BRITISH ISLES FOR THE EPOCH JANUARY 1, 1915. 
23 
Reduction of the Survey. 
As already indicated, observations at several points in the Western and Channel 
Islands will have to be made when the war is over. Inasmuch as the Western Isles 
are known to be disturbed magnetically and the Channel Isles are mainly of interest 
in connecting up with the mainland of France, no grave objection can be taken 
to analysing the main features of the magnetic state of the British Isles from the 
results already secured. On the contrary, certain advantages may arise from such 
procedure. 
The principle adopted for reducing tlie observed results to epoch has already been 
explained. It is, however, obvious that Greenwich Observatory could not supply 
final values for 1915 until some time after that year was completed. In view of the 
large amount of computation involved in reducing the survey, it appeared to me 
desirable to save time by adopting provisional Greenwich values for January 1, 1915, 
which might be expected to be very close to the final values. 
These provisional values were 
H = 18,520y, D == 15° 2''0 W, I = 66° 52'-0 
and the Tables I., II., III., were deduced for these values. 
The final values of Greenwich Observatory are 
H = 18,519y, D = 15° l'-4 W, I = 66° 51'-6, 
so that in order to obtain final values for the Survey the numbers in Tables I. to VI. 
require corrections— 
AH = -ly, AD = -0'-6 W, AI = -O'A 
AN = Oy, AW^ = —4y, AV = —17y. 
These corrections do not, of course, affect the values of disturbing forces. 
Buckee and Thoepe divided the British Isles into nine overlapping districts. 
There is no doubt that such a course tends to diminish discontinuities between the 
districts and in a first general conspectus it may be advantageous. But a smoothing 
process is always attended with some danger, and after careful consideration I decided 
to group the observations into non-overlapping districts. There is, in the event, 
no reason to think that this departure was unwise. The question at once arose as to 
what should be the districts. Now this is a matter of some importance. A priori 
an ideal survey would require stations forming a system of equilateral triangles. 
The stations were not so selected. Still it appeared to me desirable to choose districts 
so that they contained roughly the same number of stations, the same area, and 
bounded by whole degrees of latitude and longitude. Naturally we must compromise, 
but the division adopted appears to me quite a good compromise with the stations 
originally chosen by Ruckee and Thoepe. 
