Magnetical Observatory of DiiUln during the Years 1840-43. 87 



for January 1, 1840, is 332° 27'-27. Wherefore, the mean declination at any 



time is 



A = 332''27'-27 + 6'.06 y. n, 



n denoting the number of years, and parts of a year, reckoned from Jan. 1, 1840. 



15. Subducting the mean values for each year from those of the separate 

 months, we obtain a series of numbers which represent the course of the annual 

 variation. During the year 1840 tlie declinometer was twice readjusted; and 

 from this, and other causes, the monthly values of the absolute declination for 

 that year cannot be relied upon with certainty. The following Table contains 

 the values of the differences for the three following years, together with their 

 means for the whole period : 



Table IX. Annual Variation of the Declination for the tears 1841-43. 



The following Table contains the corresponding numbers for the succeeding 

 triennial period, with their means, and the means of all. 



Table X. Annual Variation of the Declination for the years 1844-46, 



together with the mean of all. 



