LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
33 
This experiment was repeated at Kerguelen Island, but with the ship’s head placed 
only on the eight principal points : the season and weather were unfavourable, and 
the errors of observation were consequently greater than at the Cape ; but the general 
indication is the same : the results are as follows : 
Ship’s head. 
Differences from the mean. 
Mean of the 
two ships. 
Ship’s head. 
Differences from the mean. 
Mean of the 
two ships. 
Erehus. 
Terror. 
Erehus. 
Terror. 
E. 
+ *008 
— -003 
+ -002 
w. 
+ -006 
+ -007 
+ -006 
S.E. 
- -007 
000 
- -003 
N.W. 
000 
+ -008 
+ -004 
s. 
- -016 
- -027 
— -021 
N. 
+ ‘007 
+ -005 
+ -006 
s.w. 
— -014 
+ *001 
- -006 
N.E. 
+ -014 
+ -008 
+ -Oil 
The experiments at the Cape and Kerguelen Island agree in indicating the points 
of greatest disturbance to be from S.E. (through south) to S.W., causing an augmen- 
tation, — and from N.E. (through north) to N.W., causing a diminution, — in the re- 
gular magnetic intensity ; the augmentation on the southerly points being rather 
greater than the diminution on the northerly, compensated by there being a greater 
number of the remaining points in defect than in excess ; the latter points being 
affected in a minor degree. Considering that the differences shown in the two last 
Tables necessarily combine the errors of observation with the influence of local attrac- 
tion, we may regard the effect of the ship’s iron on the intensity needle as probably 
amounting, in extreme cases, to Tooth of the earth’s magnetic force ; but on much 
the greater number of points as probably far less than that amount. In Table III., 
which contains the results of the almost daily observations made in the Erebus be- 
tween December 7, 1839 , and February 29 , 1840 , in the space of the Atlantic com- 
prised between the two portions of the isodynamic curve of 0 ’ 9 , I have employed the 
following scale of correction* : 
Ship’s head < 
^S.W. by S. to S.E. by S. 
— -008 
S.W. and S.E 
- -007 
W.S.W. and E.S.E. . . 
— ‘002 
Ship’s head < 
_ W. by S. and E. by S. . 
000 
W. by N. and E. by N. + '002 
W.N.W. and E.N.E. . + '003 
N.W. and N.E. . . + '004 
. N.W. byN. to N.E. by N. + -005 
* After the communication of this paper to the Royal Society, I received from Captain Croziek the results 
of a third experiment of the same kind, made in the Terror on the 20th of October 1840 in the river Derwent 
in Van Diemen Island. I subjoin these results in further evidence of the general correctness of the deduc- 
tions which have been drawn in regard to the influence of the ship’s iron on the intensity observations made 
at sea. 
Ship’s head. 
Correction. 
Ship’s head. 
Correction. 
E. 
+ -001 
w. 
+ -003 
E.S.E. 
— -001 
W.N.W. 
000 
S.E. 
— -005 
N.W. 
+ -007 
S.S.E. 
— -008 
N.N.W. 
+ -005 
S. 
- -008 
N. 
+ -004 
S.S.W. 
- -007 
N.N.E. 
+ -005 
S.W. 
- -005 
N.E. 
+ -008 
W.S.W. 
+ -001 
E.N.E. 
+ -002 
MDCCCXLII 
F 
