SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1891. 
5 
When these were graphically represented on maps it was found that the Horizontal 
Disturbing Forces tended towards the regions of greatest Vertical Force, and that it 
was possible to divide the United Kingdom into a comparatively small number of 
districts in each of which the north pole of a needle was urged horizontally towards 
a region or line (called by us a I'klge line) on or near to which the downward Vertical 
Disturbing Force was a maximum. 
In several cases these ridge lines were obviously connected with some geological 
feature of the countiy through which they passed ; and notably either with masses 
of basic rocks, or with places where the older rocks were knowm to approach relatively 
near to the surface. 
In 1889 we undertook two supplementary surveys. We had expressed the opinions 
that a centre of magnetic attraction existed out at sea, to the west of Skye and Mull, 
and that a well-marked locus of magnetic attr£iction ran through districts in Yorkshire 
and Lincolnshire, in which there was nothing in the nature of the superficial rocks to 
account for the phenomena. (“Mem.,” 90, pp. 300, 307, 312.) 
These conclusions were tested and verified by a number of new observations. They 
will be dealt with hereafter, but the leading facts of the confirmation of our predictions 
were communicated to the Royal Society in 1890 (‘Proc. Roy. Soc.,’ vol. 47, p. 444, 
1890). 
It was then determined to carry out the survey in greater detail, so that the 
positions of the ridge lines which we had discovered could be laid down more accurately. 
It was important that this should be done, in order that any shift or change in their 
positions should be detected as soon as possible. 
Such an extension of the survey involved the addition of a great number of new 
stations. It was desirable that the observations should be made within a short time, 
both to avoid the complications introduced by the application of the correction for 
secular change over a long interval, and also because the period of maximum sun spot 
frequency was approaching, during which the work would probably be interfered 
with by magnetic storms. 
A liberal allowance by the Royal Society from the Government Grant Fund enabled 
us to carry on the survey with the requisite speed, and on an extended scale, by the 
aid of assistants. We hope that the present communication will show that, as the 
result of ourwmvk, the main magnetic features of the United Kingdom are now know n 
with such accuracy that, if they are permanent, little remains to be done but to fill in 
details ; wdiile, if they are subject to change, it will be comparatively easy to detect 
and measure the alterations. 
