104 Prof. A. W. Riicker on Vertical Earth- Air 



The circuit was as follows : — 



Long. 6° 30' W. from lat. 52° to lat. 55°. 

 Lat. 55° from long. 6° 30' to long. 8°. 

 Long. 8° from lat. 55° to 54°. 

 Lat. 54° from long. 8° to 9°. 

 Long. 9° from lat. 54° to lat. 52°. 

 Lat. 52° from long 9° to long. 6° 30' W. 

 The area is 48 x 10 3 square kilometres. 

 The current is —0*046 ampere per square kilometre. 

 The fact that these different circuits, including areas of 

 very different magnitudes and situated in different parts of the 

 United Kingdom, all give very small values for the hypo- 

 thetical currents, is strong evidence that the smallness of the 

 calculated numbers is not due to the fact that large positive 

 and negative values mutually cancel each other. It is, for 

 instance, conceivable that the directions of the current- flow 

 might be opposed on what may be called the oceanic and 

 continental sides of the kingdom. 



If this were so, it is probable that circuits 7 and S would 

 have given results of opposite signs. By way of testing the 

 matter further, the current was calculated both from the 1886 

 and the 1891 lines for the relatively small area in the West 

 of Ireland bounded by latitudes 52° and 53°, and longitudes 

 9° and 10°. 



The results were : — 



for 1886, —0*04 ampere per square kilometre, 

 „ 1891, +0-11 

 Hence the difference of direction which characterized the 

 currents deduced from the two surveys when applied to large 

 areas, also distinguishes, and in an exaggerated degree, the 

 results obtained from a small border district. It is therefore 

 evident that either the distribution of the vertical currents 

 has entirely altered in five years, or they are too small to be 

 detected by the method employed. 



The former of these hypotheses is negatived by the fact 

 that different calculations, based on the first and second 

 survey 1891 curves respectively, lead to discordant results for 

 the same date, and we are therefore compelled to fall back 

 upon the second alternative. 



Effect of the Ellipticity of the Earth. 



The question may fairly be raised whether, in dealing with 

 such minute quantities, the ellipticity of the earth ought not 

 to be taken into account. 



In answer to this, it may be stated that the work done 



