SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY 1, 1886. 241 
In District III. we included Stonyhurst ; in District IV. Greenwich, Kew, and 
Berck- sur-Mer, and in District VII., Cherbourg. The data for the French stations 
were those given by M. Moureaux, reduced, of course, to January 1, 1886. 
The values of the constants in the equation were not finally chosen until the 
Declination had been determined by means of a preliminary formula closely agreeing 
with that given above for all the points indicated in Table VI., pp. 238-9, and the 
values given by it and by the district curves compared. Over the greater part of the 
country the agreement is extremely close. It is not necessary to reproduce the numbers 
here, as in Plate II. we have plotted down both the broken district curves, and 
also the smooth curves given by the equation. The former are dotted, the latter are 
continuous lines. The agreement is all that can be desired, except, perhaps, on the 
coast of Norfolk, and on the west and north coasts of Scotland. 
Taking, however, the English Channel first into consideration, we have calculated 
the points at which our 16° 53', 17° 53', etc., isogonals cut lat. 50°. AVhen secular 
change is thus allowed for, these correspond to the positions of the 17°, 18° isogonals 
on January 1, 1885, i.e., with those of M. Moureaux. We have also measured the 
corresponding points from his map. The result is shown in the following Table :— 
Isogonal 
Jan. 1, 1885. 
Longitude from Greenwich. 
R. and T. 
Moureaux. 
o 
17 
1 iV E. 
i 9 E. 
18 
0 55 AV. 
0 57 W. 
19 
3 7 AV. 
2 55 W. 
20 
5 19 AY. 
4 58 AV. 
The 20 ° isogonal does not pass through France, and, therefore, M. Moureaux’s 
map is not of special authority on its direction, but there seems no doubt that while 
the 18° isogonals cut latitude 50° at the same point, there is a considerable divergence 
between the others. 
According to the general formula, 27'’35 is the change of Declination per degree of 
longitude on lat. 50°. The two Channel Districts in Table V., p. 236, viz., VII. 
‘ and IV. agree in giving 28''9 for this number, and if we take this as correct and 
adhere to 55' W. for the 18° isogonal, we obtain the following values :—- 
Isogonal. 
R.and T. 
Moureaux. 
O 
O / 
o / 
17 
1 10 E. 
1 9 E. 
18 
0 55 AV. 
0 57 AV. 
19 
3 0 AV. 
2 55 AV. 
20 
! 
5 5 W. 
4 58 AV. 
MDCCCXC.—A. 
O 
I 
