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 than 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 Riode 
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, 
Nig rCh US 2G ao tep nen sanatien dain asidans 536,2. 
PSE AAO sah nts ate dicing ceneunestes 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 Praya. 
