THE AURORA. 199 



electrified stratum of the atmosphere surrounding the 

 polar area, like a circular belt. 



The weight of evidence is now in favor of the low 

 altitude ; sixty-nine miles above the surface being con- 

 sidered a fair estimate. But strict accuracy is not 

 attainable ; since it is impossible for any two observers, 

 at opposite ends of a base line of sufficient length, to 

 fix with certainty on the same point, so as to make an 

 angular measurement. But we can estimate the prob- 

 able height at which atmospheric resistance would be 

 sufficiently reduced to produce the auroral phenomena ; 

 and we have already seen that this plane of least 

 resistance must lie between the dense strata below 

 and the region of high vacuum above; both of which 

 oppose electric movement. Hence the height, given 

 above, may be approximately correct ; and yet subject, 

 doubtless, to variation, resulting from difference of 

 atmospheric pressure ; low pressure diminishing resist- 

 ance, and depressing the auroral plane, and high pres- 

 sure producing the opposite effect. 



GEOGRAPHICAL POSITION OF THE AURORA. Ob- 

 servation shows that the aurora is confined to com- 

 paratively narrow belts. It is never seen at the 

 equator, and is rarely visible in the northern hemi- 

 sphere south of latitude 40: while in higher northern 

 latitudes, it is seen to the south of the observer; and 

 decreases in frequency and brilliancy, assuming appar- 

 ently a more southerly position, as the observer moves 

 farther north. 



In Fig. 64, we have a chart, giving the results of 

 observations made in the northern hemisphere, by dif- 

 ferent European observers ; which shows that this 

 auroral belt is about 30 in width. Its southern limit, 



