224 Observations upon Auroral and Optical Phenomena. 
observed arches. ‘That part of the arch therefore which was the 
vertex to one observer, would be almost identical with the part which 
was the vertex to the other. Under these circumstances the upper 
parts of the arch, which were centered to Prof. Olmsted’s view upon 
the 4rrow, in N. Dec. 18°, were centered, to my own view, upon 
the star Alpha Lyre in N. Dec. 384°—showing a reduced parallax 
of 192°, 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 upward motion, 
which at the vertex was such as to coincide with Prof. Olmsted’s 
line of vision ; but which, being oblique to my own line of sight, 
would give an apparent southerly motion, This motion was so con- 
siderable as to reduce the final parallax to about 82°, and to increase 
the calculated final height to 1442 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. [had not myself the good fortune to witness 
tS but T have at my disposal the observations made at New Haven 
by Prof. Olmsted,—at Hartford, by Mr. P. W. Ellsworth, a late 
