GIO 
MK. A. W. EtiCKER AED DR. T. E. THORPE OX A MAGXETIC 
Nomenclature and Magnetic Contour Surface. 
Ill our previous ^lemoir we were anxious to prove that the conclusions based upon 
the Disturbing Forces were independent of the particular method bv which those 
forces were calculated. 
We therefore showed that in many cases similar conclusions could be arrived at 
by studying the values of the elements^ when these had been merely reduced 
to epoch. 
It is unnecessary to amplify such evidence in this paper. The agreement between 
the results of the two surveys is in itself sufficient to prove the validity of the 
methods. We shall therefore discuss the Disturbing Forces only, unless any special 
reason makes it desirable to refer to the actual values of the elements at any 
particular place. 
The*increasing complication of the problem under investigation has made it neces¬ 
sary to use very freely the nomenclature we have adopted to describe magnetic 
disturbances, and even in some cases to extend it. 
We therefore think it desirable to prevent misconception, by stating exactly the 
reasons why we use this phraseology and the reservations with which its application 
to the theory of “ underground mountains” of magnetic rock must be safeguarded. 
If the magnetic state of the United Kingdom were perfectly known, a model 
might be constructed by raising from every point of a horizontal map a vertical 
ordinate proportional to the Vertical Disturbing Force. The highest points on the 
surface formed by the upper extremities of the ordinates would correspond to points 
of maximum Vertical Disturbance. The projections on the map of the curves, in 
which any horizontal plane cuts the surface, would be magnetic contour lines, or loci 
of equal Vertical Force Disturbance. 
The analogy between such a surface and a model ot the hills and valleys ot a 
country would be very close, and we have freely transferred the terms which are 
applied to differences of height above the sea level to the differences of Vertical Force 
Disturbance. 
As far as we are aware, no objection can be brought against the use ot the analogy 
except the risk that it may be interpreted too literally, and that it may be imagined 
tliat the magnetic contour surface is an exact representation of the physical contour 
of magnetic matter near to, but below, the surface of the earth. 
We have from the first made it clear that our nomenclature must not be thus 
interpreted. We justified it (“ 1890 Memoir,” p. 265) as “convenient, quite apart 
from any theory of the cause of local magnetic forces,” and we expressly called 
attention to the fact that “we might have called a peak (to which the lines ot 
magnetic Disturbing Force converge) a magnetic sink, and so on, and [that], for the 
present, the terms we suggest may be taken as indicating merely points and lines 
from and to which such lines of force run.” 
