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AA^NUAL REPORT SMITHSONIAN INSTITUTION,, 1942 



from its average about 7 to 8 o'clock, then shifts westwardly passing 

 its average daily direction shortly before noon, reaches an extreme 

 eastward position about 1 or 2 o'clock, and during the late after- 

 noon hours begins to shift eastwardly back to the average direction 

 which it maintains practically during the night hours. In the 

 Southern Hemisphere the motion is reversed, the south end of the 

 compass needle exhibiting the same tendency to follow the sun's 

 apparent motion. Toward the North and the South Poles the di- 

 urnal variation is greatest, while near the Equator it is practically 

 zero. Somewhat similar behavior during the day and night is shown 

 also by the other magnetic elements. 



LOC/ll HOURS 



LOCAL HOURS 



FiQXJBE 4. — Diurnal variation of (A) magnetic declination, and (J8) inclination. 

 (After U. S. Coast and Geodetic Survey.) 



Thus it is apparent that in its main features the solar diurnal 

 variation progresses according to local mean tune. Therefore, this 

 variation is connected with the earth's rotation and, as its major 

 tendency follows the apparent motion of the sun, it may be pre- 

 sumed that the sun is an important factor in this daily phenomenon. 

 This is an indication that the earth's magnetism responds to outside 

 influences which find their origin in solar activity and act upon the 

 upper regions of the atmosphere. 



The lunar-day variation, while averaging less than one-tenth that 

 of the solar day, is quite systematic. Instead of considering the 



