On the Height of the Aurora Borealis. 83 



as mere errors of description, and a due consideration of them 

 is of great importance, as regards the subject in question. In 

 the last case, at midnight, at two places not sixty miles apart, 

 with a perfectly cloudless sky, totally dissimilar appearances 

 were seen. Now, had the appearance at Oxford been similar 

 to that at Blackheath, and the height of the arch, as it ap- 

 peared at each place, been accurately taken, a calculation 

 might have been made from these observations, and the height 

 of tlie aurora been considered as fairly ascertained ; whereas 

 the two arches may have been as totally distinct, and no more 

 to be referred to one point, than the appearances above de- 

 scribed ; consequently, the result could in no way be depended 

 upon, and I cannot conceive that any two observers, at a 

 distance from each other, can ever be certain that they are 

 looking at the same appearance, or at an appearance in one 

 and the same place. 



But the most important point, and one that I believe will 

 fully bear out my opinions, is the difference in the observa- 

 tions of Professor Chevallier at Durham, and Professor Chal- 

 lis at Cambridge, on the position of the centre of the corona, 

 as seen at those places. 



Professor Chevallier says, " But the most brilliant spec- 

 tacle was presented at ten o'clock. At this time the whole 

 of the south-western part of the sky was 'glowing wdth rose- 

 coloured light ; while bright streams arose from all sides, 

 especially from a little south of west to a little east of north, 

 passing beyond the zenitli, and converging in a flickering 

 corona in the same part of the sky as before, the briglit star 

 Mirach (/3 Andromeda;) forming now nearly the central point." 

 Professor Challis says, — " The most remarkable feature 

 of the phenomenon was the distinct convergence of all the 

 streamers towards a single point of the heavens, situated a 

 little to the east of the meridian, and to the south of the 

 zeniths Around this point a corona, or star-like appearance, 

 was formed, the rays of which diverged in every direction 

 from the centre, — leaving a space about the centre free from 

 light, on which I noticed at one time the I'apid formation and 

 disappearance of part of a circular luminous ring. It was 

 easy to fix on the central point. 



