Effect of an Aurora Borcalis on. the Magnetic JVeedle. 113 



Art. X. — Observations on the disturbance in the direction of the 

 Horizontal JVeedle. — 1. During the JJurora Borealis, visible at 

 Philadelphia, on the Mth of May, 1833; 2. During that of Ju- 

 ly 10, 1833; by A. D. Bache, Professor of Natural Philosophy 

 and Chemistry in the University of Pennsylvania. 



1. Aurora of May 17, 1833. 



Communicated to the committee of publications of the Franklin Institute. 



Gentlemen — Circumstances having prevented me from witnessing 

 more than a very small part of the unusually brilliant aurora which 

 was visible on the evening of the 17th of May last, I am indebted for 

 the following description of it to my friend J. P. Espy, Esq., who 

 has kindly furnished it to me from his journal. 



" On the 17th of May, 1833, the temperature of the air being 68°, 

 and the dew point 66°, a brilliant aurora appeared in the north, about 

 twenty or thirty degrees above the hoiizon, and extending about thir- 

 ty or forty degrees on each side of the north point. 



" I first saw it a few minutes after nine o'clock, when it was bright- 

 er than it appeared afterwards. Streamers, not in motion, were dis- 

 tinctly visible, rising from a dense light below, which seemed to rest 

 on dark clouds underneath, reaching the horizon. All the rest of the 

 sky was clear, and had been so all the afternoon. In a {qw minutes 

 the streamers disappeared, clouds, which suddenly formed, seeming 

 to take their places, the northern lights still appearing nearly the 

 same, only interrupted in part by a greater number of clouds. I con- 

 tinued to observe the aurora with intervals of but ^qw minutes, and 

 at nearly ten o'clock I discovered that a very brilliant arch had been 

 formed, passing through the zenith, and terminated by the horizon, 

 about twenty degrees south of east, and the same number north of 

 west. 



" This arch was much denser, brighter, and narrower, near the 

 horizon than in the zenith. It passed gradually towards the south, 

 and disappeared, at twenty minutes past ten, about eleven degrees 

 south of Lyra. The clouds, at the time of the disappearance, were 

 rapidly forming north of the arch, all the south being yet clear; in 

 fifteen minutes afterwards the whole sky was overcast, and the 

 light in the north was hardly visible through the clouds. The air had 

 been coming from the north in the morning, and had changed round 

 by the west, and at the time of the occurrence of the arch it is be- 



VoL. XXVII.— No. 1. 15 



