54 



DISCUSSION OF THE DIURNAL VARIATION 



marked, the morning maximum being considerably greater. These curves also show 

 the gradual shifting of the maxima and minimum to a later hour in winter than in 

 summer, a phenomenon also well exhibited in the preceding diagrams C and D. 

 The numerical values of this change of hours will be given in tabular form further 

 on. The small afternoon minimum about 9 P. M. is less distinctly marked than 

 any other feature of the diurnal curve. 



(E.) REGULAR SOLAR-DIURNAL VARIATION OF THE HORIZONTAL FORCB TOR SUMMER, WINTER, AND WHOLE YEAR. 



(In absolute measure.) 



0" 1 2 3 4 5 6 7 8 9 10 11 N'nl 2 3 4 5 7 8 9 10 11 12" 

 A. M. P. M. 



Philadelphia mean time. 



Table VIII. contains the computed values of the time and amount of the morning 

 maximum and minimum, and of the afternoon maximum. The values for the 

 secondary afternoon minimum are taken from the diagrams. The time of the A. M. 

 maximum and minimum is within the nearest eighth minute ; that of the P. M. 

 maximum within the nearest tenth minute. The time for the P. M. secondary 

 minimum is within the nearest hour. The amount of change of horizontal force is 

 expressed in scale divisions. 



