498 STAFF CAPTAIN EVANS, E.N., ON THE CHANGES OF POLAE MAGNETISM 
The interpretation of which Table is that, besides the general magnetism of the ship, 
there is a horizontal force nearly equal to one-fifth of the earth’s horizontal force, 
repelling the north end of each needle from a point situated nearly in the centre of the 
compasses. This I conceive to be the remains of the strong north pole to which I have 
referred, which it will be remembered affected the poop compass with a repelling force 
= '648, and now reduced to *200 ; so that more than two-thirds of the force introduced 
in the operations of January 1867 seem to have disappeared in the course of eleven 
months ; and the general result of these operations may be described as the introduction 
at a point in the poop-deck, a few feet abaft the poop compass, of a north pole acting 
on the compass with a force of nearly two-thirds of the earth’s horizontal force, and 
which force in the course of eleven months diminished to about one-fifth of the earth’s 
force, or to less than one-third of its original amount. 
The effect of the forces introduced by the operations of January 1867, and of the 
gradual decay of these forces in the interval between January and December, is no less 
obvious in the heeling-errors of the different compasses. This error, it may be remem- 
bered, is expressed by the deviation to windward produced by an inclination of the ship 
of 1°, when its head is N orth or South by the disturbed compass. 
In the interval between the 1st and 26th January 1867, there is a diminution of the 
heeling-error of the poop compass from 2° 1' to 0° 37', but an increase of the heeling- 
error of the starboard compass from 1° 4' to 1° 45'. The diminution is caused by the 
upward force on the poop compass ; the increase by the downward force on the star- 
board steering compass introduced in the interval. 
The changes so introduced in both compasses diminish as time passes. By the 10th 
December that of the poop compass had risen to 1° 14', that of the starboard steering 
diminished to 0° 56'. During the whole of this period the heeling-error of the Standard 
compass is hardly altered ; the slight apparent changes being not greater than can be 
accounted for by unavoidable errors of observation. 
We are now in a position to form an opinion as to the real nature of the changes 
effected by the operations of January 1867, and the advantages and disadvantages of 
these changes. 
The process was in no sense of the word one of “ depolarization,” either of the whole 
ship or of any part, of it. It was, on the contrary, the “ polarization ” to a high degree 
of intensity of a particular portion of the iron in the neighbourhood of three of the 
compasses. The iron so magnetized was iron capable of receiving only subpermanent 
magnetism, and which from its forming part of the structure of the vessel was 
subject to strains and concussions from which detached magnets are wholly free. The 
magnetism so communicated was therefore necessarily unstable and transient, and from 
its liability to change suddenly and unexpectedly, was a source of danger to the vessel. 
So strongly was I impressed with the danger that in an official report to the Admiralty 
of 31st January 1867, after a careful reduction of the observations the results of which 
are already given, I expressed the hope that, should further experiments be permitted in 
