ON THE ISOGONAL LINES IN IRELAND. 



219 



Sabine's Memoir, together with the longitudes and latitudes of the 

 places of observation. 



Taking Dublin as the origin of co-ordinates, and substituting 

 the values of A - A , /* - n , and S - S , given by this Table, in the 

 equation above given, we obtain eleven equations of condition, from 

 which the values of M and N are obtained by the method of least 

 squares. They are the following : 



Jf=0'-690, ^ 



We may now test the accuracy of these numbers, by employing 

 the formula to calculate the values of the declination at each of the 

 eleven stations. The result of this calculation gives, at Waterford, 

 a difference between the observed and calculated values amounting 

 to 34', an amount which far exceeds the probable error of obser- 

 vation. This difference is, therefore, probably due to some local 

 irregularity of the magnetic force. But, whatever be its cause, it 

 is obvious that it tends to vitiate the general result ; and that a 

 nearer approximation to the values of M and N will be obtained 

 by excluding that observation from the computation. We thus 

 obtain, from the remaining ten equations, 



And substituting these values, we find 

 ang. (tang = - ~ ) = - 37 25' ; 



0'-867. 



