190 
LINES OF MAGNETIC DECLINATION IN THE ATLANTIC. 
the compasses supplied to Her Majesty’s navy has at length received from the Ad- 
miralty the attention which was so long and so greatly wanted, and since a depart- 
ment has been expressly instituted for their proper examination and care, errors of 
such magnitude, where no such errors need exist, may no longer be found to occur*. 
I would also take this occasion to remark, that in examining the disturbance 
caused by the ship’s iron at the spot in which the standard compass is placed by the 
process of swinging the ship, the standard compass itself should be employed, and 
not, as appears to have been sometimes done, another compass substituted for that 
particular occasion. When compass errors exist, the coefficient A derived from the 
observations with one compass are inapplicable to the observations of another compass 
having a different compass error. Also, when circumstances permit, it is preferable 
that the actual disturbance on each point should be ascertained independently of that 
on other points, as is done when the bearing of an object is observed whose correct 
magnetic bearing is known or subsequently determined. Disturbances supposed to 
be ascertained, by comparing azimuths observed on each point with the means of the 
azimuths observed on all the points, are liable to be in error to the full amount of 
the value of the coefficient A, whether that value arise from compass error or from 
the disturbing influence of the iron. 
Determinations made in H.M.S. Thunder, in a passage from Nassau, New Providence, 
to England in 1841, hy Captain Edward Barnett, R.N. 
The observations in the Thunder were made with one of the Admiralty com- 
passes, fitted as a standard compass. The disturbance occasioned by the iron was 
examined at Nassau in March 1841, immediately after the arrival of the ship from 
England, and at Gillingham in the River Medway, about the 1st July of the same 
year on her return to England. The observations were as follows, viz. — 
Table III. — Disturbance of the Compass in H.M.S. Thunder. 
Ship’s head. 
Disturbances towards the west. 
Ship’s head. 
Disturbances towards the w'est. 
Nassau. 
GilUngham. 
Nassau. 
Gillingham. 
c / 
o f 
o / 
o f 
N. 
-0 5 
— 0 15 
S. 
+ 0 23 
+ 0 38 
N. by w. 
+ 1 19 
+ 1 00 
s. by E. 
+ 0 1 
-1 17 
N.N.W. 
+ 1 16 
+ 1 59 
S.S.E. 
— 0 34 
— 1 25 
N.w. by N. 
+ 2 18 
+ 3 38 
s.E. by s. 
-0 43 
— 3 35 
N.W, 
+ 2 48 
+ 5 02 
S.E. 
— 1 22 
-4 2 
N.w. by w. 
+ 3 32 
+ 6 2 
s.E. by E. 
-2 1 
-4 23 
W.N.W. 
+ 3 53 
+ 6 6 
E.S.E. 
— 2 36 
-5 19 
w. by N. 
+ 3 45 
+ 6 12 
E. by s. 
-3 9 
— 5 50 
w. 
+ 3 52 
+ 6 15 
E. 
-3 19 
-6 16 
w. by s. 
+ 3 44 
+ 6 51 
E. by N. 
— 3 26 
— 5 44 
w.s.w. 
+ 3 21 
+ 5 15 
E.N.E. 
— 3 34 
—6 28 
s.w. by w. 
+ 2 59 
+ 5 42 
: N.E. by E. 
— 3 12 
-5 57 
s.w. 
+ 2 37 
+ 5 2 
N.E. 
-2 52 
— 4 46 
s.w. by s. 
+ 1 55 
+ 3 32 
N.E. by N. 
— 2 16 
— 3 48 
s.s.w. 
+ 1 52 
+ 2 40 
N.N.E. 
— 1 56 
— 2 51 
s. by w. 
+ 0 34 
+ 2 5 
N. by E. 
-0 27 
— 0 51 
* When the Erebus left England the prism by which the graduation of the card of the Admiralty compasses 
