62 
MR. A. W. RtiCKER AND DR. T. E. THORPE ON A MAGNETIC 
depended on the standard. With the exception of two or three cases, which are 
referred to in the detailed account of the observations at the stations affected, we do 
not think that any error exceeding a minute of arc can have been introduced into the 
declinations from uncertainty as to the true time. 
Selection of Stations. 
In selecting stations we have aimed at uniformity of distribution over the whole 
area under investigation, and, subject to this condition, have chosen places at which 
observations have previously been made. We have also avoided situations where 
disturbance by so-called local attraction was known to be great, except in special 
cases to be discussed hereafter. On many tours we have carried geological maps 
which have aided us in choosing sites. 
In all we have made observations at 54 stations in Scotland, 102 in England, 
Wales,'" and the Channel Isles, and 44 in Ireland. At many of these places which 
are counted as a single station our instruments have been set up in different localities 
in order to study or eliminate the local effects of magnetic rocks. These sub-stations 
were in some instances only a few yards and in others several miles apart. The more 
important of them are indicated by letters in the lists given hereafter, and if these 
are added they bring the number of stations up to 213. In addition to these we 
observed on the island of Canna at 23 places. If we took minor changes of position 
into account this total would be considerably increased, and as we have been able to 
use the observations made at Greenwich and Stonyhurst, and the data for Cherbourg 
and Berck-sur-Mer, furnished by the survey of France, recently completed by 
M. Moureaux (“ Determination des Elements Magnetiques en France,” par M. Th. 
Moureaux. Paris: Gauthier-Villars, 1886), our conclusions are based upon 
observations made at about 250 different places. 
The average distance apart in normal districts is about 30 miles. At most of the 
principal stations we made two Dip observations, one with each needle, and a com¬ 
plete set of measurements for the determination of the Declination and Horizontal 
Force. At many places we determined the Declination twice, making two indepen¬ 
dent sets of observations on the Sim and the needle. In some cases only the 
geographical meridian or only the direction of the magnetic axis of the ngedle was 
re-determined. When time was short or the weather unfavourable the deflection 
experiment was omitted. The following Table shows the total number of observations 
made. By magnetic meridian we mean determination of the direction of the magnetic 
axis, which, when combined with the Sun observations or geographical meridian, gives 
tlie Declination. We do not include 23 Declinations taken on Canna by means of an 
azimuth compass and chart. 
* Exclu-sive of five supplementary stations along the valley of the Wye, at which Diji observations 
only were made in 1889 (vide p. 84). 
