360 
MAGNETIC SURVEY OF A PART OF THE SOUTHERN HEMISPHERE. 
“II. Determination of Index Corrections. 
“ 1. Declination Observations. 
“The compass used was one of the Admiralty compasses (B. 20). It was sup- 
plied with two cards, one considerably heavier than the other to be used in bad 
weather ; but as it was found that in all weathers the heavy card was the steadiest 
and gave the best results, it was accordingly generally used. The index corrections of 
both cards were determined at the Magnetic Observatory, Cape of Good Hope. The 
following are the means of several observations with each card; the mean monthly 
declination by the observatory declinometer being -(- 29° 07'. 
° / / 
Card A (the light card) gave . +28 20 ; correction +47. 
Card J (the heavy card) gave . +28 15; correction +52. 
“ These corrections have been applied to all the observations, according to the 
card employed. 
“2. Inclination Observations. 
“Two of Mr. Fox’s instruments were kept in constant use, one observed in the 
forenoon and the other in the afternoon. In order to distinguish them, we may call 
the one observed in the morning No. 1, the other was marked C. 9. In No. 1, 
needle 1 was mounted and used throughout, the spare needle 2 being used as a 
deflector. The index correction for 1 was determined at the Magnetic Observa- 
tory at the Cape, both before and after the Expedition, by comparing the inclination 
with the face of the instrument west (that being the way the observations were taken 
on board) with the mean monthly inclination shown by the observatory needles. 
The following are the observations with the deflectors at 40° from the apparent dip 
O / I 
November 10, 1844, needle 1, face west —53 39 ; correction + 8 
November 10, 1844, needle 1, face east —53 59 ; correction +28 
November 21, 1844, needle 1, face west —53 38; correction + 7 
November 21, 1844, needle 1, face east —53 58 ; correction +27 
The mean monthly inclination being —53° 31'. 
“ After the 1 3th of January it was found more convenient to adjust the deflectors at 
the apparent dip, and make the same observations serve both for dip and intensity. The 
index corrections to be applied in this case are given by the following observations : — 
Date. 
Observed. Inclination. — Face West. 
True Inclina- 
tion. 
Index correc- 
tion. 
Direct. 
Def. N. 
Def. S. 
Def. N and S. 
Mean. 
1844. 
December 1 ... 
December 5 ... 
1845. 
June 30 
-53 52 
-53 49 
— 54 12 
-54 08 
o / 
—53 21 
— 53 20 
-53 13 
-53 15 
o / 
— 53 44 
— 53 43 
-53 46 
— 53 46 
-53 39 
—53 38 
— 53 51 
-53 43 
— 53 39 
— 53 38 
— 53 45 
— 53 43 
j -53 31 
j —53 25 
+ 07 
+ 19 
July 2 
Mean 
-54 01 
-53 17 
— 53 45 
— 53 43 
-53 41 
-53 28 
+ 13 
