SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1886. 321 
It is probable that to these a sixth in North Wales and Shropshire should be added. 
There are indications of other minor centres at Malvern, in Derbyshire, and in the 
neighbourhood of Charnwood Forest, but the number of stations involved is small. 
The vast majority are included in the few and simple systems above described. 
These results are not the outcome of the calculations only, for all the principal 
conclusions can be drawn from the observations alone. Mr. Welsh’s survey, though 
including fewer stations at which all the elements were determined than ours, though 
omitting almost altogether stations in the Western Isles, though worked up (as 
regards the Dip and Declination) by means of a different system of geographical 
coordinates, and in the case of all three elements by means of an assumption as to 
the linearity of the isomagnetics which we have ahandoned, confirms our conclusions. 
In four only of the 28 of his stations which fall within our districts, viz., Stornoway, 
Pitlochrie, Ayr, and Dumfries is there an important difference between us as to the 
direction of the disturbing: forces. 
In the most highly distiuFed districts, at Loch Inver, Kyle Akin, Broadford, Glen- 
morven we agree as to the order of the magnitude and as to the direction of the forces. 
At Cumbrae and Fairlie we find closely neighbouring stations of positive surrounded 
by others of negative Vertical Force disturbance, and Welsh confirms us by a similar 
result at Ardrossan, not ten miles off 
These coincidences between the results of the two surveys, and between the 
magnetic peculiarities and the geological constitution of districts cannot be accidental, 
and we venture to assert that, throughout the kingdom, the lines of Horizontal mag¬ 
netic disturbing Force tend towards regions of maximum Vertical disturbing Force, 
which are themselves defined by the presence of crystalline rocks, and especially 
of basalt, either visible on the surface or concealed by superimposed masses of 
sedimentary strata. 
Beyond this general conclusion we do not wish at present to go. The detailed 
constitution of our principal magnetic districts (except in the case of the Beading and 
Wash disturbances) has yet to be investigated. We have not discriminated between 
various possible causes of a decrease in the Vertical Force, such as the removal of the 
attracting mass to a greater depth, or the formation of repulsive poles, either by 
induction or permanent magnetism. We do not think the last word has been said on 
the cause or causes of local and regional forces. All these require and will, in the future, 
no doubt, receive attention. We trust, however, that by following out to their 
logical conclusion the premises of a not improbable working hypothesis, we have 
succeeded in showing that local and regional forces obey certain simple laws, and that 
by means of these the kingdom can be divided into magnetic districts in which the 
relations between the direction of the disturbing forces and the main geological 
characteristics are so suggestive as to be worthy of careful statement and further 
investigation. 
2 T 
MDCCCXC.—A. 
