212 OBSERVATIONS OF THE TERRESTRIAL 



so that the final equations are 



149922o? + 31821 y = + 32157; 

 31821* + 55339y = - 18372. 

 From these we deduce 



x = +'3250, y=- -5196, 

 tan v = - 1-599, v = - 57 59', 

 s = -613. 



It would appear, then, that the angle which the isoclinal lines 

 in Ireland make with the meridian is on the increase a result 

 which is in conformity with the general progress of these lines, as 

 inferred from a comparison of recent observations with those of an 

 earlier date.* 



For the mean of the two years, 



x = + -3239, y=- -4950, 

 tan *? = - 1-528, = -5648', 

 s = -592. 



The lines in the annexed chart are deduced from these last 

 results ; and it appears from them that the interval of the lines 

 corresponding to a difference of half a degree of dip is 50-7 geo- 

 graphical miles. 



The lines of dip and of horizontal intensity being known, the 

 lines of total intensity may be deduced. For if / denote the total 

 intensity, h its horizontal component, and S the dip, as before, 



and differentiating, and dividing by the equation itself, 



/-' df=h~ l dh + tan S sin 1'dS. (I.) 



Now, if the values of * and y for the lines of dip and of 

 horizontal intensity be denoted by ar (8) , ae w and y w , y (h] , and if x (f] 



* This is further confirmed by the results obtained with the help of additional obser- 

 vations. See Eighth Report of the British Association for the Advancement of Science, 

 p. 118. 



