146 Disturbance of the Earth’s Magnetism. 
In April, 1831, a new series was commenced, to determine if the 
needles still indicated the same degree of magnetic intensity. No 
material difference was observed, except in the following instance, 
when a remarkable anomaly was exhibited. 
On the 19th of April, at 12 o’clock at noon, an observation was 
made with the Hansteen needle, the result of which differed only 
the fractional part of a second from the usual mean rate of this 
needle. At6 o’clock p. m. the same day, another observation was 
made with the same needle, and apparently under the same circum- 
stances; but a remarkable change was now observed in the time of 
its making three hundred vibrations, indicating a great increase in the 
magnetic 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 dis- 
covered which would tend in the least degree to explain the cause 
of the phenomenon. ‘The experiment was made at the usual place, 
with the box containing the needle resting on a post permanently 
fixed for the purpose, in the Academy Park, at a sufficient distance 
from every disturbing object, and with the usual precaution of di- 
vesting 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 ia the southern 
part of the heavens, which was shortly afterwards 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 co- 
ruscations of the aurora, shooting up past the zenith, and apparent- 
ly all converging to the same point. The actual formation of a co- 
rona might probably have been observed, 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 appear- 
ance of the aurora might possibly be connected with the magnetic 
disturbance observed at 6 o’clock ; and in order to test this, the ap- 
paratus was again placed on the post in the Academy Park, and an 
observation made during the most active appearance of the meteor. 
The result of the observation was, however, entirely different 
from that anticipated; for instead of still indicating, as at 6 o'clock, 
an uncommonly high degree of magnetic intensity, it now showed an 
intensity considerable lower than usual. 
