L1EUT.-C0L0NEL SABINE ON TERRESTRIAL MAGNETISM. 
97 
find that a value of a taken near +’026 will by no means bring the results on the N. 
and S. points, or on those approaching the N. and S. points, into accord ; and that as 
we have already found in the dip corrections of the Erebus, and in the declination 
corrections of both ships, a considerably higher value of a is required for the ob- 
servations on the return from the high latitudes, than for those when the ship was in 
progress from the lower to the higher dips. 
We have no observations at the Falkland Islands (made at the spot in the ship 
where Mr. Fox’s apparatus was used) either of the direction of the compass needle, 
or of the force acting on the horizontal needle: we must therefore obtain a and h 
directly from the observations of Inclination and Intensity. The observations gave 
as follows : — 
Ship’s head. 
Inclination observed. 
6— —51° 56'. 
Intensity observed. 
<p= 1-336. 
V' 
N. 
—52 46-5 —52 46-5 
1-320 
1-320 
N.N.E. 
N.N.W. 
-52 51 ] 
— 52 43 J 
> -52 47 
1-315 1 
1-313 J 
1-314 
N.E. 
N.W. 
— 52 47] 
— 52 45 J 
— 52 46 
1-3141 
1-312 J 
1-313 
E.N.E. 
W.N.W. 
— 52 52 ] 
— 52 38 J 
— 52 45 
1-336 
1-308 j 
. 1-322 
E. 
W. 
—52 31 1 
— 52 13 J 
■ —52 22 
1-3361 
1-324 
1-330 
E.S.E. 
W.S.W. 
— 52 16 
— 51 46 j 
> -52 01 
1-355 
1-345 J 
1-350 
S.E. 
S.W. 
-51 32] 
-51 32 
. -51 32 
1-3701 
1-359 J 
1-364 
5.5. E. 
5.5. W. 
-51 09] 
— 51 21 J 
-51 15 
1-368 
1-366 j 
. 1-367 
s. 
-50 53 
— 50 53 
1-370 
1-370 
For a, we have from equation (].), 
asim?=-^-cos<fcos^'-— cos0cos£, 
whence we obtain, from the observations on the N. and S. points, a= + '031], and 
from those on the N.N.E. and N.N.W., S.S.E. and S.S.W. points, a also —+'0311. 
In the Erebus we have found a for the spot in the ship where Mr. Fox’s apparatus 
was used =+'023. from the observations made when the ship was in progress to the 
southward ; and = +'029 at Hobarton and the Falkland Islands. The corresponding 
values in the Terror are +*028 and +'031. 
In the case of the Terror, therefore, I have employed separate tables for the cor- 
rections for the ship’s attraction, viz. a taken as +'028 in the passage from Hobarton 
to New Zealand; as +'026 in the passage to the higher latitudes ; and as +'031 
during the return from the high latitudes to the Falkland Islands. 
For b and c, we obtain from the observations at the Falkland Islands as follows : — 
In the case of b, we have from equation (2.), 
Z>cos0=-^cos4'sin£'cosec£ ; 
o 2 
