524 
MR. A. W. RUCKER AND DR. T. E. THORPE ON A MAGNETIC 
which two such detailed investigations of the magnetic peculiarities of the same 
district have been made at so short an interval of time, and, therefore, affords a good 
opportunity for estimating the accuracy of the methods. 
For this purpose the secular change in different districts must be determined 
independently of the terrestrial lines. 
In choosing a common epoch we have been influenced by the fact that as the 
number of stations in the 1886 survey is comparatively small, a great saving of labour 
is effected if we reduce the 1886 survey to the epoch 1891‘0, instead of choosing an 
intermediate date, which would have involved applying corrections to all the results 
obtained in both surveys. The sj^stematic differences (if any exist) will be the same 
in either case, and the more laborious method has, therefore, no counterbalancing 
advantage to recommend it. 
It has, therefore, been necessary to determine the secular change between 1886'0 
and 1891 '0 as accurately as possible. 
Three sets of data have been utilized for this purpose, which we now proceed to 
discuss in detail. 
(1.) Secular ChaiKje from Central Stations. 
The central stations in the nine districts from which the district lines were deter¬ 
mined differ but little in position in the two surveys. Tire small corrections necessary 
to transfer the values of the elements obtained at the central stations of the 1886 
survey to the positions of the central stations in 1891 cannot introduce important 
errors. Hence the secular change can be accurately determined at these nine points 
from the results of observation alone. 
The data thus obtained are summed up below. 
Tables VIII. to X. (“ 1890 Memoir”) are used in calculating the changes in the 
magnetic elements corresponding to the slight alterations in the positions of the 
central stations. 
