MAGNETIC FORCE IN IRELAND. 199 



with some other whose correction is already known. In this 

 manner it has been found that the mean difference of the results 

 of Needles L (1) and L (4) is & - 4 = + l'-5 ; being somewhat 

 smaller than that assigned above. But if 8 denote the absolute 

 dip, we have already found that 8 - Si = + 10'- ; and adding these 

 differences, the correction of Needle L (4) is 8 - S 4 = + ll'-S. 



The correction of Needle S (2) is inferred from the observations 

 made with that needle in Limerick, as given in Table IV. 



Limerick, July, 1835, . . dip = 71 16'-9 



Dec., . . 71 14-6 



Mean, . . 71 15 -8. 



This mean corresponds, in time, to the middle of October, 1835. 

 But the true dip in Limerick (July, August, 1834) was found to 

 be 71 9'-5 ; and when reduced to October, 1835 (assuming the 

 annual decrease to be 3'), it is 71 6'*0. The correction of the 

 needle is therefore - 9' '8. 



The corrections of the needles being determined, we may now 

 proceed to deduce the values of the absolute dip at the several 

 places at which observations have been made. These values are 

 given in the following table (Table V.). In the first and second 

 columns are written the place and date of the observation. The 

 third and fourth columns contain the corrected values of the dip, 

 deduced from the results of Tables III. and IV. by the application 

 of the corrections now explained ; and the last column contains 

 the mean dip inferred from the two preceding. In taking this mean, 

 double weight has been assigned to the results obtained with needles 

 of the ordinary construction, the number of readings with these 

 needles being double of that made with the needles whose poles 

 are unaltered. 



