Atmospheric Origin of the Aurora^ c^c. 153 



irreconcilable with the idea, that the needle was distorbed by a 

 general change in the nicignetisni of the earthi' According to 

 Capt. Back, auroral beams sometimes seem to attract each other. 

 Does not this seem like atmospheric magnetism ? 



There appears to be no reason to believe that the aurora is at 

 an invariable elevation. Calculations founded on observed alti- 

 tudes, have given results varying from a few miles to several hun- ; 

 dred. This discrepancy may be explained, partly by an actual 

 difference of height, and partly by mistakes as to the identity of 

 arches when several have been presented to different observers. 

 In the latter case, a mistake will usually lead to an exaggeration, 

 rather than to an underrating of the elevation. Suppose two ob- 

 servers, near the same meridian, but in different latitudes, to take 

 the altitudes of two arches situated north of their respective ob- 

 servers, and at so small an elevation, that the southern arch is be- 

 low the horizon of the northern observer, and the northern- arch- 

 below the horizon of the southern observer. Only one being seen 

 by each, they are liable to be presumed identical ; and the great 

 altitude of the northern as compared with the southern arch, 

 would lead the mathematician to refer the imaginary arch — con- 

 sidered as one — to an elevation greater than the actual elevation, 

 of either of the real arches. There is evidence that the above 

 case is more than a supposable one, and that similar mistakes have 

 actually occurred. The opposite error, an exaggeration of the 

 parallax, would, from the nature of the case, more rarely occur. 

 I have stated the first in a plain way, that those who are little 

 conversant with the subject may not be deterred from examining 

 the physical evidence of a theory of the^aurora, by a caveat sup- 

 posed to have been entered by the exact sciences. There are 

 facts quite as conclusive as a great parallax : such as the numer- 

 ous instances where individuals at moderate distances cannot 

 recognize the same phases, and some of them not even the exist- 

 ence of the aurora seen by the others. In such cases, it may fail 

 to be measured, simply because it is too low. 



The views which I have taken of the aurora, whilst they do 

 not require us to discredit those numerous proofs, both physical 

 and mathematical, of its occasionar situation in the inferior atmos- 

 pheric strata, at the same time, allow, or even require us to refer 

 it in most instances to elevations above (and in the lower lati- 

 tudes far above) the regions of the highest proper clouds, and 



Vol. XXXV.— No. 1. 20 . 



