322 PROFESSOR POTTER, ON THE HEIGHT 



From Mr. Clare's observations at Manchester. " 8^. 24™. — The arch 7° broad, includes 

 Dubhe, Arcturus, and Capella, so that Capella is on the extreme upper edge ; Dubhe rather 

 above the middle of the breadth, and Arcturus rather below the middle, centre of the arch a 

 little E. of S Ursse Majoris. Extent of the arch 130°." 



Now the altitude of j3 Ursae Majoris at 8^25". was 24°. 17' and | x dist of a and /3 = 8°. 4', 

 therefore altitude of summit of arch = 16°. 13' at Cambridge. 



The altitude of a Ursse Majoris (Dubhe) at Manchester at 8*. 24™. was 31°. 14', and azimuth 

 22°. 34' N. towards W. about 2j°. from the magnetic meridian. Therefore the altitude of the 

 summit of the arch = 31°. 14' + 3°. 15' = 34°. 29' nearly. 



The distance of Cambridge and Manchester projected on the magnetic meridian whose 

 variation is 24°. 30' is 119.42 English miles. 



These data give the distance of the arch from Manchester 123.27 miles, and the height 

 above the earth's surface, of the upper edge, 71 miles. The breadth subtending 7° at Manchester, 

 we find it to be 15 miles. Therefore the height of the lower edge was 56 miles. 



The above arch having disappeared, and the Streamers and Auroral light having diminished, 

 the appearances were subject to slight changes until lO*. 49^™.; when another arch was seen at 

 York by Mr. Phillips, and near Gosport by the Hon. C. Harris. 



From Mr. J. Phillips's observations at York. 



" 10^ 40™ 1 



I A low faint arch stationary, its upper edge nearly reaching to t/ and 7 Ursae 



.° „ I Majoris; its vertex under Mizar (alt. about 18° in the middle)." 

 ir. 19™.) •' 



In the Yorkshire Gazette for 21st September, 1833, Mr. Phillips states its breadth to be 4°; 



therefore the altitude of the under edge was 16°. 



From the Hon. C. Harris's observations, at 1 mile W.N.W. of Gosport. 



" 10** 40—™ 1 <j 



8 1^ Arch from N.W. to N.N.E. Its vertex under ^ Urs* Majoris, and the 



ii 4.11' i^^S^ °^ ''^ ^^^^ ^*^^ ^^y between that star and the horizon 



Now the altitude of ^ Ursas Majoris at Gosport at lo"". 57™. was 21°. 32', and therefore 

 the altitude of the lower edge was 10°. 46'. 



The distance of York and Gosport projected on the magnetic meridian whose variation is 

 24°. 30' is 197.66 miles. 



These data give the distance from York 1011.53 miles, and the height above the earth's 

 surface 389 miles. 



In the Conspectus for the Aurora of October 12th, we have from Mr. Phillip's observations 

 at York. 



" 7''. 56™ The summit of the arch was now 3° below the stars /3 and y Ursse Maj. &c. 



" 7''. 57™ Suddenly it appeared double, in consequence of the production of a very narrow 



faint arch above that seen before, and separated from it by a dark band. 



" 7''. 58™. — This upper arch rose, so as to include j3 and y Urs» Maj., in its midddle. 



" S**. 2™. It had vanished away, after rising still higher." 



From Professor Airy's observations at Cambridge. 



" 7*". 54™. The upper boundary of the bright cloud was extremely sharp ; it began to the 



left of Arcturus, passed a very little above Arcturus, below y Ursse Maj. at exactly half the 

 elevation of -y UrsK Maj. (which was its highest point) and terminated E. of the N. at about 

 half the azimuth of /3 Aurigae. &c. 



" 7"". 59°'. A black line was discoverable very near the upper boundary and parallel to it. 



The upper part rose and the lower fell a little, thus widening the black line. About Arcturus 

 the upper part rose most. 



" 8*'. 2™. — The upper part after rising considerably had wholly disappeared, &c." 



