and Intensity of the Magnetic Needle. 



359 



the hour and minute at which the observations were commenced ; 

 the third cohimn of each part contains the mean time in seconds 

 taken by the needle in its different positions, to perform one hun- 

 dred vibrations j and, in the fourth, is inserted the temperature of 

 the needle at the time of observation. 



" The above results show, that the mean of all the observed times 

 which the horizontal needle required to make 100 vibrations, was 

 2092.33 seconds, but that differences appear in these times amounting 

 to 94.3 seconds, or s^d part of the interval ; whereas, in the dipping 

 needle, in which the mean of the times required to perform 100 vibra- 

 tions was 408.65 seconds, the greatest difference is only 57 seconds, 

 or ygd part of the interval, which is a much less proportional change 

 than the former. 



" Therefore, as has been stated, the change of intensity in the hori- 

 zontal needle is due, principally, to a daily variation in the amount of 

 the dip, not to a real change of intensity in the terrestrial magnetic 

 force. This, at least, appears to be a legitimate deduction from the 

 preceding observations ; from which circumstance, and that of the daily 

 variation in the direction of the horizontal needle, we are naturally led 

 to the conception of a small variation in position of the magnetic axis^ 

 corresponding to a revolution of the polar point round its mean posi- 

 tion as a centre, produced by the action of the sun, on the magnetism 



* The dip of the needle resulting from these elements is 87° 48' 8 N. 



A a ^ 



