EO —e— 
ON THE THREE REPORTS OF THE LIVERPOOL COMPASS COMMITTEE. 99 
'-5.-That the compass-errors occasioned by the more permanent part 
of a ship’s magnetism may be successfully compensated, and that this 
compensation equalizes the directive power of the compass-needle on the 
- several courses on which a ship may be placed. 
The first two points we have already adverted to, and we fully agree 
with the Committee in coasidering that they may now be accepted as well 
established. 
_ The third point is one of the most important of the results to which the 
making, registering, and discussing the observations of deviation in iron 
ships is at present leading us. 
It is clear that when an iron ship is first launched, her magnetic cha- 
racter depends almost entirely on her position in building, but that this 
magnetic state is extremely unstable; that very great changes take place 
within a few days, or even hours, after launching; but that, after no long 
time (the length of time depending no doubt, to a great extent, on the ser- 
vice in which the vessel has been employed), what may be called the tem- 
porary magnetism gets “shaken out” of her, and the magnetism of the ship 
acquires an extremely stable character. This is a matter on which exact and 
varied observations are much wanted; but we think it may be taken af 
present as the most probable result, that after about twelve months there is 
very little change in the magnetism of a ship which has made some voyages 
_ in the interval. In some ships the stability is most striking. It must, how- 
ever, be remembered that it does not follow from this that the whole of the 
magnetism which remains, and which affects the compass, is the permanent 
magnetism of hard iron. There is in all iron ships, as shown by the amount 
of the quadrantal deviation, a large quantity of soft iron, and consequently a 
large quantity of magnetism developed instantaneously (or nearly so) by 
induction; and the magnetism developed in the soft iron by vertical induc- 
tion is not, in any given geographical position, distinguishable from the per- 
manent magnetism of hard iron. The test of the kind of permanence which 
is acquired by the magnetism of an iron ship after the lapse of the period we 
had referred to is, that her table of deviation shall always be the same when 
swung at the same geographical position. If, in addition to this, her semi- 
circular deviation in different parts of the globe is inversely proportional to 
the horizontal force of magnetism at the place, we infer that the vertically 
induced magnetism is so distributed as to produce a compensation of effects; 
and that the only cause which operates is the permanent magnetism of the 
hard iron. In some ships this appears to be the case. In H. M.S. ‘ Trident,’ 
which has been particularly discussed by Mr. Airy, the magnetism is not only 
extremely stable, but nearly the whole of the semicircular deviation appears, 
from observations made in various latitudes, to be due to hard iron. The 
same is the case with H. M.S. ‘Adventure’ and with many other iron ships. 
The practical conclusion which, it appears to us, may be drawn from 
these facts, is the importance in all iron'ships of having their magnetic history 
carefully recorded, and the observations discussed. We need hardly say that, 
to give any value to such a record, observations should be made with the 
compass in a fixed position in the ship, and not corrected in any way by 
magnets or soft iron. 
On the fourth point we have, in fact, already expressed our opinion. We 
are not satisfied that the effects here referred to are in general of appreciable 
amount in so short a space of time. as that occupied by the process of swinging 
aship. There seems, however, no doubt that the cause operates sensibly in 
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