SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, IS9I. H1 I 
In the first place, then, it need hardly be pointed ont that all the results we have 
obtained might be due to, and it has been suggested that all similar results are due 
to, an appropriate distribution of earth currents. The mere discovery of Disturbing 
Forces does not of itself prove the existence of subterranean magnetic rocks. 
We have given reasons, both in our earlier paper (“ 1890 Memoir, pp. 314-320) 
and in a lecture delivered by one of us before the Royal Institution (‘ Proc. R.I., 
February, 1892, 417), in support of the view that the theory of earth currents is 
far less satisfactory than the hypothesis that the disturbances are due to magnetic 
matter; but even if, as we believe, magnetic rocks are the chief cause of the 
phenomena, it is quite possible that earth currents may afibct and modify the lesults. 
It is necessary, therefore, to be very cautious in drawing conclusions as to the nature 
of the more deeply seated causes of local disturbances. 
In the next place, even if the simple hypothesis that all the phenomena are due 
to rocks magnetised by the earth's field, were absolutely correct, chfiiculties might 
arise in interpreting the meaning of the magnetic contour surface. Explanations of 
magnetic disturbances have generally been based only on the theory that there are 
centres of attraction or repulsion. In other words, one of the two poles of the sub¬ 
terranean magnetic matter has been supposed to be at an infinite distance. 
One of us has, however, pointed out (‘Proc. Roy. feoc., vol. 48, 1890, p. 523) that 
this theory will not suffice, if the horizontal dimensions of the magnetic rocks are 
large compared with the depth at which the temperature of the interior of the oaith 
may be supposed to be so high as to deprive matter of its magnetic properties. 
In that case the upper and lower surfaces of the rocks v'ould be oppositely 
mao-netlsed, and both surfaces would have to be taken into account. 
If, then, a slab of magnetic rock, say a circular disk 100 miles in diameter and 
12 miles thick, existed in the midst of a widespread region of non-magnetic rock, it 
would be surrounded by a ring-shaped district of negative Vertical Disturbing Force. 
This would be followed just above the edge by a smaller ring of large positive disturb¬ 
ance, which would dimmish as the centre was approached, xlence the magnetic 
contour surface would be of the shape of a circular fort. A deep ditch or valley would 
enclose a high rampart or line of hills, surrounding a cup-shaped hollow. A hasty 
conclusion from the study of the Vertical Forces alone, that this was the leal shape of 
the upper surface of the magnetic rocks, would be entirely erroneous. 
Fortunately, in so simple a case as this, the Horizontal Disturbing Forces would 
enable us to correct the mistake. If there wxre a true ridge of magnetic rocks the 
Horizontal Forces would tend towards it, both from the inner and the outer sides. If 
the phenomena were due to a circular slab, the Horizontal Forces would tend to its 
centre, so that at points some little way within the edge, the Horizontal Disturbing 
Forces would be directed from a region of higher to a region of lower Vertical 
Disturbing Force. 
Another and still more obvious caution is that it is difficult or impossible to 
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