10 
MR. GEORGE W. WALKER ON THE MAGNETIC RE-SURVEY OF THE 
the extent of my survey (two years) an elaborate system of correction was hardly 
justified. 
On the whole it appeared that I had to choose between assuming (l) that the 
Greenwich variations are applicable to the whole of the British Isles; or (2) entering 
on a somewhat speculative and elaborate system of corrections. I decided to make 
the former assumption. While some error may thus remain in the values far North 
and far West, the results are left in a form such that they may be most readily 
corrected in the future, when the relationship between the variations at any point 
and those at Greenwich has been investigated. 
The correction of the field results to epoch now becomes very simple. We simply 
have to add, to the value of the element as observed at a station, the increase in the 
value of that element as observed at Greenwich from the time of the observation to 
the epoch. In order that the final values should be expressed in terms of the 
Greenwich standard the difference between the Survey instrument and that at 
Greenwich has been incorporated in the correction. 
In the table of Horizontal Force I have included the observed values of Wq, the 
magnet moment of magnet 66a at 0“ C. These are of interest as showing how well 
the moment was maintained in spite of the large distance travelled and the varied 
conditions of temperatu-re experienced. They are also of value as giving some indica¬ 
tion of the degree of accuracy obtained from day to day. 
All the values of H were obtained by the Unifilar with the exception of those at the 
five stations Drogheda, Llandudno, Birkenhead, Stoke-on-Trent and Coalville, which 
are starred. At these places I was unable to get the Unifilar observations carried 
out, and the values have been assigned from the readings of the portable variometer 
for measuring differences of H which I carried with me in 1915 and which has been 
described in ‘ Roy. Soc. Proc.’ for 1916. 
In Tables I., II. and III. the results entered as “ observed ” are, of course, deduced 
from the field observations, and in the case of Force and Inclination involve a con¬ 
siderable amount of computation. Such computation cannot well be reproduced in- 
detail, but if any doubt arises in future as to the accuracy of an “ observed ” number, 
it may be stated that the original observation books are preserved in the Royal 
Society’s archives, and that the copies and tables of Greenwich corrections are 
preserved by the Ordnance Survey Oflice, Southampton. 
