518 



OBSEKVATIONS OF INTENSITY. 



At Rio de Janeiro, which was the first station observed at in 

 South America, the cylinder was vibrated in August 1826, Sep- 

 tember 1827, and December 1828 ; in the intervals between these 

 dates are comprised the greater part of the observations on the 

 east side of South America. There is no direct observation at Rio 

 subsequently to December 1828 ; but we are enabled to supply the 

 time of vibration, which would have been observed had the cylin- 

 der been employed at Rio on June 1, 1830, in the following man- 

 ner. We have seen that on the 15th September 1827 the time of 

 vibration v/as observed at Rio ; on the 18th December following it 

 was observed at Monte Video. These observations give the inten- 

 sity at Monte Video relatively to that at Rio, subject to whatever 

 change of magnetism the cylinder may have undergone in the 

 interval of three months. This comparison was repeated in the 

 following year, on Captain King's return from Monte Video to 

 Rio, the interval being nearly of the same duration, and the order 

 of the experiment reversed, the passage being in this instance from 

 Monte Video to Rio, it having been before from Rio to Monte 

 Video. On the supposition of an uniform, or nearly uniform rate 

 of change in the cylinder, the errors arising therefrom during the 

 two passages would be of opposite kinds, and should compensate 

 each other in a mean of the two comparisons. Calling the force at 

 Rio unity, these comparisons give its value at Monte Video respec- 

 tively as follows, namely, 



September and December 1827 ... M97 l 

 October and December 1828 ... 1-207 J 



On the 1st of June 1830, being then on his return from the west 

 coast of South America, and on the eve of sailing for England^ 

 Captain King again observed the time of vibration of the cylinder 

 at Monte Video ; whence, through the preceding comparison, we 

 obtain the time of vibration at Rio, which should belong to the 

 same date. We have thus a fourth date at Rio, which, added to 

 those enumerated above, will include the whole of the South Ame- 

 rican stations ; and we have only to distribute in each interval the 

 loss of magnetism which the observations shew to have taken place 

 from one date to the next, in the manner which may appear most 

 suitable. There is no very obvious indication that the loss was 

 other than gradual ; and by considering it uniform in each separate 



