MAJOR SABINE ON TERRESTRIAL MAGNETISM. 
149 
The present amount of the secular change of the dip in the Gulf of Guinea and 
its vicinity is so considerable, as to render it necessary to apply a correction to Captain 
Allen’s results obtained in the year 1833, to make them correspond to the mean 
epoch of the observations which the map represents, or to the beginning of 1837- 
From the best inference I am able to draw, in the absence of precise data, the 
north dip is diminishing, and the south increasing, in the vicinity of the islands of 
Fernando Po and St. Thomas, at an annual rate certainly exceeding half a degree. At 
Captain Allen’s stations in the Quorra the rate of decrease (of north dip) is pro- 
bably less, but is still very great. The correction applied has been, for three years, at 
the annual rate of half a degree for the islands in the Gulf of Guinea, and a quarter 
of a degree for those in the Quorra : the results so corrected are inserted in the 
map. At Ascension the secular change probably does not exceed 8' or 10'*: also 
Captain Allen’s observations at that island were made a year later than at tha other 
stations ; they have not therefore been corrected for epoch. The discrepancy in the 
results of dilferent observers at Ascension and of the same observer in different situa- 
tions in the island are well known to be considerable, and are naturally accounted 
for by its volcanic character. The mean of Captain Allen’s results agrees well with 
Captain FitzRoy’s in 1 836. 
Date. 
Needle. 
1833. 
Aug. 21. 
Mar. 22. 
Oct. 10. 
Nov. . 
Nov. . 
Dec. 27. 
1834. 
Jan. 1 7.f 
Jan. 21 + 
Jan. 24.§ 
2 { 
1 { 
2 { 
*{ 
2 i 
>{ 
■{ 
1 { 
Poles. 
cl direct. 
/3 reversed. 
Positions of the needle. 
Dip. 
a . 
a'. 
a". 
a'". 
CL 
7 22 
8 29 
6 59 
8 33 
0 
14 25 
16 27 
17 20 
14 59 
r 11 JO 
a 
9 43 
10 58 
10 25 
10 14 
0 
12 29 
13 52 
12 50 
12 00 
CL 
10 05 
10 17 
9 53 
10 55 
0 
16 55 
18 28 
17 27 
16 22 
r lo Ol 
CL 
- 0 45 
+ 0 20 
- 0 39 
- 0 10 
0 
-f- 1 55 
+ 2 26 
+ 1 18 
+ 1 57 
?■ U 4o 
CL 
- 0 55 
+ 0 28 
+ 0 31 
- 0 10 
0 
+ 0 57 
+ 2 33 
+ 1 51 
+ 0 52 
r" U 4o 
CL 
- 8 26 
- 8 30 
- 7 13 
- 8 52 
0 
- 7 15 
- 7 04 
- 6 46 
- 7 52 
> — 1 44 
CL 
1 03 
0 30 
1 59 
1 48 
0 
3 06 
4 37 
3 25 
3 41 
>* J ol 
CL 
2 15 
2 19 
1 12 
0 40 
0 
3 35 
3 28 
3 57 
2 46 
cl 
0 37 
0 18 
I 1 
1 15 
0 
2 05 
2 30 
3 10 
3 03 
> U 0/ 
Reduction 
to Epoch. 
-0 45 
-0 45 
-1 30 
-1 30 
' 
^ 0 00 
Dip in 
1837. 
10 58 
13 06 
- 0 42 
- 9 14 
1 57 
Station. 
Stirling. 
Egga. 
Fernando Po. 
Isla de Rolhas. 
Ascension. 
I. 
Captain Wickham’s intensity observations were made with Hansteen’s apparatus, 
and two needles Y and Z. The following Table contains their times of horizontal 
vibration at the stations, dates, and temperatures specified, taken, with Captain Beau- 
* Voyage of H.M. ships Adventure and Beagle, vol. i. p. 527. 
f On the sea-beach. X At the spot where Captain Foster observed. § On the summit of the Green Mountain. 
