Mr Potter on the Aurora Borealis. 25 



arch have been given in the Philosophical Magazine, by Dr 

 Burney of Gosport, Davies Gilbert, Esq., Captain Kater, and 

 Mr Utting. And three gentlemen of my acquaintance, who, 

 having arrived by the steam-packet, were proceeding from King- 

 stown to Dublin outside the coach, were struck with the bril- 

 liancy of the arch, which passed almost exactly over their heads. 

 And it is rather an unaccountable circumstance to those who 

 adopt Lieut. Hood's and Dr Richardson's views, that the 

 whole of the observers speak of seeing only one arch. 



To meet the objection, that the observers at different places 

 are generally viewing different arches, I, a short time since, 

 caught the idea, that, if we knew the direction which these 

 arches, (the only part of the phenomena which afford sufficient 

 data for determining the height in the first instance with any 

 degree of exactness,) take over the surface of the earth, then 

 when they are at different elevations, their apparent curvature 

 cannot be the same for every different one ; but on the assump- 

 tion that we have found the law of their true curvature, their 

 height and distance might be determined from their apparent 

 one. On further considering the subject, I found that, if the 

 law of the magnetic variation were exactly ascertained, and 

 could be reduced to a general formula, then the height of an 

 arch must be determined accurately from its apparent curva- 

 ture when it was symmetrical ; or at an even distance from the 

 earth in every part ; the only reason we can give for the very 

 symmetrical appearance they frequently take ; and it is univer- 

 sally allowed that they then are found at right angles to the 

 magnetic meridian. 



With this basis we may now examine the different theories 

 which have been advanced, and compare them with trigono- 

 metrical computations. Lieut. Hood, Dr Richardson, and 

 Mr Farquharson consider the whole phenomena as at low ele- 

 vation ; then, as small portions of the arches on that view 

 could only be seen, and being at right angles to the magnetic 

 meridian, they would not, in the whole of their visible extent, 

 differ sensibly from a great circle ; and Captain Kater says he 

 traced the one of the 29th September 1828, to coincide nearly 

 with a great circle. The problem of determining the height 

 of the aurora from one observation, by considering it as an arc 



