l62 



NA TURE 



[December i6, 1897 



in the United Kingdom which are bounded by lines of latitude 

 and longitude corresponding to whole degrees. 



Thus, if starting on the meridian of Greenwich a traveller 

 were to go due north from lat. 51° to 52°, that is from mid- 

 Sussex to the north of Hertfordshire, then were to go due west 

 until long. 1° W. was reached near Buckingham, thence due 

 south along long. 1°, until when near Petersfield he turned 

 homewards due east along lat. 51°, his route would include an 

 area of, in round numbers, 2800 square miles, or of about 7000 

 square kilometres. In each such circuit the average current 

 expressed in hundredths of an ampere per square kilometre has 

 been determined, and the results are shown on maps both for 

 i886 and for 1891. 



These maps are given in Fig. 2, A and B. The numbers 

 indicate the average flow in hundredths of an ampere per square 



north, in the west and south, while in the midlands and the 

 east the general tendency is from above to below. 



But in spite of this apparent agreement, I am very doubtful 

 whether these conclusions can be trusted. In the first place the 

 currents are very minute. The whole flow of electricity passing 

 through an area of 2800 square miles is less than that con- 

 centrated by Prof. Moissan in a few square inches within an 

 electric furnace. The forces to be measured are so small that 

 they must be seriously affected by the inevitable errors of 

 observation and reduction. 



Again, the observations which were made at nearly 900 places 

 scattered all over the kingdom, are affected by local disturb- 

 ances, due to quite other causes than those we are now discussing, 

 and the magnetic state of the whole area, such as it would be if 

 these disturbances were removed, can only be deduced by an 



H W 9 8 7 6 5 4 3 2 ' 



e~ i-~l 



SURVEY OF 1886. 



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Fig. 2, A. 



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SURVEYS or 189J vvo 18S(^ 



kilometre. The figures are underlined where the flow of the 

 current is from above to below. In other cases the direction is 

 upwards. 



Fig. 2, A, shows the result for January i, 1886, deduced from 

 the 200 stations which were then available. In Fig. 2, B, all 

 the facts obtained in the two surveys are worked up to a final 

 result for the date January i, 1891. 



If we compare the two maps thus obtained from the two 

 surveys, the conclusions arrived at are, in some respects, not 

 very different. In both the larger currents occur near the 

 boundaries of the land area to which the observations were 

 necessarily confined. If the maps are to be trusted, the 

 largest currents exist in the extreme north of Scotland, in the 

 east of England, and in the far west of Ireland. It is also in 

 favour of the trustworthiness of the results that in both cases 

 the upward currents occur in the same parts of the kingdom. 

 The figures indicate that the currents flow upwards in the far 



elaborate .«.ystem of averaging the results obtained at different 

 places. This process of taking means is least accurate near the 

 boundaries of the survey, and thus the larger currents which 

 are indicated near the shores of our islands have probably no 

 real physical existence, but are due only to the relative un- 

 certainty of our knowledge of the magnetic slate of the par- 

 ticular localities in which they appear to flow. From this point 

 of view, therefore, it appears to be unsafe to trust to any 

 particular figure, and that a better result will be obtained if we 

 deal with larger areas and content ourselves with taking the 

 mean of all the currents which appear to flow within them 

 through the surface of the earth. 



Adopting this plan, the general conclusions to be drawn from 

 the two maps are very nearly identical. If for the moment we 

 neglect the question as to whether the currents are flowing up 

 or down, their average magnitude in any considerable area in the 

 United Kingdom is about five-hundredths of an ampere per 



NO. 1468, VOL. 57] 



