OF THE MAGNETIC FORCE. 



45 



and winter season, the diagram exhibits the diurnal variation in summer to be a 

 curve of but one maximum and one minimum occurring about noon and midnight 

 respectively, whereas in winter the double feature of the curve becomes very con- 



(A.) DiTJKNAL Variation of the Yertical Force in Summer, Winter, and for the 



WHOLE Year. 



+ 500 



400 



300 



200 



ICO 



■ 0.000000 



I 100 



1 200 



I 300 



1- 400 



— 600 



01" 2 4 6 8 10 g 14 16 18 20 22 24i' (+20°'.6) 

 Philadelphia mean time. 



spicuous, the secondary maximum and minimum occurring about 2 and 6 A. M. 

 respectively. The phenomenon, in the two seasons, changes therefore from a single 

 to a double crested curve. 



The semi-annual change of the diurnal variation is better shown in diagram (B) , 

 which contains the difference from the annual curve in summer and winter, viz. : — 











































/ 





■■.._ 

























/' 



— 

























// 



y 



^•-- 



N N 



\ 

















. 





y^.^ 



— 





\ > 



I \ 











- 



4' 









' 







\. 



Vt 











■p- 



■"? 



^1^-' 











\ \ 











.i.^ 





/ 













\ 





^ 



^'^ 





~^f 



,^'y 



















































,..■' 





















'' 







































O" 



2 



4 



6 



8 



10 



Noon. 



14 



16 



18 



20 



22" 



(+20-.6) 



Summer 

 Winter 



—182 

 +181 



—201 

 +202 



—90 

 +91 



+28 

 —28 



+46 

 —47 



+93 

 —95 



+142 

 —142 



+105 

 —103 



+168 

 —165 



+101 —71 

 —100 +71 



—136 

 +135 





(B.) Semi- Annual Inequality in the Diurnal Variation of the Vertical Force. 



+ 200 

 160 

 120 

 080 

 040 

 0.000000 

 040 

 080 

 120 

 160 



— 200 





























- 



^^ 



\ 

 \ 





















y 







\ 









< / 



■\ 



/ 



\ 







/ 









■^ 







,^rA 



ly 



s 



^ 



\ 



I 



/ 













\ 















\/ 















\ 



''" 













v 













A 















A 















/ 





>N 











i\ 



s 











/ 







S 



V 



_, 



\ 



y 



1 



\ 











/ 









N 



''' 



\ 



/ 







\ 







-^^ 



/ 





















\ 

































0" 2 4 6 



10 g 14 16 18 20 22 24 (+20°>.6) 



Philadelphia meau time. 



