232 On the Variation of the Magnetic Needle. 



served during the year, of an arch completely spanning the heavens. 

 As very careful observations were made upon it at Dartmouth Col- 

 lege, which lies almost due north from New Haven, at a distance of 

 about one hundred and sixty four miles in a right line, we have the 

 materials for calculating its height. This arch at Dartmouth Col- 

 lege, appeared in the south at eight o'clock, having an altitude of 

 38°. At New Haven, at the same time, its altitude was 75°, from 

 which we at once obtain its height to be about one hundred and six- 

 ty miles. 



On the evening of Nov. 18th, there was a slight repetition of the 

 aurora. A diffuse light was spread all along the northern horizon, 

 and rose to a considerable elevation. The appearances, however, 

 were at no time splendid. I was absent from my room during the 

 principal part of the evening, and could not therefore observe the 

 needle constantly ; yet at seven, eight and eleven o'clock, the needle 

 was as regular as usual. 



In England an aurora was observed on the night of the 17th, and 

 early in the morning of the 18th, and so much did the appearance 

 resemble a natural fire at a distance, that we are told at London, 

 * sixty men and twelve fire engines hastened towards some dreadful 

 conflagration.' About midnight, clouds intervened, and the fire be- 

 came extinguished, but the aurora again burst forth about 3 A. M., 

 so that the firemen were again on the alert. On the evening of the 

 18th, the aurora was uncommonly splendid, consisting of beams and 

 coruscations which shot up to the zenith. The light was, how- 

 ever, almost entirely white. (Loudon's Magazine for 1836, pp. 

 23—36.) 



The preceding catalogue contains all the instances in which the 

 aurora was observed here during the year, and also all the instances 

 in which the needle was decidedly irregular. These observations 

 lead us to the conclusion, that auroras are most common during the 

 months of November and December. That when the aurora con- 

 sists merely of a baTik of light like the dawn, and rises but little 

 above the horizon, the disturbance of the magnetic needle is very 

 little, and is generally proportioned to the vividness and extent of 

 the aurora. The needle has sometimes appeared to veer towards 

 the point of greatest brightness, and sometimes to recede from it. 

 This is a question which deserves more consideration. 



Auroral beams cause a disturbance of the needle, at least, when 

 the beams are themselves in active motion. 



