380 A. B. Bache on the Horizontal Component of Magnetic Force. 



"With the exception of the month of November, the values given 

 above for the annual variation are tolerably regular in their pro- 

 gression, and considering the delicacy of the test applied to the 

 observations in deducing the annual variation, this exceptional 

 irregularity in the November value will not affect the general 

 conclusion. We have as the general results a greater horiijontal 

 force in summer (April to August), and a smaller horizontal 

 force in winter (from September to March) than the average an- 

 nual value. The maximum occurs in July (at Toronto in June) 

 and the minimum in January (at Toronto in December). 



For Toronto we have the expression for the annual variation, 



3-531+0-002sin(^4-3UG°) 

 For Philadelphia : (omitting the November value), 



4-l'76+0-001 sin ((9+312°), 

 the angle 6, in both equations counting from Jan. loth. 



The annnal range is 0-0021 (in absolute measure) the transi- 

 tion appears to take place about the time of the equinoxes or a 

 short time before. 



Table XIII contains the monthly normal values of the hori- 

 zontal force in absolute measure obtained by adding (algebra- 

 ically) 4-1760 to the values in the last column of table XIL 

 These numbers, it will be observed, are corrected for secular 

 change ; if we apply the same we obtain the result! 

 '^ the horizontal force answering 1 



Jaauary 18i3. The quantity A, mentioned in the explanatory 

 remarks to table No, Yll, is given i ' 



epoch, 



^ anatory 



last column of table 





Table No. XIII. 









Monthly ™««n^ 





Corr. for secular change 



affected with .ec. cb. 



July. - - 



4-f^ 



4-1787 



August, - - 



4-1760 



4-1769 









October. - - 



4-1753 



4-1758 



November. 



4-1776 



i]',it 



December, 









4-. 739 



Febmary, - - 

 March,' - . 



t^ 



4-l?63 



^ : : 



4-1757 

 4-1775 



4-1750 

 41766 





4-i76t 



4;i75o_ 



^1^, : : 



4 7^ 





