O F T HE MAGNETIC F O R C K . 



(A.) DIURNAL VARIATION OP THE HORIZONTAL FOBCE IN SUMMEK, WINTER, AND FOB THE WHOLE YKAII. 



47 



9 



"o 







0" 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24" 

 A. M. P. M. 



Philadelphia mean time. 



This semi-annual change in the diurnal amplitude is more conspicuously repre- 

 sented in the annexed diagram (B), derived from diagram (A) by straightening out 

 the annual curve and using it as an axis of abscissae for laying off the differences 

 between the annual values and the summer and winter values at the same respective 

 hours of the day. 



(B.) SEMI-ANNUAL INEQUALITY IN THE DIURNAL VARIATION OP THE HORIZONTAL FORCE. 



Philadelphia mean time. 



This diagram (B) may, with advantage, be compared with the analogous one 

 representing the annual change of the diurnal variation of the declination as given 

 in Part II. of this discussion. The construction is the same in either case. 



At 6 A. M. there is hardly any change throughout the year. The maximum 

 variation, in the course of a year, takes place at 9 A. M. (range 0.00194 in absolute 

 measure); about 11| A. M. there is an epoch of no variation ; at 2 P. M. a second 

 maximum is reached (range 0.00107); again at 7| and IIP. M. points of no 



