502 EDITOR'S NOTES. 



when the sun has south declination to cause the north 

 end of the magnet in both hemispheres to be more to 

 the west in the forenoon, and more to the east in the 

 afternoon, than it would otherwise be. The range of 

 deflection from one solstice to the other is about 3' or 

 4' at the extreme or turning hours ; and the daily range 

 from the forenoon to the afternoon is throughout the 

 year also about 3' or 4', except about the equinoxes when 

 the opposite phases pass into each other. The transition 

 from the one phase to the other takes place rapidly, 

 and is nearly completed a few days after each of the 

 equinoxes. 



We may now consider the general effect of the super- 

 position of the semi-annual inequality upon the mean 

 diurnal variation stated in the preceding section. In 

 the middle latitudes of the northern (magnetic) hemi 

 sphere, the eastern extreme of the mean diurnal varia- 

 tion has been described as being reached about 7 or 8 

 A.M., and the western extreme about 1 or 2 P.M. ; the 

 mean range during the year being about 9' or 10': 

 whilst in the middle latitudes of the southern (mag- 

 netic) hemisphere, the contrary extremes are attained 

 nearly at the same hours ; and the mean range is the 

 same, or nearly so. Now, as the range of the semi- 

 annual inequality is less than 9' or 10', its superpo- 

 sition upon the mean diurnal variation in the middle 

 latitudes in either hemisphere does not alter the cha- 

 racter of the forenoon or afternoon extremes in any 

 part of the year, but it does alter their amount. 

 During the greater part of the half year in which the 

 sun has north declination, the semi-annual inequality 



