242 
MR. A. W. RtiCKER ARD DR. T. E. THORPE ON A MAGNETIC 
These are in much better accord than those obtained from the general formula, 
and the conclusions which may be drawn from them are that in latitude 50°, 
(1.) M. Moureaux’s lines are certainly not too far to the west ; 
(2.) And our 17° isogonal is not too far to the west. 
Both these conclusions are important. M. Moureaux believes that there is what 
we should call a considerable regional disturbance in Brittany. The isogonals drawn- 
by him sweep out to the west in the western part of France and do not resume their 
normal course until they have reached the English coast. In the English Chaimel, 
therefore, they are deflected to the west, and as both our district lines, and to a much 
more marked extent our terrestrial lines are still further west, it is evident that 
M. Moureaux has not in any way exaggerated the westerly tendency of the lines in 
the wmstern parts of the Channel, and that, therefore, our observations tend to confirm 
his view. It is probable that our terrestrial curves are in this district a little too far 
to the west, but it must be remembered that lat. 50° is almo.st outside the region of our 
survey, and that in the Channel Isles, which with Cherbourg, are our only stations 
to the soutli of it, there are considerable disturbances. 
In fig. 2 the continuous lines are our terrestrial isogonals, which on January 1, 
1886, corresponded to M. Moureaux’s 17°, 18°, etc., isogonals on January 1, 1885, 
the secular change being about 7t M. Moureaux’s curves are shown with dashes 
and dots, and the hypothetical connections in dots. A satisfactory comparison on the 
borders of the areas of two surveys cannot, however, be made unless there is a closer 
agreement in the methods of working up the results of the observations than there 
is between our own and M. Moureaux’s. 
Coming next to the 17° isogonal, we find that on the coast of Norfolk the calculated 
Declinations are generally much lowmr than the observed values. This would be 
remedied by moving the 17° isogonal further to the east, but the fact that it is 
already to the east of M. Moureaux’s line is a strong argument against such a course. 
It is noticeable that Sir Fred. Evans appears to have found some difficulty in 
this part of the country ('Phil. Trans.,’ 1872, vol. 162, p. 330). His curves are shown 
in Plate II. The points at which the isogonals cut lat. 52° are given below, 
and it will be seen that the distance between them is a maximum in the centre 
of England. 
Isogonal (1872). 
Long. 
Difference of 
Long. 
o 
25 
o 
9-54 W. 
o 
24 
7-86 
1-68 
23 
GT6 
1-70 
22 
4-40 
1-76 
21 
2-40 
2-00 
20 
0'40 
2-00 
19 
1-47 E. 
1-87 
