BEITISH ISLES FOR THE EPOCH JANUARY 1 , 19in. 
29 
I ought to mention that in the solution adopted the mean longitude differs by 
0''3 from the mean for the districts. This really arose from the fact that after a large 
amount of reduction had been done the longitude of Worthing was found to have 
been entered with the wrong sign. The values for District VIII. had to he re-com¬ 
puted, but the net result for the British Isles was as stated above. The change in no 
case amounts to ly except by the turn of the decimal point, so that the original 
solution has been retained. In any case it is an empirical solution. But I venture 
to think it cannot be substantially improved until detailed survey of clearly disturbed 
regions has been made. 
The solution is then 
N = 16,714-6-864 AX-+-1-633 A/, 
W= 5,163-0-726 AX-2-283 A/, 
V = 44,601 + 7-297 AX-1-633 A/, 
where AX and A? are the differences of latitude and longitude in minutes of arc 
measured from 
Xo = 53° 3B-8 N, I, = 3° 23'-0 W. 
The charts (Nos. 1, 2 and 3) of equal values of N, W and V have been drawn at 
convenient intervals. In order to show how the general solution differs from the 
corresponding district solution, extra isomagnetics representing the mean for each 
district, have been drawn both from the general solution and the district solution, 
the latter being shown by a dotted line. 
Although lizies of equal Declination liave little theoretical importance, yet they are 
of considerable practical utility, and therefore a chart (No. 4) of equal declination 
has been prepared. 
The lines have been computed from the general potential solution by the formula 
tan D = W/N. 
The approximate solution, correct to the first power of AX and AZ, viz., 
D = 17° 10'+0-00436 AX-0-00872 AZ, 
where D is in degrees and AX and AZ in minutes of arc from the mean co-ordinates 
53° 3B-8 N, 3° 23'-0 W, is not accurate enough for the whole of the British Isles. 
On the chart are shown the differences between the observed and the calculated 
values of D in minutes of arc. I have further indicated the probable positions of the 
true isogonals. At some places ambiguity arises on account of the complexity of the 
disturbing forces, and the line drawn is thus a personal guess at a simple solution 
with which others may not agree. 
