Solar -diurnal Variation of the Magnetic Declination at Pekin. 475 



If we examine these figures, we perceive that the motion from west 

 to east, commencing at the turning hour between 1 and 2 in the 

 afternoon, though comparatively slow during the hours of the night, 

 is continuous and uninterrupted until the extreme easterly elonga- 

 tion is reached between 8 or 9 in the following morning, and 

 that no other turning hours intervene between those of the extreme 

 easterly between 8 and 9 a.m. and the extreme westerly between 

 1 and 2 p.m. 



The phases of the solar-diurnal variation, as they are shown by the 

 Pekin Observations, may be stated as follows : — The north end of the 

 magnet is at its extreme eastern elongation about half-past 8 in 

 the morning ; at this hour it begins to move to the west, and moves 

 rapidly in this part of its daily course, completing its whole move- 

 ment in that direction in five hours, and reaching its extreme western 

 elongation at about half-past 1 p.m. From this hour it returns, 

 somewhat less rapidly than in its forenoon excursion, until about 

 6* p.m., when the rate of progression is considerably lessened, but 

 continues in the same direction through the hours of the night, until 

 about 5 a.m., when it again accelerates until the eastern extreme 

 is attained, as already stated, about 8^ a.m. There is thus 

 a very unequal division of time in the direction of the motion, 

 which takes five hours in the progress from east to west, and 

 nineteen hours in returning from west to east through the same 

 arc. We find a more equal division of time if we regard the 

 greater or less rapidity of the motion : there are about twelve 

 hours in which the motion is comparatively quick, and twelve hours 

 in which it is comparatively slow ; the quick hours being those of 

 the day, the slow hours those of the night. 



Thus far the notice we have taken of the Pekin results has been 

 limited to the diurnal variation which we find when we take an 

 average of the whole year, and which we may theoretically suppose 

 would take place in every month of the year if the sun were always 

 in the plane of the equator. But similar investigations had already 

 made known to us the existence of a semiannual inequality, having 

 opposite phases according as the sun has north or south declina- 

 tion; with turning epochs about the times of the solstices, and 

 the phases passing into each other about the times of the equinoxes. 



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