324 PROFESSOR POTTER, ON THE HEIGHT 



" 9^. 6" Arch in the same place place, Sic." 



The distance of the projections of Armagh and Manchester on the magnetic meridian with 

 variation 25°. 30' is 128.76 miles. 



By a careful examination of the course of the arch as seen at Armagh on a coelestial globe, 

 the altitude of the summit must have been about 60°, and the altitude at Manchester as above 



was 19°. 57'. 



These data give the distance from Armagh 74.25 miles, and the height above the earth's 

 surface 64.47 miles. 



If we took the altitude at Armagh as 59°, and allowed 

 1°. so' for the point of the arch which appeared the highest at 

 Armagh, not corresponding with that which appeared the highest 

 at Manchester, on account of the elevation being so great at 

 Armagh, as shewn by the figure, we should have the height of the 

 arch above the earth's surface 66.5 miles, and the distance from 

 Armagh 78.69 miles. 



From Dr. Robinson's observations at Armagh. 



4iph , jm — Upper edge of arch has risen to Lyra and Capella, and a new arch has risen 

 beneath it, &c." 



From my own observations near Manchester. 



"()'>. 14i"". — n Ursae Majoris in the upper edge of the arch, the height of which by 

 measure = 21°. lO'." 



The altitude of tj Ursae Majoris at Manchester at 9^ l*^"'- was 21°. 6' confirming the altitude 

 I obtained by an instrument made purposely for observing the Aurora; as however there is a 

 discrepancy between the height above the earth's surface deduced from these observations and 

 the previous ones, I will suppose the extreme upper edge had an altitude of 22°, that we may 

 be certain the discrepancy does not arise from an under valuing of the altitude at Manchester, 

 but must be sought in other causes. 



From Dr. Robinson's observation, the altitude of the upper edge must have been 71-i°, 

 from which we may deduct 1^° for parallactic effect. 



These data give the distance from Armagh 69.59 miles, and the height above the earth's 

 surface 65.4 miles. 



These results are remarkably in accordance with the others for the same places, but 

 considerably different from the calculations for other places for nearly the same time ; so that 

 probably the method of projecting places of which the magnetic meridians are so distant as 

 Armagh and Manchester upon an intermediate magnetic meridian to obtain a base line, is 

 only approximately correct, from the course of the arch over the earth's surface, rather than 

 for geometrical reasons. 



Another arch was observed from lO*. 34"°. to lo"". 45"". at Dent, Guisborough, York, Man- 

 chester, and Heron Court. 



From Professor Sedgwick's observations we have, 



"lO*". 40™. — The bright space arranges itself into an arch, commencing nearly N., passing 

 through tj Ursae Major. ; about 25° high near the magnetic meridian (measured only by a 

 geological clinometer)." 



From the Hon. Charles Harris's observation. 



" lO**. S"" A low arch again formed, its base scarcely 5° above the horizon, extending to 



about 7°, &c." 



The distance of the projections of Dent and Heron Court on the magnetic meridian with 

 variation 25°, is 232.52 miles. 



