72 ME. G. W. WALKER ON THE MAGNETIC RE-SURVEY OF THE BRITISH ISLES, ETC. 
the same principle to the construction of a similar apparatus for the vertical 
component. Even if the instruments required comparison with a standard from 
time to time, their introduction would much reduce the field work of the survey and 
give greater accuracy. They would be specially useful in making detailed surveys 
of special regions and in proving if a station is really representative and not subject 
to disturbance of very local origin. 
