Ross.—On the Deviations of Ships Compasses. 13 
(usually in the former), also at a distance determined by trial, for correcting 
the deviation when the ship’s head is N.E., S.E., S.W. or N.W. For the 
same ship the mass of unmagnetized iron, if adjusted at one port, will produce 
its due effect in all parts of the world, without ever requiring change or 
adjustment. The quadrantal deviation may thus be accurately corrected, 
leaving only the polar-magnet deviation uncorrected. The elements of polar- 
magnet deviation are liable to changes, but of very different amounts in 
different ships. 
* [t is therefore," says Professor Airy, *imperatively the duty of every 
captain of a ship, particularly of an iron-built ship, to examine the state of the 
compasses at every opportunity. For the correctness of the compasses may be 
vitiated, not only by changes in the polar-magnetism of the ship, but also by 
changes in the intensity of the magnets used for the correction. But as the 
quadrantal deviation is not liable to any doubt whatever, it is sufficient, for 
ascertaining the existence and recording the amount of error of the polar- 
magnet deviation, to observe the error when the ship's head is N. or S., and 
when it is E. or W." 
From whatever cause the changes in the elements of polar-magnet- 
deviation may arise (whether from a real change in the sub-permanent 
magnetism of the ship, or from the variation of that part of induced 
magnetism which is similar to polar magnetism, but which changes in different 
magnetie latitudes) they may be precisely corrected by re-adjusting the 
position of the magnets, leaving the unmagnetized iron undisturbed. And 
the change (if there is any) in the intensity of the correcting magnets will 
also be corrected, as to its effect on the compass, by the same re-adjustment 
of position. The re-adjustment can always be effected in harbour in a very 
short time. Or it may probably be done at sea by reference to a compass 
carried high up the ship’s mast. 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. 
Professor Airy denounces as dangerous 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; and 
expresses an opinion that it ought at once to be discontinued. It does not, in 
the smallest degree, provide against the effects of possible change in the ship’s 
sub-permanent magnetism during the interval in which no observations are 
obtained (which, with sometimes a minute change in the power of the magnets, 
is the only risk to which the method of mechanical correction is liable), and 
as it does not recognise the effect of the variation in the magnitude of 
terrestrial-horizontal magnetism at different places (which alters the compass 
