SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY 1, 1891. 627 
Thus ten points, which are on the average about 16 miles apart, are found between 
pairs of places, the average distance between which is 12 miles, so that the mean 
error will not exceed d:: 6 miles. 
It is also evident from the direction of the arrows at Thorne, Butterwick-on-Trent, 
and Barton-on-Humber, that there is a centre of attraction to the east of the first, 
and to the north of the second place. At its extreme west end, near Barrow, 
the ridge line under discussion meets that from the Lake District, which deflects it 
to the north. A well-defined valley line runs from Barrow to Giggleswick. On the 
opposite side of the ridge line is a station of low Vertical Force (Aysgarth), and, 
though there is no further evidence that the valley line crosses the ridge, the part of 
the ridge to the west of Aysgarth may be regarded as a separate section. In 
this neighbourhood the region of high Vertical Force is very narrow, and indeed its 
existence has only been directly proved during our recent work. 
In our previous paper a narrow region of low Vertical Force was shown in Plate 13 
as joining Manchester and Appleby, the connecting station being Stonyhurst. 
At that time we had only the record of the Stonyhurst observatory at our disposal, 
which had not been reduced to our survey standard by direct comparison with our 
instruments. We have since been allowed by the courtesy of the Bev. Father 
SiDGREAVES to make observations at Stonyhurst, with the result that the Vertical 
Disturbing Force comes out a small instead of a large negative quantity. This is a 
striking example of the necessity of direct comparison between the instruments used, 
if accurate results are to be obtained. 
Apart, however, from the magnitude of the Disturbances at Stonyhurst, the main 
ridge line has now been traced across the supposed magnetic valley, and the regions 
of low Vertical Force to the north and south of it respectively have been clearly 
separated by a line of stations of positive Vertical Disturbing Force. It will, however, 
be observed that the Horizontal Disturbances at the coast stations are directed east, 
and it is therefore probable that the line we are discussing is not continued under the 
sea, but turns northwards towards the English lakes, though a spur may project 
southwards from near Kendal. 
To the east of the line which joins the low' Vertical Force Stations, Giggleswick 
and Aysgarth, the Vertical Disturbing Forces increase rapidly, reaching a maximum 
at Harrogate, near to which is a focus or centre of attraction. 
At this point the ridge is crossed by another, which can be clearly traced by means 
of the Vertical Disturbing Forces from Pickering in the East Biding, to Rochdale in 
Lancashire. 
The Horizontal Forces show that it runs to the east of the latter place, but to 
trace it further, at present, would lead us away from the main ridge line. We have, 
therefore, taken advantage of some very faint indications of a line of relatively low 
Vertical Force, running from Leeds to Stonyhurst, to separate—for the purposes of 
discussion—the regions to the north and south of it. But as the physical reality of 
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