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STAFF COMMANDER E. W. CREAK ON THE DEVIATIONS OF THE 
are of constant magnetic moment, it would be reasonable to expect that the constants 
P and Q should remain unchanged in value, except the small decrease due to time. 
This is nearly the case, yet there are small fluctuations in them which demand notice. 
It is known that if an iron vessel be placed in dock for any length of time in one 
direction with respect to the magnetic meridian, the values of P, and especially Q. 
undergo small changes dependent upon that relation. If, in addition, the vessel be 
subjected at the same time to concussion, from whatever cause, the change is greater. 
On the return of the vessel to her anchorage, or on proceeding to sea when the 
direction of her head varies frequently, P and Q return slowly to their original 
value. 
It may therefore be inferred, that although P and Q as shown in the tables are for 
the most part due to permanent magnetism in the hard iron of the ship, there is a 
small part which is sub-permanent and subject to alterations from concussion, or the 
vibratory motion caused in the ship by powerful steam-engines when proceeding in a 
given direction for several days. On the removal of the cause inducing the change in 
P and Q they gradually return to their original values. 
In the turret ships of the Poyal Navy, where the standard compass is necessarily 
placed on a thin iron superstructure, this temporary dislocation of parts of P and Q, 
caused by the concussion of firing heavy guns and subsequent gradual recovery, is 
well known and provided for on board by constant observation for deviation of the 
compass. 
Before considering the constants c and f which represent the chief part of the 
changes which take place in the deviation of the compass on change of magnetic 
latitude, a few preliminary remarks appear to be necessary. 
In “ Contributions to Terrestrial Magnetism,” No. IX.,* Sir Edward Sabine records 
the result of his investigations as to the effects of a change of magnetic latitude on 
the deviation of the Standard Compass of some wooden sailing ships of forty years ago. 
lie concluded that their deviations were caused by vertical induction in soft iron, that 
they did not change directly in proportion to the dip, but there was a lagging behind 
proceeding from a slowness in the soft iron to part with its induced magnetism. For 
example, a ship passing quickly from 40° N. to 20° N., dip, would find the deviation 
due to vertical induction in 20° N., dip, to be that of 30° or 32° N. 
In considering how far this theory applies to modern armour-plated and iron vessels, 
it may help to clear the question to note the rapidity with which horizontal soft non, 
when magnetised by the earth’s horizontal force, takes up and parts with its induced 
magnetism, as exemplified in swinging a ship for deviation of the compass. 
In this operation—during which the direction of the ship’s head passes through a 
complete circle—the deviation caused by horizontal induction in soft iron attains two 
maxima in an easterly and two in a westerly direction within an hour and a half. 
* See Phil. Trans., 1849. 
