SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY 1, 1880. 265 
however, both the maximum and minimum values are less, while in the latter case 
they are gneater than the normal values at the stations at Avhich they occur, and 
if one maximum and minimum only are formed, the disturbances at once decide 
whether they are to the north or south of a centre of attraction. 
The lines which separate regions of positive disturbance in which the observed 
is greater than the calculated value from those in which it is negative are also of 
importance. Thus, in the case of the Horizontal Force, if such a line is approximately 
perpendicular to the magnetic meridian, and if in passing over it from south to north 
we leave a region in which the disturbance is positive and enter one in which it is 
negative, we either pass over a centre or line of attraction or pass from the range 
of the influence of one centre of repulsion into that of another. 
If the centre is not at a great depth below the surface, and if the magnetic matter 
is not widespread at its minimum depth, it will cause a sudden reversal in the sign of 
the disturbances which will, however, be large near the centre on both sides. On the 
other hand, passage from the region of influence of one centre to that of another wdll, 
if the distance between them is considerable, be marked by a transition from small 
positive to small negative disturbances, or vice versa. 
Similar remarks may be made with respect to the disturbances of the Declination, 
but they are most easy to interpret when the line which separates a positive from a 
negative region runs approximately north and south. If such a line meets another 
which separates a southern region of positive from a northern region of negative 
Horizontal Force disturbance, all the disturbing forces in the neighbourhood tend 
tow’ards the point of intersection. It will be convenient to speak of such a point as 
a peak, and to call a line which divides regions of positive from those of negative 
disturbance of the Declination or Horizontal Force, so as to indicate attraction towards 
it, a ridge line. In like manner, a line which separates the regions of influence of two 
attractive centres may be called a valley line. 
In the choice of these terms we are, no doubt, influenced by our views of the facts 
to be hereafter set forth, but they are convenient, quite apart from any theory of the 
cause of local magnetic forces. It will be proved beyond doubt that in some cases 
these forces emanate from matter below the surface of the earth. If this is so, an 
increment in their intensity must be due either to a closer approximation to the 
matter or to an increase in its magnetisation. We have no magnetic test to 
discrimate between the tw'o, and therefore, without prejudice, adopt a nomenclature 
which is perhaps most consistent with the first hypothesis. If we wished to keep 
absolutely free from all expression of an opinion as to the causes of the phenomena, we 
might have called a peak (to which the lines of magnetic disturbing force converge), 
a magnetic sink, and so on, and for the present the terms we suggest inay be taken 
as indicating merely points and lines from and to which such lines of force run. 
The disturbances of the Declination and Horizontal Force, together with the ridge 
and valley lines are shown in Plates IX. and X. 
mdcccxc.—A. 2 M 
