IN THE IRON AND ARMOUR-PLATED SHIP NORTHUMBERLAND. 
499 
Her Majesty’s ships for depolarizing their hulls, the so-called “ depolarization ” should 
not be allowed “ within 20 feet of any compass that may be placed for the navigation 
of the ship.” 
Nor could the effect produced, even if it had been much more permanent than it 
proved to be, be considered an advantage. In two out of the three compasses to 
which it was applied the semicircular deviation was reduced within the limits which 
make tabular corrections possible, not within those which allow it to be dispensed with. 
In the third — the starboard steering compass — the effect, though considerable, was 
rather a change in the direction, than a reduction of the amount, of the semicircular 
deviation ; for it exceeded 36°, and a reduction by a magnet was still necessary. In the 
two compasses in which the process was effective to the extent we have mentioned, the 
requisite reduction might have been effected with infinitely greater ease and certainty, 
as well as permanency, by the application of a single magnet to each compass. 
In dismissing this subject it may seem that some apology is necessary for occupying 
the time of the Society with the details of a process which had so little to recommend 
it, and which has proved injurious, not beneficial ; it is, however, a process to which many 
persons looked with hope, and from which no one apprehended danger ; both were mis- 
taken, and in both respects it is desirable that the results of the trials should be known. 
The Tables and the discussion will also 1 hope be accepted as an interesting example 
of the method now constantly practised in the Royal Navy of supplementing the obser- 
vations made in “ swinging ” a ship, by observations of deviation and horizontal and 
vertical force made on one azimuth. 
As regards the observations generally, the conclusions to be drawn from them, or 
rather which, having been already drawn from numerous other observations, are sup- 
ported by those made in the ‘ Northumberland,’ seem to be — 
1. That in an iron-built ship, and in that part of her within which the Standard 
compass is generally placed, the polar force is that from the magnetism of the whole 
body of the ship, and is nearly uniform. 
2. That we cannot escape from the action of that force by any care in the selection 
of a place for the compass. 
3. That though positions may be found where from the magnetism of particular 
masses of iron counteracting that of the ship, the deviation will be small, yet that such 
positions are in general to be avoided, as the change of magnetic force in such positions 
will probably be larger and less regular than when the compass is only acted on by the 
general magnetism of the whole ship : — any attempt to produce this counteraction by 
magnetizing artificially masses of iron in the vicinity of a compass is to be deprecated. 
4. That in iron-built ships, as at present constructed, the ship’s polar force is generally 
so great as to make it necessary to employ magnets to equalize the directive force on 
different azimuths of the ship’s head, even at the most carefully selected position ; but 
that the use of correcting magnets does not dispense with the necessity of ascertaining 
from time to time by observations, the amount of the remaining deviations. 
