504 
MR, A. W. RUCKER AUD DR. T. E. THORPE OH A MAGNETIC 
CALCULxVTrON OF THE IsOMAGNETIC LiNES. 
(1) The Isogonal Lines. 
The plan adopted for the determination of the terrestrial isogonals ^Yas precisely 
the same as that employed in the first survey. 
The country was divided into the same nine overlapping districts. The stations 
were weighted so as to make the weighted number of stations per unit of area about 
the same everywhere. 
The point corresponding to the means of the weighted latitudes and longitudes of 
all the stations in a district was called the central station, and the mean of the 
weighted declinations was taken as the declination at that point. 
The. rates of change of the declination per degree of latitude and longitude were 
then calculated on the supposition that they were uniform over the whole of each 
district. 
The numerical calculations were heavy, and were entirely carried out by j\[essrs. 
Gray and Watson. At first the Scotch stations were divided into two groups of 
about seventy each, uniformly distributed over the country. The results obtained 
were as follows : — 
Central station. 
Declination. 
dc 
dc 
Latitude {Iq). 
Longitude (Aq). 
^0- 
dl ■ 
d\' 
56 43 0 
56 33 1 
4 22-2 
4 20-8 
20 .58-9 
20 57-9 
13-3 
13-1 
30-7 
33-9 
We had hoped that labour might have l)een saved by using only some of the stations 
in the formation of the terrestrial lines, but the above values of dh'dX were hardly 
in sufficient accord to justify such a course. Nearly all the stations were therefore 
used at which observations had been made since the previous survey up to that time 
(January, 1892), the total number thus employed being about 450. About ten 
stations at which great magnetic disturbances were known to exist were omitted.’^ 
The following table contains the boundaries of the nine districts, the latitudes and 
longitudes of the central stations, the values of the changes of declination per degree 
of latitude and longitude {dS'jdl and dS'/dX), both expressed in minutes of arc. 
* One or two stations were included at wliicli there are large disturbances which had not been 
detected at the time when the calculations were made, as the normal values could not be determined 
before the terrestrial lines were drawn. It is probable that the differences between the results given by 
the two groujis of stations would have been much reduced if these stations had been omitted. 
