82 DR. A. H. COX: REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
essential features that Melton, Rempstone, Nottingham, and in a slighter degree 
Pen Hill, are maxima of downward disturbance, and therefore in the vicinity of what 
Rucker called “ magnetic peaks.” Taking into account the distribution of the 
vertical and of the horizontal disturbing forces, the presence of a pronounced line of 
disturbing matter running nearly east and west from Melton to between Rempstone 
and Loughborough is indicated. 
Rucker and Thorpe"^ have pointed out that the disturbing forces may be due either 
to permanent or to induced magnetisation of the rocks. It appears from their work 
that in many cases the hypothesis of induced magnetism is consistent with the 
observations, but that there are cases in which a minor portion of the effect may be 
due to permanent magnetism. 
On the hypothesis that the disturbing forces are due to the earth’s normal field 
acting by induction on the rocks, it is better to avoid elaborate calculation and to 
have a simple if only approximate rule to guide one in looking at the map. The 
following rule, wdiich can be justified analytically, I have found useful as a rough 
test. The number expressing the disturbing force is given in units 10“^ of an absolute 
unit of force, and similarly it is convenient to use a unit 10“^ of an absolute unit to 
express the susceptibility of the rocks. . The rule is that the number giving the 
observed disturbance which we wish to explain, is an approximate measure of the 
lowest susceptibility by which we could hope to provide a quantitative explanation. 
Thus, if the disturbance is 100, then 100 is the order of magnitude of the minimum 
susceptibility which a rock must possess if it is to account for that particular 
disturbance. The actual minimum value possible for the susceptibility might be 
somewhat less than 100, perhaps even as low as 70, but a rock with a susceptibility 
of only 50 would, in general, have to be ruled out as a possible cause for a disturbance 
measured by 100. Moreover, if the rock attains only this minimum susceptibility, 
its upper limit will have to be situated close up to the surface of the ground, and it 
would have to extend in depth and area to distances determined by the disturbances 
at neighbouring stations, granting that the disturbances at adjoining stations have 
the same cause. 
My general inference from the forces at Rempstone and Melton Mowbray with 
their neighbouring stations is that there are two main magnetic centres situated 
along the line running westwards from Melton Mowbray which has been particularised 
above, and that the depth of these can hardly be less than 1 km. (say 3000 feet), 
while it may be as much as 2 or 3 km. 
As already indicated further observations are necessary in order to get precise 
information as to the distribution of the disturbing magnetic material. This raises 
a point of some importance. Absolute observations do not give the most accurate 
results and in any case take too long. If further details are to be carried out, 
I think it will be necessary to prepare a set of portable variometers on the principle 
* ‘Phil. Trans.,’ Ser. A, vol. 181 (1890), p. 314. 
