28 Captain Sabine's Experiments on the 



and myself, by a similar process. The six needles employed 

 between London and Paris made the ratio of the horizontal 

 force at Paris to unity in London as follows : — 



1.0722 



1.0675 



1.0731 



1.0726 



1.0709 



1.0717 

 Whence the needle, which has served as a general term of 

 comparison for the different stations, and is supposed to make 

 its time of vibration in London in the direction of the dipping- 

 needle 300", would have its corresponding time of vibration at 

 Paris as follows : — 



The dip in London, in the spring of 1827, (the period at 

 which the comparison was made,) is considered to have been 

 69° 49'; and in Paris, 67° 58'. 



The stations, common to M. de Humboldt's series and mine, 

 are TenerifFe and Trinidad; (in the latter case his observations 

 were made at Cumana, in lat. 10° 27' N., and long. 64° 16' W. ; 

 and mine at Port Spain, in Trinidad, in 10° 39' N., and 

 long. 61° 35' W.) M. de Humboldt did not employ horizontal 

 needles, but observed the number of vibrations which a dipping- 

 needle, freely suspended in the magnetic direction, made in ten 

 minutes at each station. This number was in Paris 245 ; at 

 TenerifFe 238 ; and at Cumana 229. If, then, we suppose the 

 relative intensity at Paris, London, TenerifFe, and Trinidad, to 

 have been the same in 1822, as it was twenty-four years antece- 

 dently, (which supposition is probably not strictly correct, but 

 sufficiently so for the present comparison), the needle which 

 makes its time of vibration in London 300", and in Paris 302".2, 

 would make, according to M. de Humboldt, the corresponding 



