6 
ME. GEOEGE W. WALKEE ON THE MAGNETIC EE-SUEVEY OF THE 
azimuth obtained when the A point of the needle dipping with face to instrument 
face was on the cross-wire of the lower microscope when set at 90° in the vertical 
circle. (An error of 1° in azimuth means an error of 0'‘2 in Inclination.) Further, I 
saw no adequate gain in using two needles. Accordingly I used only one needle 
(32 readings), but I carried a second needle in reserve so as to be fresh if the first 
should go wrong. I found this precaution valuable twice in 1914, viz., at King’s 
Lynn and Portree, but unnecessary in 1915. 
With regard to accuracy the azimuths given by the O.S.O. are believed to be not 
more than 0'’5 in error, and so I think the absolute Declinations ought not normally 
to be more than 1' wrong. 
In Horizontal Force I consider that the absolute values are normally not more 
than 15y wrong. The unit of Force is ly = 10~® gauss. 
In Inclination it would be unsafe to suppose that the absolute values are correct to 
less- than P, and from Portree to Oban I suspect they may be several minutes 
wrong. 
The following constants were used in calculating the force :— 
Induction coefficient, /x . . log m = 0’83793. 
Temperature coefficient of magnetic moment = O'OOOGO per 1° C. 
Moment of inertia, K . . log tt^K = 3‘42610-t-log (l-t-0‘000022^), 
where t = temperature centigrade. 
Deflexion distance r = 24'9977 (l 4-0’000017^). 
log (1 + P/r2+Q/D) = 0-00342. 
When a station had been “ observed ” the calculations were made, and a copy of 
the data sent to the O.S.O. for preservation and verification of my calculations. The 
O.S.O. then sent enquiry slips to Greenwich for the corresponding magnetic- values 
at the times of my observations. The variation of H at Greenwich between the^ 
vibration and deflexion experiments was assumed to be applicable at all points in the 
British Isles, and was used to correct the observed value at the station. 
We may now consider the comparative results at Greenwich. From the tables we 
have the following :— 
Declination. 
January 14, 1914. 
January 15, 1914. 
May 14, 1914. 
Sm-vey value. 
o / 
15 11-7 
1°5 ll'-O 
15 s'-8 
Greenwich value. 
15 12-3 
15 12-2 
15 9-4 
Difference. 
- 0-6 
- 1-2 
-0-6 
Mean . 
- o '-8 
