SURVEY OP THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1886. 283 
Let us now turn to the calculated disturbances of the elements. In the accom¬ 
panying map (fig. 16) the figures represent the differences between the observed and 
calculated values of the Declination expressed in minutes of arc, and taken as positive 
when the needle is turned to the west. There is a sharply marked boundary between 
the regions of positive and negative disturbance which passes through the focus. To 
the west of it the needle is deflected to the east and vice versa. 
Fig. 16. 
Declination disturbances in minutes of arc. 
+ Indicates that the observed westerly Declination is greater than the calculated value. 
-- Curve of no disturbance. 
To the south of the focus the needle is oppositely deflected on each side of a line 
which runs nearly due north and south. To the north the efiect on the Declination 
needle dies out. 
If the cause of the disturbance were a mass of “ magnetic matter ” below the 
surface of the Earth symmetrical with respect to the isogonals, the line which divided 
easterly from westerly disturbances of the Declination would intersect that which 
divided positive from negative disturbances of the Horizontal Force over the focus or 
centre. The next map (fig. 17) shows that this condition is very nearly fulfilled, the 
two points being only 7 or 8 miles apart. Lastly, the maximum disturbance of the 
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