8 
ME. GEOEGE W. WALKEE ON THE MAGNETIC EE-SUEVEY OF THE 
The survey values of H ought to be reduced by 2y in view of the determination 
of the value of log (l+P/r^ + Qr^) referred to, and it is possible that a further 
reduction might be made if the better form is (l+P7r^). But without expressing 
an opinion as to whether the Greenwich values are more correct in an absolute 
sense than the survey values, the only rational course in reducing the survey to 
epoch was to adopt the Greenwich standards, so that if in the future these require 
change the magnetic survey as reduced will be changed in a simple and definite 
way. 
We now compare the Cambridge observations with the corresponding Greenwich 
values. 
Declination. 
1913. 
1914. 
1915. 
November 10. 
November 18. 
1 
November 19. 
November 24. 
November 25. 
May 25. 
September 25. 
March 24. 
April 22. 
Cambridge survey value . . . 
15 19'-9 
19'-0 
17'-8 
2l'-0 
IS-7 
13-1 
9'-9 
o / 
6-0 
2'-4 
Greenwich value. 
15 14-0 
13-1 
13-9 
14-8 
14-0 
9-9 
4-9 
14 59-6 
57-4 
Difference . 
+ 5-9 
5-9 
3-9 
6-2 
4-7 
3-2 
5-0 
6-4 
5-0 
Mean 
. +5-1 
Divergence . 
+ d-8 
+ 0-8 
- l'-2 
+ l'-l 
-0-4 
-T-9 
-0-1 
+ 1-3 
-o'-i 
Horizontal Force. 
1913. 
1914. 
1915. 
November 10. 
1 
November 28. 
December 2. 
1 
December 3. 
December 8. 
May 25. 
September 25. 
March 24. 
April 22. 
Cambridge survey value . 
18,162 
18,188 
18,168 
18,161 
18,174 
18,176 
18,190 
18,137 
18,136 
Greenwich value. 
18,505 
18,518 
18,527 
18,518 
18,516 
18,514 
18,526 
18,486 
18,484 
Difference . 
343 
330 
359 
357 
342 
338 
336 
349 
348 
Mean 
. 
345 
Divergence ..... 
+ 2 
+ 15 
- 14 
- 12 
+ 3 
+ 7 
+ 9 
-4 
-3 
