SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1891. 
23 
Year. 
Corrections applied to Magnetometers. 
60 
61 
70 
73 
1884-88 
0 
0 
1889 
+ 7 
+ 8 
1890 
-tl3 
+ 16 
+ 46 
1891 
+ 13 
+ 16 
+ 46 
1892 
+ 15 
+ 24 
+ 38 
-1-29 
Comijarison at Kew of Dip Observations made with the Survey and Kew Instruments. 
In our earlier survey, we made at Kew a number of Dip Observations sufficient to 
prove that our two instruments were in accord, but we compared them only with 
the absolute measurements made with the Kew Dip Circle as they were at that time 
published, i.e., uncorrected for diurnal variation or disturbance. As the corrections 
are small, this was sufficient for our purpose, and it is further justified by the fact 
that the measurements made at Kew with all the Dip Circles we have since- 
employed have proved to be in such close agreement that no corrections have been 
considered necessary to reduce them to a common standard. 
In our later work, however, we have adopted the plan of comparing our uncorrected 
Dip Observations with the angle whose tangent is the ratio of the Vertical and 
Horizontal Forces, as given by the magnetograph at the time of the Dip Observations. 
It is evident that this method introduces the errors of the base lines of the self¬ 
registering instruments, while it has the advantage of making the indications 
compared with those of the Dip Circle contemporaneous with the Dip Observations. 
The result is to bring the Survey Dip Circles into close agreement with the Kew 
Instruments used in this way. 
