CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 315 
Borneo, at Batavia, and the western portion of the island of Java ; these observations 
are included in pages cxxx and part of cxxxi. 
On my return to Batavia at the latter end of the year, I found the three dip circles, 
which, having been thoroughly repaired, had been sent out by Mr. Barrow ; the 
observations are contained in pages cxxxii and cxxxiii*. 
Before commencing the survey of part of Sumatra, I determined to make use of 
needles with their poles unchanged, and to apply a correction, if necessary, for the 
true dip, as the large magnets for reversing the poles were often brought in danger- 
ous proximity to the small cylindrical magnets used on the Survey. Having deter- 
mined the true dip at Padang with nine needles, I fixed upon Dip Circle No. 1 with 
three needles, A 1, A 1 L and A2 L, by first combining A 1, A 1 L, and subsequently 
A 1 L with A 2 L. 
The mean result gave me as a correction . . . -}-8'’9 
The mean of the nine needles with the poles changed . = — 18° 31 '*7 
With the three combined as above and the poles unchanged 18° 40''6 
4-8''9 
So that in the Survey in Sumatra, at page cxxxv in the last column, is given the 
corrected dip by subtracting 8''9 from the mean of the three needles. 
This series of three needles with poles unchanged and correction +8''9, terminates 
at Natal in Sumatra. 
On my return to Singapore in February 1848, a long series of observations was 
taken, to determine by direct means the difference in the inclination a.m. and p.m., 
the former being taken with four needles at 10 and 11 a.m., the hours of maximum 
horizontal intensity, and the latter at 4 p.m., the hour of minimum horizontal inten- 
sity ; the poles of the needles employed were changed at the commencement or the 
termination of each day’s observation, so that the a.m. and p.m. observations were 
taken with the needles under the same magnetic condition ; two days consequently 
were necessary for the determination of the true dip. From the mean result at page 
cxxxiv, there is a difference of 0'*8 increase in the afternoon, whilst the probable 
error of the mean value of each set is the same, viz. +0''4. 
At the Coeos or Keeling Islands, the poles of the needles of Box No. 1 being un- 
changed, the correction to be applied to the three needles was as follows : — for 
A 1, — 3''0; for A 1 L, — 6''0; and for A2L, — 3'‘0 ; equal value in determining the 
mean has however been given to these needles, as double the number of readings 
have been taken. 
At Pulo Binding the poles of the needles of Dip Circle No. 1 were again changed, 
* There were originally four needles, A 1 , A 2, A 1 L, A 2 L, to each dip circle numbered 1 , 2 and 3 ; but 
three of the needles having soon become rusty, there were left three needles with Dip Circle No. 1, four needles 
with Circle No. 2, and two needles with Circle No. 3 ; and it was in consequence of Dip Circle No. 2 being 
complete, that it was used for the survey of Java. 
2 S 2 
