174 EIGHTH REPORT 1838. 



being manifestly affected by disturbing action. The disturb- 

 ance at the two latter stations is obviously due to the presence 

 of trap rocks. 



In deducing the lines of total intensity, I have been guided 

 by the principles laid down in page 95 and seq.., and have ac- 

 cordingly assigned double weight to the results in Dublin and 

 Limerick, the weight of each of the other comparisons being 

 taken as unity. The results of the computation are as follows : 



L = l-0268, M= + -0000748, N= +'0000501; 

 M=-33°48', r = -0000900; 



L denoting the intensity at the central station (Lat. = 53° 21', 

 Long. = 8° 0'), the intensity at London being unity; M and N 

 the increase of the intensitj^, corresponding to each geographical 

 mile of distance in the direction of the two coordinates ; n the 

 angle which the isodynamic line, passing through the central 

 station, makes with the meridian ; and r the increase of the 

 intensity in the direction perpendicular to that line. 



The lines of horizo)ital intensiti/ rest upon a somewhat 

 broader basis, there being four stations where the horizontal 

 force was observed without the dip. In deducing them, I have 

 given a weight of two to the results obtained at Dublin, Lime- 

 rick, and Markree, the weight of each of the other determinations 

 being unity. We find, accordinglj^, 



L = '9290, M=— -000190, N= — -000368; 

 M=— 62°40', r=-000414. 



Captain Ross's observations are contained in the following 

 table. They were made in the autumn of the year 1838, with 

 a single cylinder, designated as R (X) in the following pages. 

 The stations are twelve in number, and are distributed uniformly 

 over the island. The permanency of the magnetism of the 

 cylinder during this series, and its time of vibration at West- 

 bourn Green, near London, have been already shown in 

 Table LXIII. 



