LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
155 
From the observations at E.N.E. and W.N.W., 
\p' = — 13° 36' at E.N.E. , and — 22° 06' at W.N.W. ; 
4 = — 17° 51' ; £' = 6/° 30', and £ = 63° 15' ; whence a = *0261. 
The mean of the two pairs of observations gives a = + ’0263. 
Whilst within the Antarctic Circle only a single opportunity occurred of observing 
the inclination otherwise than on board, and thus of obtaining a from a tan 0 by 
having an assured value of 6. This was on the 8th of January 1841, in lat. — 68° 30', 
long. 176° 35', where the inclination observed on the ice with a needle in which the 
observation was complete by the reversal of the poles, was found to be — 83° 35'. 
The declination was observed on board on the same afternoon and following morn- 
ing, as nearly as could be in the same geographical position, with the ship’s head on 
several points, from which we may select for a determination of a those nearest to 
the east and west points. We have then the following observations : — 
] . At W. | N. 4' = — 46 02 4. At E. f N. 4' = — 20 51 
2. At W. by S. 4' = - 46 32 5. At E. by S.|S. ^'=- 19 58 
3. At W. by S.iS.4'=- 47 17 6. At E. by S. f S. ^' = — 20 22 
From 1. and 4. we have 4 = 
4 ' + 4 /' 
= - 33 27‘5 
4 *' 4 * 4 *' 
From 3. and 5. we have 4 = — ^ — = — 33 3 7’5 
>Mean — 33 32 - 5. 
Hence o ; 
\ = — 1°2 29-5 
\ = - 12 59-5 
S 3 = — 13 44-5 
and having thereby the values of £, as we 
h 4 = + 12 4T5 
\ = + 13 34-5 
\ = + 13 10-5 ; 
have those of £' by observation, we obtain 
a x = + -0249 
a 2 = + -0274 
a 3 — + '0274 
Means ... 4- '0266 
a 4 = + '0259 
a b = + '0275 
a 6 = + '0266 
+ -0267 
The deviation of the compass observed on board the Erebus during the stay of the 
Expedition at Christmas Harbour, Kerguelen Island, in July 1840, when the ship’s 
head was on the N.E., S.E., N.W , and S.W. points, and at the points on either side 
of those points, viz. N.E. by N., N.E. by E., S.E. by S., S.E. by E., &c., will furnish an 
additional determination of the value of b : 
MDCCCXLIII. 
Y 
