64> Cambridge Philosophical Society. 



discussion being given in the Number of this Magazine for December 

 1833. 



The observers of the display of September 17, were Mr. J. Phil- 

 lips, at York ; Mr. Clare, Mr. Hadfield and the author, at or near 

 Manchester; Professor Airy, at Cambridge ; and the Hon. C. Harris, 

 near Gosport. 



The observers of that on October 12, were Professor Sedgwick, at 

 Dent ; Mr. W. L. Wharton, near Guisborough ; Mr. J. Phillips, at 

 York ; Mr. Clare, Mr. Hadfield and the author, at or near Man- 

 chester ; Dr. Robinson, at Armagh ; Professor Airy, at Cambridge ; 

 and the Hon. C. Harris, at Heron Court. 



The observations of the aurora of September 1 7 at Cambridge at 

 8 h 25 m Greenwich time, taken with those at Manchester at 8 h 24 m , 

 give the height of the lower edge of the arch 56 English miles, and 

 of the upper edge 71 miles. 



The observations of another arch, seen from 10 h 49 m to l] h 19 m 

 at York, and from 10 h 49§ m to ll h 4± m near Gosport, give the 

 height of the lower edge 3S9 miles. 



The observations on October 12, at 7 h 56 m at York and at 7 h 54 m 

 at Cambridge, give the height of the upper edge of an arch 72*2 

 miles. 



The observations at Guisborough at 8 h 20 m , and at Heron Court 

 at 8 h 22 m , give the height of the under edge of the arch seen at that 

 time 70*9 miles, and of the upper edge 85'5 miles. 



The observations at Dent at 8 h 55 m , taken with others at Man- 

 chester at 8 h 54 m , give the height of the upper edge of that arch 

 84-97 miles. 



The last arch remained stationary about a quarter of an hour, and 

 therefore the observations are the more valuable ; but combining an 

 observation at Armagh with those at Manchester, the height comes 

 out only 64*47 miles ; and even with the utmost allowable latitude 

 to the deductions from the observations, the height comes out 66*5 

 miles. 



The last arch having been noticed to have risen to a higher alti- 

 tude at the same places, a calculation with the corresponding data 

 gives the height 65*4 miles. 



These last three results are remarkably in accordance with each 

 other, but considerably different from those for other places at nearly 

 the same time ; so that probably the method which was used, of ob- 

 taining a base line by projecting the places of observation upon an 

 intermediate magnetic meridian, is only approximately correct, from 

 the* course of the arch over the earth's surface, rather than geome- 

 trical reasons. 



Another arch was noted by most of the observers from 10 h 34 m 

 to 10 h 45 m . The observations at Dent at 10 h 40 m , and Heron Court 

 at 10 h 37 m , give the height of the upper edge 59'4 miles. 



An observation made by the author on the extent of the arch, on 

 September 17, upon the horizon at 8 h 40| m , and its altitude, for ap- 

 plication to the formula he has given in the Edinburgh Journal of 

 Science, before it was joined with the Philosophical Magazine, for de- 



