L1EUT.-C0L0NEL SABINE ON TERRESTRIAL MAGNETISM. 
153 
a ship changes her magnetic latitude, the corresponding change in the magnetism of 
the ship, or more strictly in that portion of it which is derived from induction, fol- 
lows, but does not always, or altogether, take place instantaneously. It would accord 
with this supposition, that the disturbance of the compass should be less in the 
Erebus on her first arrival at Hobarton in 1840, than on her return there in 1841, 
because in 1840 she had recently passed through the lowest magnetic latitudes, and 
in 1841 she came immediately from the highest. The observations in 1840 give a 
less value for a tan 9 than those of 1841, and taking the dip at Hobarton as the value 
of 6, to which the induced magnetism of the ship on both occasions should strictly 
correspond, we should have a less value for a in 1840 than in 1841 ; whereas if with 
the same dip we take a mean between the disturbances of the compass on the first 
arrival and on the return, by which we may be conceived to neutralize in a great 
measure the temporary influences which have been supposed, we find the value of a 
to be almost identical with the result of the former experiments at Gillingham. From 
this accordance in the value of the constant in dips which differ so greatly as from 
+ 69° to — 70 °, we should infer the probability, — first, that the local attraction of the 
Erebus was due to induced magnetism alone, the influence of any portions of iron 
which, in the strict sense of the term, were permanently magnetic, being insensible ; — 
and secondly, that no material change affecting the standard compass had taken 
place in the distribution of her iron. These inferences are by no means inconsistent 
with the supposition above suggested, that some portions of her iron might be of a 
quality intermediate between that of perfectly soft iron which undergoes instanta- 
neous change, and that of iron which acquires permanent magnetism, and that such 
portions should be liable, in regard to their magnetic condition, to be more or less in 
arrear of the ship’s magnetic position. I abstain from entering further into this 
question at present, because a fitter opportunity of doing so will be afforded when 
the whole of the observations of the Expedition shall be collected, including those 
which have yet to be made at Rio de Janeiro on the return from the high latitudes 
of the south, and in England after passing through the low magnetic latitudes of the 
equatorial region. Should it prove that the induced magnetism of a ship due to any 
particular dip requires time for its full development, more or less according to the 
various quality of her iron, the corrections to be applied may possibly in some ships 
be considerably complicated thereby : fortunately in the Erebus the difference in the 
amount of the disturbance on the two occasions at Hobarton, which gave rise to this 
discussion, is not of any serious consequence ; and we may employ without any 
material inconvenience for our present purpose the mean of the two series as appli- 
cable generally between their respective dates, for which interval we specially desire 
the corrections. 
