MAGNETIC SURVEY OF GREAT BRITAIN. 51 



science, the actual state of the phaenoniena of the magnetic dip 

 and intensity in the British islands ; and furnishing for distant 

 times the means of a comparison, whereby the secular changes 

 of these elements may be correctly judged of. 



It has been found convenient to divide the report into two 

 parts, the first comprising the observations of the Dip, the se- 

 cond those of the Intensity. 



Division I. — Dip. 



In the memoir on the magnetical observations in Ireland 

 (British Association Reports, vol. v.), Mr. Lloyd has noticed 

 the discrepancies which have been occasionally found in the re- 

 sults of observations of the dip made at the same station with 

 different instruments. The observations of Captain Ross at 

 Westbourne Green, which are there related, place these discre- 

 pancies in the strongest light. Captain Ross employed eight 

 needles, making from eight to ten observations with each, each 

 observation consisting of eighty readings; i. e. of ten in each 

 of the eight usual positions. The dip at Westbourne Green, 

 resulting from each of these needles considered separately, va- 

 ried from 69° 01'-5 to 69° 42'-6. On these discordances Mr. 

 Lloyd remarks as follows : "Thus it appears that there is a dif- 

 ference amounting to 41' in the results of two of the needles 

 used; and that the diffei-ence is very far beyond the limits of the 

 errors of observation, will appear from the fact, that the extreme 

 difference in the partial results with one of these needles, B (1), 

 does not amount to 4'^, while with the other, (P), the extreme 

 difference is only 2'. In fact, it so happens, that these very 

 needles which differ most widely in their vnecm results are those 

 in which the accordance of the partial results is most complete. 

 Of the eight results obtained with needle P, there is one only 

 which differs from the mean of the eight by a single minute; 

 and yet the mean of all the observations with this needle differs 

 by more than 20' from the mean of any of the others, while its 

 excess above the mean of tlie entire series amounts to 25'. 



" These differences cannot be ascribed to any partial mag- 

 netism in the apparatus, for three of the needles (I, P and R) 

 were of the same dimensions, and were used with the same cir- 

 cle, and yet their results, as we see, are widely discordant. We 

 must seek then in the needles themselves the cause of these 

 perplexing discrepancies; and we are forced to conclude that 

 there may exist, even in the best needles, some source of con- 

 stant error which remains uncorrected by the various reversals 

 usually made ; and that accordingly no repetition of observa- 



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