the aurora to terrestrial and atmos- 

 pheric electricity present a problem of the deepest inter- 

 est and importance, whose satisfactory solution must 

 render clear many questions now involved in doubt and 

 obscurity. Hence, during the last fifty years, it has 

 been carefully observed, and a number of important facts 

 in regard to it ascertained. The laws which govern it 

 are still far from being understood, and much con- 

 flict of opinion exists in regard to many points; but its 

 electric origin may be regarded as fully established. 



This phenomenon occurs in zones surrounding the 

 northern and southern magnetic poles. And obser- 

 vations have been chiefly confined to its occurrence in 

 the north. The northern aurora is known as the 

 aurora borealis, the southern as the aurora australis, 

 while the term aurora polaris, or simply the aurora, is 

 applied to either. 



In the United States it is usually first seen 

 at from 8 to 10 P. M., though often beginning much 

 later : and it continues from three to four hours. Its 

 occurrence during the day, also, is probable ; though 

 it can only be inferred from coincident effects; the 

 brilliancy of the daylight rendering it invisible. 



