218 



AURORA BOREALIS. 



ber of observations we should infer that whenever an aurora is seen at Ilobarton, 

 where the magnetic dip is — 70°, an aurora occurs at some place in the north- 

 ern hemisphere as far south as where the magnetic dip does not mucli exceed 

 75° ; in other words, an unusual auroral display in the southern liemisplwre is>-^ 

 always accnmpunied hy an unusual display in the northern hemispJterc ; or an 

 exhii)ition of auroral light about one magnetic pole of the earth is uniformly 

 attended by a simultaneous exhibition of auroral light about the opposite mag- 

 netic pole.* 



Height of the aurora. — The great auroral exhibition of August and Septem- 

 ber, 1S59, was very carefully observed at a large number of stations, and these 

 observations afford the materials for determining the height of the aurora above 

 the earth's surface. The southern limit of these auroral displays was not the 

 same upon all meridians. la North America, the aurora • of August 28 

 appeared in the zenith as flu* south as latitude 36° 40' ; and it attracted general 

 attention as far south as latitude 18°. In Central Europe this aurora extended 

 to the zenith of places as far south as about latitude 45°. It was brilliant at 

 Rome in latitude 42°, but was not noticed at Athens in latitude 38° ; neither 

 was it seen in western Asia in latitude 40°. 



In North America, the aurora of September 2 appeared in the zenith at 

 places as far south as latitude 22° 30', and attracted general attention in latitude 

 12° ; and if the sky had been clear, some traces of the aurora might probably 

 have been detected even at the equator. In Euroj)e this aurora was noticed at 

 Athens in latitude 38°. Both of these auroras conformed to the general law of 

 auroral distribution already explained, the region of greatest auroral action be- 

 ing in America, about 15° further south than in eastern Europe. 



At the most southern stations where these auroras were observed, the light 

 rose only a ii^.w degrees above the northern horizon; at more northern stations 

 the aurora rose higher in the heavens ; at certain stations it just attained the 

 zenith; at stations further north the aurora covered the entire northern heavens, 

 as well as a portion of the southern ; and at places further north the entire visi- 

 ble heavens, from the northern to the southern horizon, were overspread with the 

 auroral light. The following table presents a summary of a few of the most 

 definite observations on the aurora of Attgust 28, 1859, at about 8'^ 42'" p. m., 

 New Haven time : 



Tablr I. 



The following table presents a summary of observations of the same aurora, 

 made at the same hour, at places where the auroral light covered the entire 

 northern heavens, as well as a portion of the southern : 



* American Journal of Science, n. s., v. 32, pp. 11-14. 



