518 OBSERVATIONS 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 was 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 ... 1:197 
October and December 1828 ... 1.207 } Mean 1-202. 
On the Ist 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 
