of an Aurora Borealis on the Evening of March 21, 1833. 425 



judge how nearly it might be taken as contemporaneous with 

 that of Professor Forbes. This, it will be seen, is necessary 

 to be attended to, for the observations at Armagh and Edin- 

 burgh agree in giving the southward motion as very nearly at 

 the rate of 2° in 5 minutes. The calculation referred to gave 

 195*77 English miles for the height at 8 h 57 m 41 s Greenwich 

 time. This difference of the height, in 13 m 4 s difference of 

 time, is not otherwise than I should have expected, as I believe 

 I have good grounds for the opinion that it will eventually be 

 found that the arches generally descend nearer to the earth as 

 they move southward. 



As the locality, within certain limits, in which the aurora 

 takes place must now be considered, by impartial persons, 

 well determined, as well as the direction both of the beams 

 and the arches, I will now state some points to which ob- 

 servers should pay attention, in order to extend our knowledge 

 still further, and enable us to deduce from the phenomena 

 of the aurora, results which will assist us in the study of the 

 important subject of the earth's magnetism. Amongst the 

 most important points to which an observer can attend, is 

 that of determining, with every accuracy the subject is capable 

 of, the azimuth in which each end of an arch cuts the horizon, 

 and as nearly as possible at the same time its altitude. When 

 this shall have been determined, together with observations 

 at other places sufficient to determine the height by common 

 trigonometry, then we should have sufficient data for dedu- 

 cing the position of the magnetic axis of the earth, continued 

 to the region in which auroral phaenomena occur. We may 

 naturally expect that those anomalies in the magnetic varia- 

 tion, &c, which occur at the earth's surface will disappear at 

 those great altitudes, and especially if they arise from varia- 

 tion in the direction of thermo-electric currents. 



When observations are made with respect to the visible 

 horizon of any place, they should be corrected to the real ho- 

 rizon of that place by the observer, and also the effect of re- 

 fraction should be allowed for. This latter, however, will, of 

 course, be unnecessary when the azimuthal extent of arches, 

 or their altitudes, are determined by a comparison with stars, 

 both being then equally affected, and the places of such stars 

 are easily found with accuracy. 



The late splendid displays on September 1 7th and October 

 12th, have been observed over a great distance of country \ 

 but the aurora has frequently a more useful character for ob- 

 servations than these had, when even it has a much less im- 

 posing appearance. 



I may here state, that in my former papers in the Edinburgh 

 Third Series. Vol. 3. No. 18. Dec. 1833. 3 I 



