26 



We may now test the accuracy of these numbers, by em- 

 ploying 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', — which far exceeds the 

 probable error of observation. 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 viti- 

 ate the general result; and that a nearer approximation to the 

 values of M and N will be obtained by excluding that observa- 

 tion from the computation. We thus obtain, from the re- 

 maining ten equations, 



M=0'-689; 2V= 0-527. 

 And substituting these values, we find 



ang. ('tang = - |D = - 37° 25'; V (M* + N*) = 0- 867. 



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

 north, making an angle of 37° 25' with the meridian of Dub- 

 lin ; 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 

 longitude and latitude are known, is given by the formula 



8 - g = 0'- 527 (A - X ) + 0'- 689 - /i ) cos A ; 

 the declination at Dublin, § , being supposed known. Or, if 

 we substitute for cos A the value corresponding to the mean 

 latitude (A =53° 1 70, 



8-S o = 0'-527(A-A o ) + 0'-412( iU - Mo ). 

 " The mean decimation at Dublin, for the year 1850, is 



* " This result agrees very closely with Colonel Sabine's map of the iso- 

 gonal lines in the Atlantic, as to the direction of the lines ; but gives a more 

 rapid rate of increase." 



