228 On the Variation of the Magnetic Needle. 



a similar effect on the needle, but have never seen the effect so great 

 as is here stated. This disturbance of the needle by an aurora is 

 not merely occasional, but almost invariable. During the continu- 

 ance of my magnetic observations, I paid particular attention to the 

 aurora, and in every instance when the aurora was considerable, 

 there was a palpable agitation of the needle, and almost always a 

 deflection to the amount often, twenty or thirty minutes, and in two 

 instances of more than a degree. On the other hand, whenever the 

 needle has experienced any unusual deflection, I have uniformly 

 seen reason to ascribe it to an aurora. The aurora, indeed, has not 

 always been visible ; and there are several reasons why it should 

 not be. It might occur in the day time, when it would be wholly 

 invisible, or during moonlight, or a cloudy night, when it would be 

 nearly if not wholly obscured. But there has not occurred an in- 

 stance, during the period embraced in these observations, when the 

 needle has suffered an unusual deflection, without an aurora being 

 visible, unless observations were frustrated by one of the causes 

 above mentioned. I will now enumerate all these cases, and in the 

 chronological order : 



JVov. 8, 1834.— 6 h., A. M., Declination 5° 41' ; 7 h., 5° 41'; 

 8h., 5° 57'; 9 h., 5° 47'; 10 h., 5° 51'; 10 h. 50 m., 6° 4' ; 11 

 h., 6° 1' ; 12 h., 5° 49' ; 1 h., P. M., 5^ 45'. During the remain- 

 der of the day, the needle was tolerably regular, although not quite 

 so much so as usual. Professor Olmsted observed about 8 o'clock 

 last evening, an uncommon brightness, like the dawn in the north, 

 much brighter than the common aurora — lasted with fluctuations all 

 night. At 5 o'clock in the morning, it was nearly as bright as it 

 had been in the evening. The needle was not observed during the 

 evening of the 2nd ; nor did I notice the aurora myself; if I had, 

 I should have watched the needle at the same time. On the eve- 

 ning of the same day, a very brilliant aurora was seen in England, 

 of which a description was given in the New York Observer of De- 

 cember 27, 1834. It is described as an arch of light, six or seven 

 degrees in breadth, extending from the eastern to the western hori- 

 zon, nearly through the zenith. The observers represent it as un- 

 usually splendid. See also Loudon's Magazine for 1835, p. 94. 



Nov. 5.-6 h., P. M., Declination 5° 42' ; 7 h., 5° 30' ; 7 h. 

 40 m., 5° 28' ; 8 h., 5° 30' ; 9 h., 5° 38' ; 10 h., 5° 36'. At half 

 past seven in the evening, although cloudy, the entire horizon, from 

 the west point almost to the east, was lighted up like the dawn, 



