25 



ang. 



tang = --^ 



and the increase of declination corresponding to each geogra- 

 phical mile of distance, in a direction perpendicular to this 

 line, is 



" It is evident then that, to obtain the values of ilf and N, 

 observation must give the values of the declination at three, or 

 more, stations. The observations of Sir James Ross were 

 taken at twelve stations, well distributed throughout the 

 island ; and as they were all made during the months of Octo- 

 ber and November, 1838, no correction is required to reduce 

 them to a common epoch. For convenience of reference, they 

 are here extracted from Colonel Sabine's Memoir, together with 

 the longitudes and latitudes of the places of observation. 



STATION. 



X 



P 



a 



Valentia, . . . 



51°56' 



10° 17' 



28° 42' 



Killarney, . . 

 Westport, . . 

 Limerick, . . . 



52 2 



53 48 

 52 40 



9 30 

 9 29 

 8 36 



28 11 



29 9 

 28 3 



Cork, .... 



51 54 



8 28 



27 44 



Markree, . . . 



54 14 



8 28 



29 15 



Shannon Harbour, 



53 14 



7 53 



28 3 



Edgeworthstown, 



53 42 



7 33 



28 8 



Londonderry, 

 Waterford, . . 



54 59 

 52 15 



7 19 



7 8 



28 47 

 26 44 



Armagh, . . . 



54 21 



6 39 



28 8 



Dublin, . . . 



53 21 



6 15 



27 35 



" Taking Dublin as the origin of co-ordinates, and substi- 

 tuting the values of A - A , ju-u , and S - 8 , given by this 

 Table, in the equation above given, we obtain eleven equa- 

 tions of condition, from which the values of M and N are ob- 

 tained by the method of least squares. They are the following : 

 M= 0-690, N= 0-585. 



