OF THE MAGNETIC DECLINATION. 



15 



The table includes only the series of bi-hourly observations ; the reduction of the 

 numbers from incomplete years to the correct sum for the whole year being effected 

 by means of ratios as in the discussion of the number of disturbances. For com- 

 parison the average value of a disturbance at Toronto is added. It must be 

 remarked, that the amount of deviation from the normal, constituting a disturb- 

 ance, was nearly but not quite the same at Toronto as at Philadelphia, so that the 

 ratios of the corresponding numbers in the last two columns should be compared. 



The eleven year period is well marked in the aggregate value of the disturbances, 

 as well as in their average value in the different years; and the year 1843 is 

 decidedly indicated as the minimum. To find a more precise value for the epoch 

 of the minimum, the formula, 



= T.09 0'.930 (t 1840.5) + OU49 (t 1840.5) 2 , 

 has been constructed, which represents the observed values as follows : 



The first and last value have only half weight. According to the formula, the 

 minimum took place in August, 1843. (See Fig. 3.) 



3 

 a 





(Middle of each year.) 



As the resulting epoch from the differential observations with the declinometer 

 we find the month of June, 1843, by giving double weight to the result deduced 

 from the inequality of the diurnal amplitude. 



