296 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



(a) The diurnal variation. Each magnetic element is subject 

 to a daily periodic change. This is called the diurnal variation. 

 Thus, the curves in Figs. 6a and 6b show the diurnal variation of 



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Fig. 6a. 



Fig. 6b. 



the magnetic declination at the United States Magnetic Observa- 

 tory at Baldwin, Kansas. Figure 6a shows the diurnal variation 

 in mid-summer and Fig. 6b shows the diurnal variation in mid- 

 winter. 



(&) The annual variation. Each magnetic element is subject 

 to an annual periodic change which is called the annual variation. 



W 24 2.2 20 18 16 14 2_-_ 10- 8,6 4 2. 024 6 



