CnAr. Ill] INERTION. 51 



direction in which the earth moves in its orbit ; and 

 also, on the side of the earth turned from the sun 

 the motion of axial rotation is in the same direction 

 as that in which the earth is moving in its orbit, and 

 on the opposite side of the earth in the reverse direc- 

 tion ; so that in March at midday this force acts 

 about ENE. -wards, and at midnight about WNW.- 

 wards ; whereas in September, at midday it acts 

 ESE. -wards, and at midnight WSW. -wards ; all 

 intermediate degrees of variation lying evenly be- 

 tween the extremes of daily and annual variation 

 respectively. Thus, this force always acts eastwards 

 in the daytime, and westwards at night, and always 

 northwards in March and southwards in September ; 

 oscillating continually backwards and forwards be- 

 tween the extremes of daily and annual variation 

 respectively. The annual variation is confined within 

 about four points of the compass ; that is, between 

 23^° north and 23^° south, and the daily variation 

 extends over about fourteen points of the compass ; 

 that is, from 66^° east to 66^° west, the greatest 

 combined variation being from any one point to the 

 opposite point of the compass, that is, the sixteen 

 points. 



In the annual variation this force turns more and 

 more northwards from the 23rd of September to 

 the 20th of March, and then from the 20th of March 

 to the 23rd of September it turns more and more 

 southwards. And in the daily variation the change 



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