20 DISCUSSION OF THE VERTICAL FORCE. 



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

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

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



Mean amount of disturbances. 



In 1841-42 2306 div. 



" 1842-43 1521 



" 1843-44 959 



" 1844-45 63G 



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 distvirbances, their diurnal and annual 

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



Aiimtal IiiequaUfi/ in the numher 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. 





Table VII.- 



—Annual Inequality in the Number or Disturbances. 























Mean ratio 



Mean ratio 





1841-1842. 



1842-1843. 



1843-1844. 



1844-1845. 



Means 



Vert, force 





Declination 



Vert, force 



Hoi-.forceaud 













from four 



ratio. 



ratio. 



ratio. 



iior. force and 



declination. 













years. 







declination. 





July 



64 



24 



4 



22 



28 



0.9 



1.1 



0.9 



1.0 



0.9 



August 



21 



34 



10 



47 



28 



0.9 



0.9 



1.6 



1.3 



1.1 



September 



40 



57 



20 



7 



31 



1.0 



1.4 



1.4 



1.4 



1.2 



October 



28 



12 



12 



10 



15 



0.5 



1.4 



2.1 



1.7 



1.3 



November 



37 



13 



40 



5 



24 



0.8 



1.0 



1.1 



1.1 



1.0 



Deceinber 



84 



15 



33 



23 



39 



1.3 



1.0 



1.0 



1.0 



1.1 



January 



76 



58 



40 



9 



46 



1.5 



0.6 



0.8 



0.7 



0.9 



February 



86 



61 



16 



3 



39 



1.3 



1.0 



0.5 



0.7 



0.9 



March 



S6 



44 



53 



34 



42 



1.4 



1.1 



0.7 



0.9 



1.1 



April 



51 



50 



42 



12 



39 



1.3 



1.1 



0.9 



1.0 



1.1 



May 



47 



36 



4 



14 



25 



0.8 



1.0 



0.6 



0.8 



0.8 



June 



37 



16 



26 



4 



21 



0.7 



0.6 



0.5 



0.6 



0.6 



Mean 



51 



34 



25 



16 31 



1 



The months of maximum disturbance are ]\Iarch 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, gii^o the maximum number 

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

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



