Electric Currents in the United Kingdom. 103 



hypothetical currents are due to small errors in the determi- 

 nation of the exact positions of the lines. 



In the final calculation of the results of our survey, we have 

 taken the means of the positions of these two sets of lines as 

 the isomagnetic lines for 1891, hence the mean values of the 

 currents deduced from them by the 



(4) Mean 1891 Curves 



are +0'001 and — O005 ampere per square kilometre for 

 the a and ft circuits respectively. 



(5) The True 1891 Curves. 



We have further checked the above results by means of the 

 true curves, taking two circuits — one (7) in England and 

 Scotland, and the other (8) in Ireland. 



The first of these was as follows : — 



Long. 1° E. from lat, 51° to lat 53°. 

 Lat. 53° from long. 1° E. to 1° W. 

 Long. 1° W. from lat. 53° to 55°. 

 Lat. 55° from long. 1° W. to 3° W. 

 Long. 3° W. from lat, 55° to 53°. 

 Lat. 53° from long. 3° W. to 4° W. 

 Long. 4° W. from lat. 53° to 51°. 

 Lat. 51° from long. 4° W. to 1° E. 



The area is 1*054 x 10 5 square kilometres. The values 

 of the horizontal force and declination for every 10 / of 

 latitude or longitude were read on from the maps on which 

 the values at the different stations were entered, and the 

 true isomagnetics drawn. This operation was performed 

 by Messrs. Kay and Whalley and checked by myself. The 

 northerly or westerly component of the force was then calcu- 

 lated for each of these points, and the average value for each 

 short section was assumed to be equal to the mean of the 

 values at its initial and final points. 



No difficulty arose except at a point in Wales, where the 

 curves are closed, and where it was therefore necessary to 

 assume an average value for a section of the line on which a 

 maximum occurred. 



The result of the calculation gave a current of — 0'008 

 ampere per square kilometre. 



The second circuit was taken in Ireland. It traversed the 

 district of Antrim, in which there are violent local disturb- 

 ances, and is interesting chiefly as showing to what extent 

 the result may be affected by such causes. 



