32 
LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
Minimum Intensity . — In the passage from England to the Cape of Good Hope the 
Expedition traversed three times that large space of the Atlantic in which the mag- 
netic intensity is less than in any other part of the surface of the globe ; first in a 
southerly course, in and about the meridian of 330 ° E. ; a second time in beating up 
to St. Helena, in and about the meridian of 345 ° E. ; and a third time in the course 
from St. Helena to the Cape of Good Hope, in and about the meridian of 350 ° E. 
Before we examine more particularly the results of the observations made daring 
these traverses, it will be proper to clear them from the effects of the ship’s iron, as 
far as the data furnished will enable us to do so. 
It is obvious, on a simple inspection of the results in the tables, that, in the 
southern hemisphere, when the ship’s head was on the points from S. E. to S.W., the 
intensity observed was generally slightly in excess, and on the contrary, when on 
the points from N.E. to N.W., slightly in defect ; and that such was the case in both 
ships. At St. Helena and at the Cape of Good Hope, an endeavour was made to 
ascertain more precisely the effect of the ship’s iron in modifying the results, by 
placing the ship’s head successively on the principal points of the compass, and ob- 
serving the intensity in each position. At St. Helena, the experiment failed, owing, 
apparently, to the disturbing influence of the island itself, which, even at the distance 
at which the vessels were anchored, was found to be sufficient to mask the local at- 
traction of the ship, and to produce anomalies which were not experienced at sea. 
At the Cape, the geological character of the land interposed no such difficulty. The 
following Table shows the differences found at Simon’s Bay between the intensity 
observed on each of the sixteen principal points of the compass, and the arithmetical 
mean of the whole. Each difference has the sign prefixed which would be required 
for a correction to the arithmetical mean. Allowing for discrepancies incidental to 
single observations, the general aspect of the differences is sufficiently systematic to 
justify us in regarding them as principally occasioned by the influence of the ship’s 
iron. 
Ship’s head. 
Corrections. 
Mean of the 
two ships. 
Ship’s head. 
Corrections. 
Mean of the 
two ships. 
Erebus. 
Terror. 
Erebus. 
Terror. 
E. 
+ -002 
+ *001 
+ *001 
w. 
- -000 
+ -oio 
+ -005 
E.S.E. 
+ -006 
+ *004 
+ *005 
W.N.W. 
000 
+ *009 
+ -004 
S.E. 
- -002 
- -004 
— -003 
N.W. 
+ *005 
+ -012 
+ -008 
S.S.E. 
4- *001 
- -004 
— -002 
N.N.W. 
- -004 
+ *008 
+ -002 
S. 
- -007 
— -015 
— *011 
N. 
+ -002 
+ -012 
+ -007 
s.s.w. 
— -015 
— -015 
- *015 
N.N.E. 
+ -003 
+ -012 
+ ’007 
s.w. 
— -012 
— -007 
- *010 
N.E. 
+ -007 
+ -004 
+ -005 
w.s.w. 
+ -005 
— *004 
000 
E.N.E. 
+ -001 
+ -001 
