SURVEY OP THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1886. 285 
disturbing forces at stations in the neighbourhood ought to be directed towards the 
lofty central mass. Close to the peak it would itself form the centre of attraction. 
At stations near to, but not over, outlying ridges, the needle might be deflected 
towards them. Immediately over a ridge the direction of the disturbance would 
change rapidly, and thus give an idea of instability. The Horizontal Forces would 
be least over the peak, would increase up to a certain distance, and would finally die 
out. 
Fig. 18. 
BIRMINGHAM ■ 
0+/5 
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+ /50» 
CLIFJOPJ 
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Vertical Force disturbances in terms of O'OOl metric unit. 
+ Indicates that the ob.served is larg’er than the calc^^lated value. 
-Contour lines of equal Vertical Force disturbance. 
In fig. 19 we have depicted the disturbing Horizontal Forces. They are drawn in 
the proper directions, and to a scale on which 0‘9 mm. corresponds to O'OOl metric 
units. They fulfil the above conditions exactly, and we think leave no doubt 
that in the south-east of England over an area of 10,000 square miles the lines of 
magnetic disturbing force tend to a centre which lies near to and probably between 
Twyford and Henley-on-Thames. We have treated this district in great detail, 
because we rely upon these results to prove that the methods of calculation and 
deduction adopted are satisfactory, at all events in districts where the surface rock 
or soil is non-magnetic. If the results attained elsewhere present greater difficulties 
