ON THE MAGNETIC INTENSITY OF THE EARTH. 17 
one selected, and regarded in the same light as a first meri- 
dian. In justice to these valuable observations, and in consi- 
deration of the great change undergone by the needle, it may 
be desirable briefly to describe the manner in which this has 
been done. 
At Rio de Janeiro, which was the first station observed at in 
South America, the needle was vibrated in August 1826, Sep- 
tember 1827, and December 1828; in the intervals between 
these dates are comprised the principal part of the observations 
on the east side of South America. There is no direct obser- 
vation at Rio subsequently to December 1828, but we are able 
to supply the time of vibration at a fourth date in the following 
manner. The intensity at Rio and at Monte Video having been 
correctly compared by a double comparison in 1827 and 1828, 
the needle was vibrated at Monte Video on the Ist of June, 
1830, immediately before Capt. King’s departure for England, 
and we thus obtain by an easy calculation the time of vibration 
at Rio corresponding to the same date. The intervals between 
these four dates include the whole of the South American sta- 
tions ; and we have only to distribute in each interval the loss of 
magnetism which the observations show 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 interval, the results are found extremely ac- 
cordant at several other stations at which observations were re- 
peated at distant intervals. The subjoined tables will enable 
the reader to judge of this for himself. In the first table are 
shown the times of vibration at Rio, corresponding to the four 
dates: Ist, the observed times of horizontal vibration reduced 
to infinitely small arcs and to a temperature of 60°; and 2nd, 
the corresponding times as a dipping-needle. The value of 
the correction for temperature has been determined for this 
needle by observations which I have recently made with it for 
that purpose, the particulars of which will be given in the 
more detailed statement in Capt. Fitz Roy’s publication. In 
the three last columns are shown,—the number of days com- 
prised in each interval,—the increase in the time of vibration 
owing to loss of magnetism in the needle,—and the resulting 
daily correction on the supposition of the loss in each interval 
being uniform. 
_The second table contains the corrected times of horizontal 
vibration at each of the South American stations at the dates 
respectively inserted ;—the dips observed by Capt. King ;—the 
time of vibration as a dipping-needle at Rio at the same dates, 
VOL. VI. 1837. Cc 
