Sun-Heat. 43 



has received the regard due to a mere nega- 

 tive though brilliant exhibition, whereas the 

 character, extent, and significance of its mani- 

 festations should have caused it to be greeted, 

 and studied, as the index of the operation of 

 very positive cosmical functions. 



HUMBOLDT regarded this process as "the 

 restoration of a disturbed equilibrium ; " and 

 so indeed it is, but it is an equilibrium, not 

 simply as between the earth, and atmos- 

 phere. Various observers have estimated 

 the altitude to which the aurora sometimes 

 reaches, at from 80 to 265 miles. The fact 

 that the bulk of the atmosphere reaches but 

 three miles above the earth's surface, forbids 

 it to be regarded as purely a terrestro-atmos- 

 pheric phenomenon. 



While viewing the more striking and bril- 

 liant exhibitions of the aurora, the more un- 

 demonstrative and by far the most impor- 

 tant and vital operations have been disre- 

 garded. The former may not be observed, 

 except occasionally, and fitfully, can only be 

 present when favoring meteorological condi- 

 tions admit of its disclosure. The latter, 

 more unobtrusive and even invisible to the 

 naked eye, are incessantly, and at all seasons, 



