220 ON THE ISOGONAL LINES IN IRELAND. 



Accordingly, the isogonal lines in Ireland lie to the east of north, 

 making an angle of 37 25' with the meridian of Dublin ; and the 

 declination increases as we proceed in the north-westerly direction, 

 the increase being 52" for each geographical mile, in a direction 

 perpendicular to these lines.* 



Finally, the declination at any point of the island, whose longi- 

 tude and latitude are known, is given by the formula 



S - S = 0'-527 (X - X ) + 0'-689 fc - ,,) cos X ; 



the declination at Dublin, <5 , being supposed known. Or, if we 

 substitute for cos X the value corresponding to the mean latitude 

 (\-53ir), 



S-S =0'-527 (X -X ) +0'-412 (^-^J. 



The mean declination at Dublin, for the year 1850, is 26 29' 

 west ; and as the yearly value of the secular change of the declina- 

 tion is - 6' - 06, the mean declination, in any not very remote future 

 year, will be given by the formula 



S = 2629'-6'-06x ; 



n being the number of years, counted from the present. If greater 

 accuracy be desired, the diurnal and annual variations of the de- 

 clination, corresponding to the time of the day and of the year, 

 must be added. . 



* This result agrees very closely with Colonel Sabine's map of the isogonal lines in 

 the Atlantic, as to the direction of the lines, but gives a more rapid rate of increase. ' 



