210 OBSERVATIONS OF THE TERRESTRIAL 



from which we have, by elimination, 



x = - -0001633 = A ; 

 y = + -0003748 = B. 

 Finally, substituting these values in equations (H, I) 



tan u = - 2-2952, u = - 66 28', r = - -000409.* 



The positive branches of the axes of co-ordinates having been 

 assumed to be those which stretch to the north and to the west, it 

 follows that the lines of equal horizontal intensity lie to the east of 

 north, making an angle of 66^ nearly with the meridian of Dub- 

 lin. The horizontal intensity decreases as we proceed northward, 

 the decrease being equal to the distance traversed in a direction 

 perpendicular to these lines (estimated in geographical miles, or 

 minutes of latitude) multiplied by the coefficient '000409. The 

 lines are laid down in the accompanying chart for differences of 

 O'Ol in the value of the intensity, the corresponding intervals of 

 distance being 24-4 geographical miles. 



On a comparison of the separate determinations with the result- 

 ing lines, it will be observed that the intensities in the northern 

 group are greater than those due to their position, those of the 

 western group less, and those of the south-western again greater. 

 These deviations may, in part, arise from the inexactness of the 

 assumption with which we set out in the computation of the lines, 

 and from the sensible deviation of those lines from parallelism. 

 But they are probably owing in a much greater degree to the dis- 

 turbing causes to which we have already alluded. The separate 

 results composing each of these groups were for the most part ob- 

 tained about the same time, and they are therefore probably affected 

 in the same manner, and nearly in the same amount, by the irre- 

 gular fluctuations in the direction and intensity of the resultant 

 magnetic force. Of these, the changes in the direction of the force 

 are by far the most influential. The relation between the corre- 

 sponding changes in the dip and in the horizontal intensity is ex- 

 pressed by the formula 



dh 



y = - tang sin 1'rfg; 



* The inclination of the lines of equal horizontal intensity, deduced with the 

 aid of additional observations, is = - 62 40', the rate of increase in the normal 

 direction remaining unaltered: Eighth Report, p. 174. 



