1832] WRITINGS OF JOSEPH HENRY. 61 



mean rate of this needle. At 6 o'clock p. m. the same day, 

 another observation was made with the same needle, and 

 apparently under the same circumstances; but a remarkable 

 change was now observed in the time of its making three 

 hundred vibrations, indicating a great increase in the mag- 

 netic intensity of the earth. It was at first supposed that 

 the needle had accidentally been placed contiguous to some 

 ferruginous substance ; but on a most careful investigation, 

 nothing could be discovered which would tend in the least 

 degree to explain the cause of the phenomenon. The experi- 

 ment was made at the usual place, with the box containing 

 the needle resting on a post permanently fixed for the pur- 

 pose, in the Academy Park, at a sufficient distance from 

 every disturbing object, and with the usual precaution of 

 divesting the person of all articles of iron, such as keys, 

 knives, &c. 



At about 9 o'clock in the evening, or three hours after the 

 above observation, an unusual appearance was noticed in 

 the southern part of the heavens, which was shortly after- 

 wards recognized as an arch of the aurora. It was about 

 nine degrees in breadth, with the vertex of the arch twenty 

 degrees above the horizon. At this time the northern part 

 of the sky was covered with light fleecy clouds. At forty- 

 five minutes past nine, the clouds partially disappeared, and 

 disclosed the whole northern hemisphere entirely occupied 

 with coruscations of the aurora, shooting up past the ze- 

 nith, and apparently all converging to the same point. The 

 actual formation of a corona might probably have been ob- 

 served, but for a dark cloud which remained stationary a 

 little south of the zenith. The idea for the first time now 

 occurred to me, that this uncommonly brilliant appearance 

 of the aurora might possibly be connected with the magnetic 

 disturbance observed at 6 o'clock; and in order to test this, 

 the apparatus was again placed on tlie post in the Academy 

 Park, and an observation made during the most active ap- 

 pearance of the meteor. 



The result of the observation was however entirely differ- 

 ent from that anticipated ; for instead of still indicating, as at 6 



