366 MAGNETIC SURVEY OF A PART OF THE SOUTHERN HEMISPHERE. 
“ Assuming the intensity at the Cape as unity, we get the following formulae for 
calculation : — 
Def. N 
. I'— *4692 cosec v' . w'. 
Def. S 
. I'— *4606 cosec v ' . w'. 
Mag. N and S . . . . 
. I' = *4686 cosec v ' . w'. 
Mag. N 
. I'=*4634 cosec v' . w'. 
Mag. S 
. I'— *4552 cosec v' . w'. 
Weight 1 grain .... 
. I f = *4728 cosec v'. 
Weight 1^ grain . . . 
. I' = -6974 cosec v'. 
Weight 2 grains . . . 
. I' = -9361 cosec v'. 
“ The value of the intensity at King George’s Sound by this needle is — 
By weights T688. 
By deflectors .... 1*672. 
“The intensity by the other apparatus No. 1 is 1*688. 
“ At the Mauritius the intensity is — 
By weights 1*156. 
By deflectors .... 1*155. 
And by the other instrument 1*156. 
“ It is therefore evident that needle B preserved its magnetism from King George’s 
Sound to the Cape. Comparing the results with the deflectors with those of the other 
instrument, the deflectors do not appear to have lost magnetism ; the difference at 
King George’s Sound of *01 arises probably from error of observation. As the results 
given by weights are the most accurate when the observations are made on land, 
they have been exclusively used in such cases ; at sea both weights and deflectors 
have been used. 
“Besides the correction for the effect of the ship’s iron, a second correction for the 
effect of temperature on the needle and deflectors is necessary. The observations 
have all been reduced to a common temperature of 60° by means of the formulae 
c=r • q(t'-t), 
t being taken as 60° and q being the coefficient for 1° of Fahr. Values of q for each 
needle and deflector employed, were obtained at the Magnetic Observatory, Cape of 
Good Hope, in the usual manner. The following is an abstract of the observations : — 
Needle or deflector. 
Approximate 
distance. 
Total deflec- 
tion in scale 
Mean alterna- 
tion of 
No. of 
alternations. 
Corresponding 
mean differ- 
ence of 
Corresponding 
bifilar correc- 
tion in parts 
Values of q. 
divisions. 
temperature. 
deflection. 
of force when 
+ additive. 
^ A. 1. 
ft. in. 
3 0 
497*5 
38 35 
5 
1*36 
+ *000046 
•000072 
No. l.<| 
A. 2. 
3 0 
805*2 
38 76 
5 
3*88 
+ •000004 
•000116 
Def. N 
1 0 
873*3 
40 43 
5 
4*33 
•000019 
•000123 
LDef. S 
1 0 
880*5 
40 02 
5 
2*82 
•000054 
•000081 
f A. 
1 5 
1019*0 
34 68 
3 
4*21 
-•000070 
•000117 
B. 
1 5 
1059*5 
43 35 
4 
3*18 
— •000004 
•000069 
C. 9. <t 
C. 
1 5 
1065*9 
43 46 
5 
3*67 
+ •000008 
•000079 
Def. N 
1 0 
1004*8 
45 85 
5 
7*08 
+ •000004 
•000154 
LDef. S 
1 0 
1021*7 
47 38 
5 
6*45 
— •000024 
•000133 
