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.) DIURNAL VARIATION OF THE VERTICAL FORCE IN SUMMER, WINTER, AND FOR THE 



WHOLE YEAR. 



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. : 



(B.) SEMI-ANNUAL INEQUALITY IN THE DIURNAL VARIATION OF THE VERTICAL FORCE. 



+ 



200 

 160 

 120 

 080 

 040 



| 0.000000 





040 

 080 



12 



160 



200 



0" 2 4 6 8 10 14 16 18 20 22 24 (+20.6) 



I 

 Philadelphia mean time. 



