276 Mr. Christie on the Aurora Borealis. 



From the means, it appears that the variation, during the 

 days on which the aurora was visible, was greater than during 

 those on which it was invisible ; in the month of February 

 more than double, and on a mean of the two months nearly so. 

 Taking individual observations, we have, in the month of 

 January, when the aurora was visible, six days on which the 

 variation exceeded the mean variation during the month, and 

 eight days on which it was less ; when the aurora was not 

 visible, five days on which it exceeded, and ten days on which 

 it was less than the mean : and in the month of February, we 

 have, when the aurora was visible, eleven ddys'ori which the 

 variation exceeded the mean variation of the month, and only 

 three on which it was less ; when it was not- visible only one 

 day on which' it "exceeded, and thirteen on which* it was less 

 than the mean. . Sp that, whatever may be the cause of the 

 aurora, it is evident from these observations, that, during its 

 occurrence at Port Bowen, the needle had in general a ten- 

 dency to make wider excursions, although this tendency may, 

 in many instances, have been counteracted. 



I am aware that results, directly the reverse of these, have 

 been drawn from Captain Foster's observations. In the ' Edin- 

 burgh Journal of Science,' vol. viii. p. 200, it is remarked that, 

 * In the two months during which twenty-eight aurorse oc- 

 curred, the mean monthly excursions of the needle on each 

 side of its mean position was only 1 37 J'; whereas during the 

 two months when there were no aurorse, it was almost exactly 

 double, viz., 3 18' 41". If this difference, which' is far too 

 great to be accidental, shall be confirmed by future observa- 

 tion, it will prove that, in the arctic latitudes, and in those 

 periods which abound with aurorse, the excursions of the mag- 

 netic needle are diminished ; while, in our latitudes, the causes 

 which produce auroras increase the excursions of the magnetic 

 needle.' 



If we admit this, these observations prove that, during an 

 aurora, magnetic forces are developed, a supposition which they 

 have been considered to refute ; but I think .we cannot allow 

 that the difference here noticed, in the extent of the variation, 

 can be fairly connected with the aurora. The mean variation 

 for the month of March, during which the aurora was only 



