20 



DISCUSSION OF THE VERTICAL FORCE. 



If we take the monthly aggregate amount of the disturbances, all referred to a 

 uniform series of bi-hourly observations, and form a table of these values for each 

 year (Table VIII), the mean aggregate amount for each year is as follows : 



In 1841-42 

 " 1842-43 

 " 1843-44 

 " 1844-45 



Mean amount of disturbances. 



. 2306 div. 

 . 1521 



959 



636 



This again points to the end of the year 1844 for the epoch of the minimum 

 amount of disturbances, and considering the three elements, declination, horizontal 

 and vertical force, the spring of 1844 might be assumed as the time of the minimum 

 magnitude of the magnetic disturbances. 



Altogether, the inequalities in the diurnal amplitude and in the number and 

 magnitude of the disturbances of the magnetic elements, as observed at Philadelphia, 

 fix the end of the year 1843, or the beginning of 1844, as the epoch of the minimum 

 of the eleven (or ten) year inequality. 



We now proceed with the analysis of the disturbances, their diurnal and annual 

 inequality in number and amount, and for increasing and decreasing values. 



Annual Inequality in the number of Disturbances. The numbers for each month 

 have been referred to a uniform series of bi-hourly observations as explained above. 

 The ratios of the monthly means to the annual means is given, and also, for com- 

 parison, similar ratios found for the horizontal force and declination. 



The months of maximum disturbance are March and September (the high value 

 in January and the low one in October appear anomalous, and would no doubt dis- 

 appear in a longer series of observations). The minimum occurs in June ; there is 

 no well expressed second minimum. The horizontal force and declination ratios, 

 as well as the ratios of the three elements at Toronto, give the maximum number 

 of disturbances at the equinoxes, and the minimum number at the solstices, and as 

 the winter solstice minimum only is wanting in the Philadelphia vertical force 



