SURVEY OE THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1891. 613 
A 'nioc/ 7 ict'ic pccih is a point at which the vertical disturbance is a inaxiinuin, when 
compared with that at all neighbouring stations. It is generally a more or less 
j powerful centre of attraction, especially if it is an isolated point of high Vertical 
' Force. A inagnetic peak is generally found at a point where two I'idge Imes cross. 
I A magnetic col is a point on a ridge line at which the Vertical Disturbing Force is 
a minimum as compared with that at neighbouring points on the 7'idge line. A well- 
marked magnetic col generally occurs at the crossing point of a 7nagnetic ridge and 
vcdley line. 
\ 
General Kesults of the Survey. 
' Having thus explained the chief terms which we intend to use, it is desirable to 
indicate the nature of the additions to our previous knowledge which wm should 
d prio 7 'i expect to follow from a large increase in the number of places at which 
: observations were made. 
1 These may be summed up as follows :— 
! (1.) The positions of the principal magnetic ridge and valley lines wmuld be more 
I accurately fixed. 
, (2.) The details of their constitution would be more clearly denned, so that varia- 
1 tions in the magnitude of the attractive force wmuld be discovered, wdiich would 
enable us to specify, not only the general position of the ridge line, but also to detect 
I the existence of magnetic peaks and cols, 
^ (3.) Minor magnetic districts more or less completely separated from the others by 
magnetic valley lines would be discovered. 
I All these results have followed, but, though their boundaries have been modified, 
the principal magnetic regions into which we originally divided the United Hingdom 
' still stand out as connected wholes, and as far more Important than any of the 
' comparatively large number of minor magnetic divisions wdiich must now be added to 
j them. It is true that some of the latter enclose an area ecj^ual to that of two or three 
j small counties, but they do not rank with the principal magnetic regions, in some of 
which a clearly-marked ridge line can be followed for nearly 200 miles. 
The only important change which we are inclined to make in our published list of 
the chief magnetic ridge lines is rather by way of subtraction than of addition. 
W^e previously classed the line which is connected wdth the great fault maiked by 
the Caledonian Canal, and that which appeared to be related to the basalts of Skye 
and Mull as independent. 
We ate now inclined to regard the cross connection between them as so important 
as to make it better to regard them as parts of a complex but single whole. 
In addition to the increase in our knowledge of magnetic ridge and valley lines, 
which are clearly indicated both by the Vertical and Horizontal Disturbing Foices, 
the more detailed survey has enabled us to detect minor peculiarities, which would 
have escaped notice in any less complete investigation. 
