MAGNETIC SURVEY OF GREAT BRITAIN. 141 



Major Sabine s Observations. — The needle S 2 with which 

 these observations were made, has been already described in 

 the Reports of the British Association, vol. v. pages 141 — 149. 

 It is 11^ inches long, on Professor Lloyd's statical principle, and 

 is used in a circle made by Nairne and Blunt. The observa- 

 tions, together with the deduced values of the intensity, are 

 contained in the subjoined table, No. XLV. Each observation 

 is a mean of forty readings, taken in four positions of the needle. 

 The thermometer by which the temperature was registered was 

 always enclosed with the needle in the dip circle. The values of 



-: — 77; TT are reduced to a standard temperature of 60°, in the 



sm (b — ff) 



manner described by Mr. Lloyd in the Transactions of the 



Royal Irish Academy for 1836 ; the coefficient of t — t' in the 



reduction, or the value of M a experimentally determined, is 



•000024. (See 6th Report, British Association, pp. 11, 12.) 



The observations with this needle at Tortington, in Sussex, 



in the summer of 1837, repeated in the autumn of 1837 and 



summer of 1838, and lastly in the autumn of 1838, produced 



on each occasion an almost identical result, and afford most 



satisfactory evidence of the unaltered state of its magnetism 



cos 

 during the whole of the present series : the values of -: — 75 rr, 



*' '^ sm (0 — a) 



resulting from the observations at Tortington at the three 



epochs alluded to are as follows : 



Mean. 

 May to September, 1837 . . . 0-95390 

 October 1837 to July 1838 . . 0-95361 

 October 1838 0-95375 



cos u 

 To obtain the value of -r — p; rr- in London, to serve as 



sm (6 — 6) 



the unity of the series, observations were made on three several 

 occasions, and in three different localities ; namely, in the gar- 

 dens of the Little Cloisters, Westminster ; in the nursery gar- 

 den in the Regent's Park ; and in the gardens of the palace at 

 Kew. The results were as follows : 



Little Cloisters . . . 0-95245 

 Regent's Park . . . . 0-95684 

 Kew Gardens .... 0-95479 



Mean. . . 0-95469 



