24 Mr Potter on the Aurora Borealis. 



Amongst our countrymen, Mr Cavendish had, by trigono- 

 metrical measurement, computed the height of an arch to be 

 between 52 and 71 miles, and afterwards Mr Dalton, by the 

 same method, on a base of 22 miles, found the height of an 

 arch to be about 150 miles ; see his Meteorological Essays, 

 page 175 ; and from the observations of others, he has since, 

 see Phil. Trans, for 1828, brought out the height of another 

 arch to be at least 100 miles. These computations would un- 

 doubtedly place the meteor at a distance from the earth at 

 which the atmosphere, if it reaches it at all, must be of the 

 greatest rarity, and where the aurora must be almost beyond 

 the immediate effect of the ordinary changes near the surface. 



An opinion has been adopted by Lieut. Hood and Dr Ri- 

 chardson, from observations made during the northern over- 

 land expedition, that the meteor occurs in the region only just 

 above the clouds, and the same view has been supported by 

 Mr Farquharson of Aberdeenshire, — see his papers in the 

 Phil. Trans, for 1829 and 1830. These observers have made 

 no sufficient trigonometrical measurements to give even a plau- 

 sibility to their views ; they seem to have been governed in 

 their opinions by hypothetical assumptions of the aurora being 

 an electrical phenomenon, depending on the condensation of 

 aqueous vapour ; instead of first seeking correctly its height, 

 and then forming their theory. 



It is so easy a thing to any person who sees an aurora which 

 passes nearly over his head, to convince himself, even without 

 either instruments or possession of the ordinary mathematical 

 knowledge, that it is more than a few miles from him ; that I 

 consider it a reflection on any one's capacity and judgment, who, 

 having once seen it so situated, comes to the conclusion that it 

 is only just above the region of the clouds. And of those who 

 have not observed the aurora, and would yet be anxious to 

 support the same opinion, I should say unhesitatingly, that 

 they are either ignorant of what has been done on the subject, 

 or entertain an unphilosophical bias, which prevent their exa- 

 mining it correctly. 



I have myself watched many Aurora Boreales, and had the 

 good fortune to observe the one on the evening of the 29th 

 September 1828, of which I left a memorandum of particulars 

 with Mr Dalton, the next morning. Accounts of the same 



