180 
LINES OF MAGNETIC DECLINATION IN THE ATLANTIC. 
responding in character to those of the Erebus and Terror. When a ship is rapidly 
changing her geographical position, or when she has just arrived in port after making 
a recent considerable change of geographical position, her magnetism is always in 
arrear (if I may so express myself) of the change which would be equivalent to the 
change in the terrestrial dip ; but after she has remained in the same locality a 
period, which may be supposed to depend in some measure on the rapidity and 
amount of the change of dip that she has passed through, as well as on the parti- 
cular degree of retentiveness of her iron, I have found in all cases that have hitherto 
come under my examination, that the amount of disturbance in north and south dips 
of equal amount becomes ultimately the same, but with the opposite sign. 
The practical bearing of these conclusions is considerable. If the whole disturbance 
be due to induced magnetism, — and if when changes of geographical position are made, 
the disturbance is found to conform fully to the laws of induced magnetism after an 
interval which maybe considered brief in comparison with a ship’s frequent detention 
in different places, whilst during that interval it is in continual progress thereto, — 
permanent magnets are wholly inappropriate for the purpose of supplying a com- 
pensating force in ships making considerable changes of geographical position ; and 
if correctly applied in the one hemisphere they may even double the error they were 
intended to correct when the ship is in the other hemisphere. On the other hand, 
the compensation by means of soft iron, if correctly applied in the one hemisphere, 
may become after a time an equally approximate compensation in the other hemi- 
sphere ; but in the passage from the one hemisphere to the other, and generally when 
a ship is changing rapidly her geographical locality, the compensation may be very 
imperfect ; and errors thus resulting are the more likely to be prejudicial when a 
compass is supposed to be compensated, because the habit of watching for them is 
then impaired. The counteraction of the disturbance by the introduction of a mag- 
netic force which should at all times counterbalance that of the ship, would seem 
therefore to be a more complicated problem than it has been supposed to be : for 
neither permanent magnets, nor iron which changes simultaneously, can afford 
separately or conjointly suitable compensation for disturbances which are in part at 
least a function of time. 
Nor are these conclusions without a practical bearing on the applicabilities of the 
formulae which have been derived from theoretical investigations, for the purpose of 
supplying corrections for the disturbing influence of a ship’s iron on her compass, and 
on other magnetical instruments employed on board ship: for it becomes necessary 
to take into account, in addition to the two qualities of iron previously recognised 
and for which terms were provided, a third portion which is of an intermediate 
quality between the other two, and of which the magnetism is neither permanent on 
the one hand, nor are its changes simultaneous with or immediately consequent on 
changes of the terrestrial dip. Even in the most simple case of the disturbances 
being occasioned chiefly or wholly by induced magnetism, the data which are 
