88 
LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
Deductions of the Constants a and h in the Corrections for the Ship’s attraction. 
1. In the Erebus. — For the constants a and b to be employed in computing the 
corrections of the declination, we have the observations on each of the 32 principal 
points of the compass at Hobarton, in October 1840 and June 1841. We have also 
a similar series at Port Louis, in the Falkland Islands, in August 1842. The ob- 
servations at Hobarton have been already discussed in No. \ * Those at Port Louis 
were as follows : — 
August 19, 1842. 
Ship’s head 
by compass. 
Disturbance 
towards the 
west. 
Ship’s head 
by compass. 
Disturbance 
towards the 
west. 
Ship’s head 
by compass. 
Disturbance 
towards the 
west. 
Ship’s head 
by compass. 
Disturbance 
towards the 
west. 
N. 
+ 6 12-7 
w. 
— 2 15-8 
s. 
+ 6 00-1 
E. 
+ 2 07-4 
N. by w. 
— 0 04-1 
w. by s. 
— 2 21-2 
s. by e. 
+ 0 43 9 
e. by n. 
+ 1 54-0 
N.N.W. 
-0 33-6 
w.s.w. 
— 2 21*3 
S.S.E. 
+ 1 12-7 
E.N.E. 
+ 1 44-0 
n.w. by n. 
-0 50-1 
s.w. by w. 
— 2 4-3 
s.e. by s. 
+ 1 41-4 
n.e. by e. 
+ 1 16-5 
N.W. 
— 1 02-3 
s.w. 
-1 8-0 
S.E. 
+ 1 55‘5 
N.E. 
+ 0 50-9 
n.w. by w. 
-1 00-6 
s.w. by s. 
- 1 3-3 
s.e. by e. 
+ 2 06-9 
n.e. by n. 
+ 0 40-5 
W.N.W. 
-1 49-3 
s.s.w. 
-1 17-3 
E.S.E. 
+ 2 18-9 
N.N.E. 
+ 0 41-2 
w. by n. 
-2 09-6 
s. by w. 
-0 38-6 
e. by s. 
+ 2 16-4 
n. by e. 
+ o 27-7 
The values of the constants deduced from the observations at Hobarton were, 
a= f' 027'2 ; b— -j-"986. The values from the observations at the Falkland Islands 
are, +’0292 ; b—- p984. 
The values of a at Hobarton were derived from two series, one in October 1840, 
when the ship had recently passed through the low magnetic latitudes, and the other 
in June 1841, on her return from the highest magnetic latitudes of the southern hemi- 
sphere ; the two series separately considered give a— J r 4)235 in 1840, and ‘0309 in 
1841 ; we have therefore the following values : — 
-f- *02 67 in the Thames, where the ship had been stationary for several years. 
+ •0235 at Hobarton, on her first arrival from the low latitudes. 
+ •0305 on her return to Hobarton from the very high southern magnetic latitudes. 
+ •0292 at the Falkland Islands in 1842, on her second return from the very high 
southern latitudes. 
The variations in these values is in accordance with the view expressed in the 
preceding Number of these Contributions'!', that when a ship changes her magnetic 
latitude, the corresponding change in the induced portion of her magnetism may not 
be instantaneous ; that some portions of her iron may be of a quality intermediate 
between perfectly soft iron, which would undergo instantaneous change, and iron 
permanently magnetic ; and that when changing rapidly her geographical position, 
she may be liable to be more or less in arrear, in regard to her magnetic condition, 
of her actual locality at any particular time. In a ship in which this should be the 
case, a table computed with any one value of a would not apply equally to one portion 
* Philosophical Transactions, 1843, Part II. pp. 152-154. 
f Ibid. pp. 152, 153. 
