SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY 1, 1891. 523 
The map proves that, in the centre of each of the regions we previously marked out, 
the boundaries of which are shown by red lines, is a ridge or locus of high vertical 
force, bordered on each side by districts of^low vertical force (valleys), and that the 
Horizontal Forces point from the valleys to the ridge lines. 
As this is the only fact on which we are at present dwelling, the Horizontal Forces 
are not shown at the stations on the ridge lines. They will become important when 
we come to details. For the moment it is sufficient to insist that there are two 
independent tests of the existence of a ridge line, viz., high Vertical Force over it, and 
Horizontal Disturbing Forces directed from each side towards it; and that these two 
conditions are both satisfied with regard to the same districts by tAvo perfectly 
independent surveys taken five years apart. 
When it is remembered that a Horizontal Disturbing Force of 0'5 per cent, of the 
earth’s horizontal field is fairly large, the result affords a satisfactory confirmation of 
the validity of methods of calculating the normal undisturbed forces, from which the 
Disturbing Forces are in turn deduced. The Vertical Force Disturbance is in general 
negative at the stations (joined by blue lines), whence the Horizontal Forces point to 
the central attractive regions. In some cases it is positive, but it is then less than at 
neighbouring stations which are more directly over the ridge line. 
Sometimes (as at places in Antrim) the Vertical Force is less than its calculated 
value within the attractive region, but the negative disturbances in tlie surrounding 
magnetic “valleys” are then correspondingly large. 
In a few cases the Horizontal Disturbing Forces are so small tliat the directions in 
which they point are quite uncertain. Sometimes also—as at Pontefract—the 
station lies in the valley between two ridge lines, and points away from that the 
existence of which the map is intended to demonstrate. At all such stations the 
arrows liave been omitted. 
The only district in which our previous conclusions appear not to be in exact accord 
with the facts is near the Hebrides. We formerly inclined to the view that a ridge 
line ran up the Minch. Our recent observations seem to make it more probable 
that a ridge line crosses it from Skye. The fidl discussion of the point must, 
however, be postponed to a later section of this memoir. 
Secular Change between 1886 and 1891. 
Having thus proved that the results of the new survey, when referred to the 
terrestrial lines deduced from it for the epoch 189 t’O, confirm the conclusions as to 
the ridge lines which we had drawn from our earlier Avork, Ave must next combine 
the tAvo surveys by reducing them to a common epoch. 
This o]jeration is not only necessary if the tAvo surveys are to be treated as one, but 
it affords a means of testing the amount of error Avhich may attach to the process ol 
calculating the district and terrestrial lines. The present is the first occasion on 
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