ON THE MAGNETIC VERTICAL FORCE. 



67 



of the annual means give equal weight to the results from the six months of hourly 

 observations in 1845, and to the results from the twelve months of bi-hourly obser- 

 vations in 1842; compared with these results, those of 1844 have the weight two. 



If we represent these values graphically, we find the general shape of the curve 

 to be similar to that of the horizontal component, it is double crested with a prin- 

 cipal maximum a little before the upper culmination, and a principal minimum 

 about 3| hours after the lower culmination of the moon; the average epoch 

 of the vertical force tide is, therefore, about one and a half hours apparently in 

 advance of the culminations. The secondary wave is very feeble, its greatest value 



(A.) LUNAR-DlURNAL VARIATION OP THE VERTICAL FORCE, 1841 TO 1845. 



I 



U. C. L. C. 



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