520 



OBSERVATIONS OF INTENSITY. 



deducted, from my determination of the dip^ 12' for the probable 

 change between 1822 and 1826. At Teneriffe the dip has been 

 frequently observed ; but the values assigned by different observers 

 vary so much as to indicate a more tiian usual frequency of local 

 disturbance, which might also be expected from the geological 

 character of that island. It would be unsafe therefore to employ 

 any dip for that station but one which was certainly obtained at 

 the same spot at which the horizontal intensity was observed. 



The dates of the observations at these three stations fall between 

 the observations at Greenwich in March 1826, and those at Rio de 

 Janeiro in August of the same year. Having the intensity at 

 Greenwich = 1,372 and at Rio = 0^884, and the dip at Greenwich 

 69° 52', and at Rio 14° 00, we have the time of vibration of Cap- 

 tain King's cylinder as a dipping needle at Rio at the respective 

 dates as follows, namely, 



March 1826 536,2. 



August 1826 537,0. 



It appears therefore that but a very slight change took place in 

 the magnetism of the cylinder during the outward voyage, and we 

 may take 536,6 as the time of vibration at Rio, corresponding to 

 the dates of the intermediate observations; and this is done in the 

 table for Madeira and Port Pray a. 



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