SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1886. 247 
(3.) Lines of equal Horizontal Force. 
These lines were treated in the same way as the isogonals and isoclinals, and in the 
following Table the constants for the nine districts are tabulated as before. As Professor 
Balfour Stewart did not deduce the lines of equal Horizontal Force from Mr. Welsh’s 
observations, there was not as much reason in this case as in the others for taking the 
whole of Scotland as District I. We therefore included in this district only all 
stations north of lat. 56°. 
District. 
Central Station. 
H ' 
_ dB. 
® dl ' 
II 
Lat. N. 
Long. W. 
I. 
5°7 26-3 
4 30h 
1-5580 
-0-03930 
-0-00575 
II. 
55 27-3 
3 41-6 
1-6363 
-0-0.3324 
-0-00882 
III. 
53 26-7 
2 26-0 
1-7164 
-0-03868 
-0-00661 
IV. 
51 47-7 
0 17-4 
1-7970 
-0-04374 
-0-00620 
V. 
.54 2-9 
7 36-5 
1-6650 
-0-04.380 
-0-01046 
VI. 
53 29-0 
5 43'0 
1-6979 
-0-03771 
-0-00439 
VII. 
50 49-3 
2 57-6 
1-8212 
-0-04134 
-0-00698 
VIII. 
52 57T 
8 13-1 
1-7053 
-0-04237 
-0-00628 
IX. 
51 49-6 
4 47*4 
1-7694 
-0-04212 
-0-00598 
By means of these data a Table like Table VI., pp. 238-9, was prepared, and lines 
of equal Horizontal Force were drawn on curve paper as in the case of the isoclinals (see 
p. 245). These at once showed that the lines above and below that corresponding to 
1'7 units were differently disposed. The mean direction of each of the southern lines 
can be accurately represented by a linear function of the latitude and longitude, their 
departure from parallelism is not great, and their mean distances are nearly the same. 
On the other hand, though the lines inDistrict I., i.e., in the extreme north of Scotland, 
are parallel to the 1‘7 line, which runs from the neighbourhood of Miltown Malbay to 
that of Scarborough, the intermediate isodyjiamics make smaller angles with the geo¬ 
graphical meridian, and their average distance is greater than in the south. It is, 
however, difficult to decide what is the direction of the terrestiial lines. Too much 
weight must not be attached to the fact that the lines in the north of Scotland agree 
with those in the Midlands and south of England, as they are deduced from a single 
district of irregular form, and the seat of great local and I'egional disturbances. There 
IS also reason to suppose from what is known of the lines of equal Horizontal Force on 
the continent that the distance between them increases towards the east, and those 
crossing England are so represented on M. Moureaux’s map. It may well be, there¬ 
fore, that the diverging lines are those which agree most closely with the terrestrial 
lines of equal Horizontal Force. 
