326 CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 
.intensity deduced from the three declinometers, to the horizontal intensity found by 
means of the portable declinometer, is constant. 
As the instrument was left at Madras, I have no means of measuring the true values 
of r, r', &c. ; but if four stations be taken, Singapore at the commencement, Batavia 
and the Cocos during 
the Survey, and Madras at its termination, we find the compa- 
rison to be as follows 
— 
Singapore, 1845 
Horizontal intensity 
from three declinometers. 
8-0947 
Horizontal intensity 
from portable declinometer. 
7-9495 
Ratio. 
1-0182 
Singapore, 1846 
8-121 
7-951 
1-0214 
Singapore, 1848 
8-114 
7-991 
1-0154 
Batavia, 1847 . 
7-897 
7-784 
10145 
Cocos, 1848 
7-2745 
7-167 
1-0150 
Madras, 1849 . 
. . . . . 8-0784 
7-951 
1-0160 
Mean ratio . . . 
1-0167 
by which ratio 1‘0167, all the values of the horizontal intensity found by means of 
the portable declinometer, are multiplied for the true value. 
The seventh column contains the total intensity. The total intensity has not been 
determined by direct observation, but from the formula h=f cosh, 
or 
f= - — i—h . sec 
where 
h is the horizontal intensity. 
^ the dip. 
and 
f i\\c total intensity. 
The eighth column contains the variation or magnetic declination, found by means 
of the altitude and azimuth instrument and collimator magnet. The following Table 
is given in explanation of the manner in which the variation was determined, the 
station being on Direction Island at the Cocos ; each observation is the mean of three 
sets of altitudes and azimuths, with the vertical limb in the direct and reversed 
positions. 
Reading of the true meridian on the limb. 
Mean of the 
two sets, or 
true meri- 
Mean of 
magnetic 
axis on the 
Mean 
magnetic 
variation. 
Limb direct. 
Limb reversed on its axis. 
Date. 
Time. 
1. 
2. 
3. 
Mean. 
1. 
2. 
3. 
Mean 
limb. 
limb 
West. 
Aug. 28. 
A.M. 
211 
18 
30 
211 
17 
10 
211 
18 
00 
211 
17 
53 
211 
34 
10 
211 
34 
40 
211 
34 
00 
211 
34 
17 
211 
26 
05 
210 
16 
28 
1 
09 
37 
29. 
A.M. 
211 
29 
47 
211 
29 
27 
211 
30 
17 
211 
29 
50 
211 
32 
23 
211 
30 
13 
211 
32 
57 
211 
31 
51 
211 
30 
50 
210 
18 
06 
1 
12 
44* 
31. 
A.M. 
211 
27 
40 
211 
26 
50 
211 
27 
10 
211 
27 
13 
211 
26 
00 
211 
25 
04 
211 
25 
00 
211 
25 
21 
211 
26 
17 
210 
16 
16 
i 
10 
01 
Sept. 7. 
A.M. 
211 
23 
53 
211 
23 
23 
211 
25 
27 
211 
24 
14 
211 
30 
07 
211 
29 
53 
211 
29 
53 
211 
29 
58 
211 
27 
06 
210 
17 
06 
1 
10 
00 
9. 
A.M. 
211 
25 
30 
211 
25 
16 
211 
26 
20 
211 
25 
42 
211 
29 
40 
211 
27 
30211 
28 
03 
211 
28 
24 
211 
27 
03 
210 
17 
20 
1 
09 
43 
19. 
A.M. 
211 
28 
05 
211 
28 
14 
211 
30 
54 
211 
29 
04 
211 
23 
40 
211 
25 
40 
211 
23 
59 
211 
24 
26 
211 
26 
45 
210 
18 
37 
1 
08 
08 
27. 
A.M. 
212 
10 
17 
212 
10 
40 
212 
12 
44 
212 
11 
14 
212 
08 
30 
212 
07 
40 
212 
09 
47 
212 
08 
39 
212 
09 
50 
210 
55 
18 
1 
14 
38t 
Mean of the variation 
1 
10 
42 
* New spider’s web, and instrument readjusted. 
t New level put on the vertical limb, and position of the altitude and azimuth changed. 
