LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
159 
as a case of very unusual observation error. Were we to omit this result, the mean 
would become — 112° 39'. When the corrections for local attraction become so 
great, it is necessary to be very accurate in noting the direction of the ship’s head 
at the same instant that the azimuth is observed, as at the points where the changes 
of c$ for changes of are very great, an error of a degree in the direction of the ship’s 
head will make nearly the same error in the correction ; on such occasions therefore 
the result is liable to an additional source of observation error of serious magni- 
tude. 
We have seen that when the inclination is — 87° 53', the sum of the devia- 
tions at east and west amounted to 92° 26'; with 10' increase in the dip, their 
joint amount would have become 126° 52'. The scale which the compass needle pre- 
sents for the deduction of the inclination is consequently a very large one, when 
the inclination is so great as that which we are now considering ; and it continues 
to increase in magnitude, until the compass ceases altogether to indicate the direc- 
tion of the horizontal component of the terrestrial force, and points unchangingly, 
under every alteration of the ship’s head, to the direction of the general resultant of 
the ship. 
The terrestrial dip observed with a dipping-needle on board on the 16th of February, 
and corrected for the ship’s attraction, was — 88° 20' ; that corresponding to the 
magnetism of the ship was, as we have seen, — 87° 53', being a little in arrear, in a 
magnetic sense, of her then position. 
For the constants c and d in the formula for the correction of the inclination, we 
have to take into account, in the first instance, a series of observations of the inclina- 
tion with the ship’s head successively on the sixteen principal points of the compass, 
made on board the Erebus at Hobarton in November 1840, before her departure for 
the Antarctic Circle, and a similar series made at the same place in June 1841 on her 
return from the south. The inclination observed on shore was — 7 0° 40'. 
Ship’s head 
by compass. 
Inclination observed. 
Ship’s head 
by compass. 
Inclination observed. 
1840. 
1841. 
Mean. 
1840. 
1841. 
Mean. 
N. 
O / 
— 71 52 
O / 
-71 59 
—71 55-5 
S. 
— 69 49 
— 69 19 
-69 34 
N.N.W. 
— 71 55 
— 72 00 
-71 57-5 
S.S.E. 
—70 00 
-69 41 
— 69 50-5 
N.W. 
-72 03 
— 71 45 
— 71 54-0 
S.E. 
— 70 22 
— 70 04 
-70 13 
W.N.W. 
—71 30 
— 71 24 
-71 27 
E.S.F.. 
-70 45 
-70 33 
-70 39 
W. 
-71 16 
— 70 55 
-71 05-5 
E. 
-70 58 
— 71 08 
-71 03 
w.s.w. 
— 70 55 
-70 30 
-70 42-5 
E.N.E. 
-71 33 
— 71 32 
-71 32-5 
s.w. 
-70 20 
— 69 56 
— 70 08 
N.E. 
— 71 35 
-71 57 
— 71 46 
s.s.w. 
-70 07 
-69 44 
-69 55-5 
N.N.E. 
-71 42 
-71 56 
-71 49 
Employing for the observations between N.N.W. and S.S.W., and N.N.E. and 
8.8. E. the formula 
c cos £ -f d tan 6 = b sin £ cosec tan Of, 
