Effect of an Aurora Borealis on the Magnetic Needle. 1 17 



tal. In the interval of an hour and a half, between 9 and lOj P.M., 

 the needle had moved to the eastward 12', or one-fifth of a degree; 

 and the observations during the early part of the phenomena tend to 

 show, if they do not prove, that this motion took place in the latter 

 part of the interval, the mean hourly rate of motion, as shown by the 

 observations at 8 J and 9 P. M., being only three minutes. I regret 

 that the observations were not more regular; but as no particular in- 

 terest attached to the evening, the observer, as I have already stated, 

 not being aware of the presence of the aurora, I considered myself 

 fortunate in the frequency of those observations which were made ; 

 this being a part of the evening in which, usually, there is nothing to 

 require regular observations, being the interval between the evening 

 minimum and the night maximum. 



The observations on the horizontal needle out of doors do not con- 

 tradict any of the remarks just made, and they show further that at 

 11 P.M., forty minutes after the disappearance of the arch, the ef- 

 fect on the needle was still strongly marked ; the westerly variation 

 at 11 P.M., having been ten minutes less than on the same hour of 

 the next succeeding evening. The very rapid formation and dis- 

 appearance of clouds during the evening, and the low stratus which 

 formed about 11 o'clock, would all, in ordinary cases, have produced . 

 slightly marked changes in the variation, but nothing of the character 

 of those noted in the table. The temperature having remained sta- 

 tionary, within doors, during the evening, no part of the changes in 

 the position of the horizontal needle noted in the third column, were 

 due to variations of temperature. The results, in the absence of cor- 

 rection for these changes, are therefore the more valuable. 



The dip, recorded in the sixth column of the table last given, has 

 its minimum at 4 P. M. ; a rise then begins, which is so very irregu- 

 lar as not to permit any inference from it ; diminishing between 7 J 

 and 8i P. M., it increases between 8J and 9, decreases between 9 

 and 10^, and subsequently increases to 11 o'clock. These changes 

 do not seem to attach to the different phases of the aurora, and are 

 not more considerable than ordinary meteorological phenomena 

 would produce, such, for example, as are recorded in the first table. 



My aim having been merely to establish that a decided disturbance 

 of the horizontal needle took place during the aurora of the 17th, I 

 have not thought it necessary to apply the corrections for the tem- 

 perature of the needles, which the successful establishment of the 

 changes in diurnal variation will require. 



