Height of the Auroral Arch of March 20, 1865. 381 



at the other extremity to a height of 77 miles over Ireland, a 

 short distance inland from Louth, upon the Irish coast. (See 

 Fig. 2.) 



At 8h. 20m. p.m. the arch was nearly vertical at Man- 

 chester, extending from between e Tauri and the Pleiades, 

 across Capella, and across a Ursce Majoris, to e Bootis. As, 

 soon after this time, the position of the arch underwent a 

 change, by revolution in a direction contrary to the sun or to 

 the hands of a watch, it may be supposed that at the time of 

 the second observation at Hawkhurst (8h. 25m. p.m.) the 

 apparent position of the medial line at Manchester had ad- 

 vanced to e Tauri in one direction, and three or four degrees 

 north of e Bootis in the other. The second observation at 

 Hawkhurst, compared with that at Manchester, gives nearly 

 the same height as before — namely, the height of the centre 

 108 miles over Preston, in Lancashire, and the height of the 

 two extremities, 75 miles over the North Sea, and 90 miles 

 over the Irish Channel between Milford Haven and Queens- 

 town. (See Fig. 2.) 



The apparent flexure of the luminous arc at Hawkhurst, 

 where it was seen in profile, was greater than what would 

 arise from exact conformity of the arch to the curvature of the 

 earth. This circumstance of a greater distance from the earth 

 at the centre of the arch than at the two extremities, might 

 lead to a different view of the constitution of the arch from that 

 already advanced from the movement of the magnetic needle. 

 It might, for example, be inferred that the arch was occa- 

 sioned by a current of galvanic electricity flowing across 

 the magnetic meridian, from the atmosphere of Denmark to 

 the atmosphere of Ireland, or the reverse (see Fig. 2), and 

 seeking the line of least resistance by rising to a summit in 

 the atmosphere ; the small change of vertical force compared 

 with the great change of horizontal force, seen in the curves 

 of the magnetometers (Fig. 1), appears to favour this con- 

 clusion. To establish the result of a greater height at the 

 centre than at the two extremities as a general condition of 

 their form, would require repeated observations, and these 

 arcs are, unfortunately, numbered among the rarer features of 

 the Aurora Borealis. 



The following heights of three auroral arches, calculated 

 by Dr. Dalton, are taken from the Philosophical Transactions 

 for 1828, where they were pointed out to me since writing 

 the above description. 

 (1). 1819, October 17th. Height between 100 and 102 miles, 



observed at Keswick and Gosport. 

 (2). 1826, March 29th. Height between 100 and 110 miles, 

 observed at Jedburgh and Warrington. ' 



