MAGNETIC FORCE IN IRELAND. 211 



(IS being expressed in minutes. Hence when 8 = 71 0', the change 

 of the horizontal intensity, , corresponding to a change of one 



minute of dip, is - '00084 ; and for a variation of 12' in the dip, the 

 corresponding variation of the horizontal force is -01. 



In deducing the lines of dip from observation, it seems advis- 

 able to separate the results of the two years. For the weights we 

 shall assume 



Limerick (1834), .... weight = 5, 

 Armagh, .... = 2; 



the weights of each of the other determinations being unity. 



Making the computations for the year 1834, we obtain the fol- 

 lowing results : 



S(urf) = 86660, S(icab) = + 36129, S(wb z ) = 64303, 



S(wac) = + 11043, 8(wbc) = - 18633. 

 The final equations accordingly are 



86660* + 36129y = + 11043; 

 36129* + 64303y = - 18633. 



From which we deduce 



a? = + -3228 = cos t>, 

 y = - '4705 = s sin v ; 



in which v denotes the angle which the isoclinal line makes with 

 the meridian of Dublin, and s the coefficient which determines the 

 rate of increase of the dip in the perpendicular direction. Dividing, 

 squaring and adding, we find 



tan v - - 1-458, t? = - 55 33', 



- -571. 



The following are the results of calculation for the year 1835 : 

 S (a 2 ) - 149922, 8(ab) = + 31821, S(V) - 

 S (ac) = + 32157, S (be) = - 18372 ; 



