196 



AURORA BOREALIS. 



being divided by the first two digits (for they will be sufficient) 

 of thrice the square of the said fust figure, will immediately 

 shew which of the ambiguous figures should be taken for the 

 second figure of the root. Thus, if the proposed number be 

 465,484,375, here the first and last digits of the root are 7 and 5: 

 A=5andB>=5j any odd number, therefore, multiplied by 

 A will give B j but if the cube of 7=343 be taken from 465, 

 and the remainder 122 be divided by 14 (the first two digits 

 of 7 2 X 3) it will be instantly seen that 9 is too great, and 5 is 

 manifestly too little j there only remains 7, therefore, for the 

 second digit of the root. The same method will easily clear 

 the ambiguity when the proposed .cube ends with an even 

 digit. 



I am, &c. 



O. 



nection be 

 ^ween the 

 aurora borea 

 lis and mag- 

 netism. 



Facts. 



IX. 



On the Appearance and Disappearance of the Aurora Borealh, 



By M. Cotte*. 



Supposed con- ^CITTHETHER there be any relation or agreement between 



V v the progressive changes of magnetical variation in 



a given latitude, and the times at which the aurora borealis 



appears, or ceases to manifest itself, is a question entitled to 



discussion. It is proved by observation, 



1 . That the declination of the magnetic needle is not con- 

 stant ; that in our latitudes it was easterly before the year one 

 thousand six hundred and sixty-six ; and since that time it has 

 more or less slowly increased to the west. 



2. That the phenomenon of the aurora borealis, of which 

 the western part of the atmosphere is also the seat, is seen fre- 

 quently during certain epochas, and very seldom during others. 



3. That when this phenomenon appears, it sometimes has 

 an influence on the magnetic needle, so as to produce an irre- 

 gularity of motion, or unsteadiness in the variation of the 

 needle. The same thing sometimes happens in stormy weather, 

 or when much electricity predominates in the atmosphere. 



It must be remarked, that this influence of the aurora 



Agitation of 

 the needle. 



Journal de Physique, lxxiii. 



borealis 



