18S 



DESCRIPTION OF A REMARKABLE AURORAL ARCH. 



BY DANIEL KIKKWOOD. 



During the evening of the 7th of April, 1847, the northern sky was 

 illuminated by a brilliant aurora ; the streamers sometimes extending at 

 least sixty degrees above the horizon. These disappeared, however, 

 about 9 o'clock, and shortly after, the auroral light itself partially sub- 

 sided. This was followed, about 10 o'clock, by an extraordinary and 

 magnificent phenomenon — the formation of a white, luminous arch, 

 having a striking resemblance to the tail of a comet, and spanning the 

 heavens from a point about 20° south of east, to another directly oppo- 

 site, or 20° north of west. When first observed, its summit was a 

 few degrees south of the zenith, which position it preserved with 

 the exception, that shortly before its disappearance, which occurred 

 about 11 o'clock, it gradually moved somewhat further southward. — 

 The arch was generally about four or five degrees in breadth, and was 

 observed to be agitated by a rapid motion from the east toward the 

 west. 



As this appearance was undoubtedly of the same nature as the ordi- 

 nary aurora borealis, it furnishes an opportunity of determining the im- 

 portant and much disputed question, whether that meteor is ivithin or 

 beyond the limits of the atmosphere. On this subject, Brande's Ency- 

 clopoedia has the following statements : 



"There is great difficulty in determining the exact height of the au- 

 rora borealis above the earth, and accordingly the opinions given on this 

 subject bv different observers are widely discordant. Mairan supposed 

 the mean height to be 175 French leagues. Bergman says 460 miles, 

 and Euler several thousand miles. From the comparison of a number 

 of observations of an aurora that appeared in March, 1826, made at dif- 

 ferent places in the north of England and south of Scotland, Dr. Dal- 

 ton, in a paper presented to the Royal Society, computed its height to 

 be about 100 miles. But a calculation of this sort, in which it is of 

 necessity supposed that the meteor is seen in exactly the same place by 

 the different observers, is subject to very great uncertainty. The obser- 

 vations of Dr. Richardson, Franklin, Ilood, Parry, and others, seem to 

 prove that the place of the aurora is far within the limits of the atmos- 

 phere, and scarcely above the region of the clouds ; in fact, as the diur- 

 nal rotation of the earth produces no change in its apparent position, it 

 must necessarily partake of that motion, and consequently be regarded 

 as an atmospherical phenomenon. " 



In the present instance, it is evident that the difficulty here referred 

 to does not exitst, and that a few observations made at points considera- 



