224 Observations upon Auroral and Optical Phenomena. 



observed arches. That part of the arch therefore which was the 

 vertex to on observer, would be almost identical with the part which 

 was the vert. " to the other. Under these circumstances the upper 

 parts of the ai which were centered to Prof. Olmsted's view upon 

 the Arrow, in iV. Dec. 18°, were centered, to my own view, upon 

 the star Alpha Lyr(je in N. Dec. 38^° — showing a reduced parallax 

 of 19|°, in the first position, and a height of sixty miles above the 

 earth's surface. The parallax however appears to have been con- 

 tinually diminishing ; for the vertex as seen by Prof. Olmsted was 

 stationary, while the same, as seen by me, was constantly and pro- 

 gressively, although rather irregularly changing southward. This at 

 first view perplexing circumstance involves no difficulty, except that 

 it compels us to suppose that the arch had a rapid upivard motion, 

 which at the vertex was such as to coincide with Prof. Olmsted's 

 line of vision ; but which, being oblique to ray own line of sight, 

 would give an apparent southerly motion. This motion was so con- 

 siderable as to reduce the final parallax to about 8|°, and to increase 

 the calculated final height to 1441 miles. In computing the latter 

 height, the southern observer's change of place by the motion of the 

 boat has been estimated ; — and the base accordingly assumed at 25 

 miles S. 23° W. from New Haven. The results of these calcula- 

 tions are to be taken only as near approximations, in consequence of 

 the impossibility of fixing exactly my own situation on Long Island 

 Sound. They are, however, doubtless, near the truth, and are com- 

 plete for the object of demonstrating the great height of the region 

 in which this auroral arch had its formation, and its changes. This 

 arch was narrow, — not more than three degrees in apparent breadth ; 

 yet that breadth cannot have been less, in absolute distance, than an 

 equal number of miles. It was visible at the extremities not quite 

 down to the horizon ; yet the part which was in view to either ob- 

 server, cannot be estimated in the lowest position of the arch at less 

 than seven hundred miles, and in its highest position it must have 

 equalled eleven hundred. 



Auroral Arch of May, 1836. 



This appeared in the evening of May 8th. It has been mentioned 

 by Prof. Olmsted in his memoir, published in the last No. of the 

 Journal of Science. I had not myself the good fortune to witness 

 it ; but I have at my disposal the observations made at New Haven 

 by Prof. Olmsted,— at Hartford, by Mr. P. W. Ellsworth, a late 



