30 



SOLAR DIURNAL VARIATION. 



Diagram (H) exhibits the diurnal variation of the total force for summer, winter, 

 and the whole year. 



(H). SOLAE-DIURNAL VARIATION OP THE TOTAL FORCE BETWEEN 1841 AND 1845. 



Philadelphia mean time. 



The diurnal curve is single crested in summer, and on the average during the 

 year, but in winter it assumes a double crested form, having a secondary maximum 

 between 2 and 3 A. M., and a secondary minimum about 6 A. M. The principal 

 maximum in summer occurs about 2 P. M., and in winter about one and a half hours 

 earlier; the principal minimum in summer occurs about 10 P.M., and in winter 

 about two hours earlier. The summer range is about 0.0009, and the winter range 

 about 0.0004 parts of the force. 



The following table contains the computed times of the principal maximum and 

 minimum, together with the elapsed time, amount and range of variation in force 

 for each month and season, also the time and amount of range of early morning 

 secondary wave as taken from the diagrams. 



