Mr Potter on the Aurora Boreal if. 31 



and west to east, alternately. At 20' past 8 the vertex of the 

 arch attained an elevation of about 15° above the north-western 

 horizon, from which it did not deviate while it continued visi- 

 ble. The arch was about 5° broad, and was bisected by the 

 magnetic meridian. Although the sky continued perfectly 

 cloudless prior to 10 o'clock, this was the only arch seen, and 

 there was not the slightest appearance of an aurora in any 

 other part of the heavens." This arch was observed also by 

 Dr Burney of Gosport, to whom the world is indebted for so 

 many valuable meteorological observations. He says, in the 

 Philosophical Magazine, that at the time he estimated the alti- 

 tude at 8° ; but having written to him to know if he meant 

 this for the upper or the under edge, I am favoured by his 

 reply, stating, that, on further consideration, he thinks 7° for 

 the upper edge might be nearer its altitude, as he only esti- 

 mated it hastily to be about 8° at half-past 8 o'clock. 



Though these low arches are not the most desirable for 

 trigonometrical computations, yet this has the advantage of 

 having been for a long time stationary, and Gosport and Man- 

 chester are very nearly on the same magnetic meridian. Where 

 we do not estimate nearer than whole degrees, we may disre- 

 gard the effect of refraction ; and, calling the chord of the arc 

 of the meridian, between the places of observation, 193 miles, 

 and adding 1° 23' to the altitude at the former place, and sub- 

 tracting the same quantity from that at the latter, on account 

 of the effect of the curvature of the earth, we find the distance 

 of the highest point of the arch from Gosport to have been 

 498 miles if its altitude were 7°, or 615 miles if 8°, and its 

 elevation above the surface of the earth 99 miles in the former 

 case, and 134 in the latter. 



The arch which was seen on the 29th September 1828, at- 

 tained at Gosport an elevation of 70°, or about 20° above a Po- 

 laris. With me it was visible south of the zenith, and was in 

 quick motion during the whole time I saw it. It passed over 

 /3 Pegasi at about 15' past 8 o'clock, and, when its northern 

 edge was about 5° south of that star, it was become hardly 

 perceptible. Its breadth was very nearly 5°. Hence its low- 

 est apparent altitude may be taken at 54°, or, if Dr Burney 

 only gave its altitude of 70°, whilst still of considerable bright- 

 ness, we must allow an altitude of G0° for its southern ed<re at 



