IN WOOD-BUILT AND IRON-BUILT SHIPS. 
81 
ing part by unmagnetized iron, as is described above for the Erebus; and then the 
correction would be complete in all parts of the earth. 
9. But, practically, it will perhaps always be easier and safer to readjust the 
positions of the magnets (as in art. 6.) whenever the directions of one of the mag- 
netic points N. and S., and one of the magnetic points E. and W., can be truly ascer- 
tained. This can always be done in harbour, in a very short time. Probably this 
can also be done at sea, in fine weather, by reference to a compass carried high up 
the ships’ masts. It can also be done with the aid of astronomical observations and 
of a knowledge of the local “ variation ” or “ declination.” In all cases, the mere 
adjustment of the magnets is an extremely rapid process. 
10. On reviewing the results of the preceding examinations, I think that I am jus- 
tified in denouncing any system of navigating a ship by forming a table of compass- 
deviations at the starting port, and using that table until means of correction can be 
obtained from observations, as dangerous ; and I think that it ought to be at once 
discontinued. It does not in the smallest degree provide against the effects of 
possible change in the ship’s subpermanent magnetism during the interval in which 
no observations are obtained (which, with sometimes a minute change in the powers 
of the magnets, is the only risk to which the method of mechanical correction is 
liable) ; and, as it does not recognize the effect of the variation in the magnitude 
of terrestrial horizontal magnetism at different places (which alters the compass- 
deviation by changing the proportion of the ship’s subpermanent magnetism to 
the terrestrial horizontal magnetism, upon which proportion the compass-deviation 
depends), it gratuitously introduces a class of errors which are entirely avoided by 
correcting the compass by magnets and soft iron. Thus, in the instance of the Tri- 
dent (24) and (25), sailing from Greenhithe to Rio Janeiro : suppose that there had 
been no good opportunity of making observations of azimuth on the voyage; on the 
ship’s arrival at Rio Janeiro, the table of deviations formed at Greenhithe would have 
been found erroneous by 6° or 7° in one direction with head eastward, and erroneous 
by 8° or 9° in the opposite direction with head westward. But if the compass had 
been corrected by magnets and soft iron at Greenhithe, it would have been correct 
at Rio Janeiro without an error approaching to a single degree. The change of 
compass-deviation, in fact, has been produced, not by the change of the ship’s sub- 
permanent magnetism (which has been sensibly constant), but by the change in the 
magnitude of the earth’s directive magnetism, which change has altered the proportion 
of the ship’s invariable magnetism to the earth’s variable magnetism ; and if this pro- 
portion had been reduced to zero by neutralization of the ship’s magnetism by means 
of magnets, the variation of the proportion as depending on the variation of the 
earth’s magnetism would also have been destroyed. What has been said in regard 
to the errors arising during the whole voyage, applies, in a proportionate degree, to 
the errors arising during a part of the voyage : if there had been valid observations 
MDCCCLVI. M 
