Atmospheric Origin of the Aurora, §e. 153 
irreconcilable with the idea, that the needle was disturbed by a 
general change in the magnetism of the earth? | According to 
Capt. Back, auroral beams sometimes seem to attract. ar 
Does not this.seenr 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 vatying 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 dina’ 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 cither 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. 
{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 - 
posed to have- been entered. by the exact sciences. 
facts quite as conclusive as a great parallax : such as pag a 
ous instances where individuals at moderate distances cannot 
recognize the same phases, and some of them not even the’ exist- 
encé of the aurora seen by the others. . In such cases, it may fail 
to be meastired, 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 occasional 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 
Von. XXXV.—No. 1. 20 
