OP THE DIP. 



23 



Diagram (D) exhibits the diurnal variation of the dip for summer, winter, and 

 the whole year. 



(D). SoLAR-DlUENAL VARIATION OF THE MAGNETIC DiP BETWEEN 1841 AND 1845. 



+1.'0 

 .8 

 .6 

 .4 

 .2 



Philadelphia mean time. 



The general character of the above three curves is the same ; the annual curve 

 has its greatest value about 11 A.M., and its least value about 5 A.M., with a 

 range of about 1.'2; in summer the epochs occur a little earlier, with a range of 

 about 1.'5; in winter the epochs are a little later, and the range is contracted to 

 about l.'O. There is also a secondary maximum between 1 and 2 A. M., with a 

 less regular secondary minimum occurring at some hour in the afternoon or early 

 in the night ; in su.mmer, however, these minima appear interchanged. 



The Toronto curves are similar to those above, the shifting of the epochs is the 

 same as at Philadelphia ; the morning minimum is less prominent, and the afternoon 

 minimum constitutes the principal minimum during summer, and also, on the average, 

 during the year. The total annual range is almost exactly the same at the two 

 places, and the times of the principal maxima also nearly coincide. 



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

 of the morning minimum, also the elapsed time, together with the amount and range 

 for each month and season ; also the times and amount and range of the afternoon 

 or early night minimum, taken from the preceding diagrams. 



13 



