70 
MR. GEORGE W. WALKER ON THE MAGNETIC RE-SURVEY OF THE 
that the horizontal disturbing force presents no anomaly. (So far as I know Lough¬ 
borough is the only case in which serious magnetic variations were occurring during 
the observations, and it is matter for congratulation that the survey has been carried 
out under such favourable conditions in this respect.) 
The horizontal forces in this region are in the main in very good agreement in the 
two surveys, but we ought to recall that Rucker and Thorpe used two stations at 
Melton Mowbray and got different results. The magnitude of the horizontal force 
was the same at both, but very different in direction. The directions intersected 
quite close to Melton Mowbray. The vertical components also differed, being +80y 
and -f-305y at the two stations. This renders doubtful the existence of direct 
correlation between the forces at Melton Mowbray and those at Loughborough and 
Coalville to the west. The vertical disturbances in the two surveys differ considerably. 
Thus taking Nottingham, Melton (a), and Manton, the old survey gives 4-179, 
4-305, 4-79, while the re-survey gives 4-230, 4-238, 4-58. 
Again, for Coalville and Leicester the old survey gives —109, —27, while the new 
survey gives 4-30 and 4-57. 
Consider next the line Nottingham, Newark, Lincoln. The old survey gives 
4-179, 4-6, 4-172, while the re-survey gives 4-230, —162, —76. 
For the line Melton (a), Grantham, Lincoln, the old survey gives -f305, 4-50,, 
4-172, and the re-survey gives 4-238, —143, and —76. 
The inferences from the two surveys are somewhat different, and I think it will 
be admitted that the values for the re-survey form a rather simpler system to 
explain. 
There is a known ridge of iron ore running from Lincoln through Grantham 
to Melton, and if I understand the matter correctly, this ore is naturally magnetised. 
The magnetic data suggest that Melton itself is the most important part of this ridge. 
Reasoning by analogy of the data, it would not be surprising if a ridge of similar 
ore exists along the line Lincoln through Newark to Nottingham, the latter being 
the most intense point. It would seem natural, further, to join up Nottingham and 
Melton by a ridge of considerable intensity passing near Loughborough. 
The possible economic importance of this inference seems sufficient in justify the 
selection of a few more stations in order to test its validity. 
The examination of other districts in the British Isles and the problems connected 
with the magnetic data has attracted my attention, but this is not the time or place 
to give detailed account, especially as the work is only in its initial stages. But 
the three cases discussed will serve to indicate the importance of the matter in its 
practical as well as in its purely scientific aspect. 
Conclusion. 
I think that I should fail in an obvious duty towards the future progress of the 
magnetic survey of the British Isles if I did not set out, for the assistance of those 
