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



milky light, four degrees wide, and considerably wider and 

 brighter at last towards the western extremity. At that part 

 the upper margin descended abruptly to meet the lower mar- 

 gin at the horizon, the lower margin itself presenting an 

 unbroken and even curve from horizon to horizon. At 8h. 

 10m. p.m. the brightest or medial line of the arch (which was 

 very near the lower margin) passed across the stars k Oygni, 

 and \, it Andromedce, from a point in the east horizon below 

 Corona, to a point in the west horizon below the head of 

 Aries, where the lower margin was met by the upper margin 

 descending, from an altitude of five or six degrees, perpen- 

 dicularly to the horizon. The upper margin was diffuse at 

 all parts excepting near the western extremity, where the 

 light was most intense, contrasting strangely with the pale, 

 narrow riband of white light stretched out to the horizon in 

 the east. In this position the arch continued stationary from 

 7h. 50m. p.m., to 8h. 20m. p.m., or nearly so ; but at 8h. 25m. 

 p.m. it was observed that the western extremity had risen 

 five or six degrees from the horizon, the eastern extremity at 

 the same time remaining unmoved, and gradually growing- 

 fainter. The medial or brightest line of the arch now passed 

 through the stars i Her cutis, 8 Oephei, j3 Andromedce, and a 

 point as far from /3 Arietis, as /3 from a Arietis. 



By comparing these observations with observations of a 

 similar nature recorded elsewhere, some idea of the height of 

 the arch above the earth may be obtained. At Burton-on- Trent 

 the luminous arc was first perceived at 7h. 50m., at an altitude 

 of 45° above the north horizon, and at the end of fifteen 

 minutes the altitude had increased to 60°. This is an indica- 

 tion, which cannot be mistaken, of the drift of the phenomenon 

 (at one time of its appearance) bodily towards the south. 

 At one time of its visibility it was nearly vertical over Win- 

 dermere, and at another time nearly vertical over Manchester. 

 Between these places the southward drift must have taken 

 place. It was succeeded by an apparent elevation of the 

 western extremity of the arch at Hawkhurst, and by a revo- 

 lution of the arch, observed at all places, by which the western 

 extremity was brought nearer to Hawkhurst, but the eastern 

 extremity retreated towards the north. (See Fig. 1.) 

 i Between 8h. and 8h. 30m. p.m. the arch passed over Winder- 

 mere, and extended in length from the tail-stars of Ursa Major 

 in the east, to between the Pleiades and Venus in the west. 

 Comparing with this the first observation at Hawkhurst, where 

 for a long time the arch was nearly stationary, it appears that 

 at this time the light floated over Sea-fell at a height of 105 

 miles, and over Durham at a height of 110 miles, declining to 

 a height of 81 miles over the North Sea, at one extremity, and 



