SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1886. 
237 
same coordinates have been employed as he used (the geographical-mile system), hence 
the values of dS'ldl and dS'/dX can only be considered as applying to the central stations. 
By means of the first the Declination at any point on the meridian through that 
station can be calculated, and for other points on the pai’allel of latitude passing 
through such a point, the formula y = //„ cos Ij cos must be employed where y^^ 
and /q refer to the central station. In all the other districts the values of x and y 
are valid for the whole district. 
By means of these formulae the Declination was cailculated for all points within the 
United Kingdom defined by whole degrees of longitude and half degrees of latitude, 
e.g., for lat. 50° 30', long. 2° E., 1° E., 0, 1° W., and so on. Where the districts 
overlapped the means of the numbers thus obtained were taken. All these values 
are given in the following table. The figures in brackets at the end of a row 
indicate the number of the district from which it was deduced. Where two or more 
districts overlap, the individual declinations are given in italics and the mean in 
ordinary type. 
Throughout the central j)arts of the kingdom the agreement between the numbers 
given by the linear formulre proper to different districts is sufficiently close to leave 
little doubt that the mean cannot be more than l' or 2' wrong. Where greater 
differences appear it is generally easy to account for them. Thus, lat. 53° 30', 
long. 10° W. is in the highly disturbed region in the west of Galway. Lat. 52° 30', 
long. 0°, is close to a remarkable and hitherto unsuspected disturbance in the eastern 
counties, of which we shall have more to say hereafter. The large differences on the 
border of Districts I. and II. (lat. 56° 30') are, perhaps, in part due to the irregu¬ 
larities in the shapes of these districts owing to which the formulas are not obtained 
under favourable conditions. Both the other elements, however, agree with the 
Declination in indicating violent local disturbance in this region, and there can be no 
doubt that the discrepancies are due to a physical cause. 
