Ross. — On the Deviations of Ships' Compasses. 13 



(usually in the former), also at a distance determined by trial, for correcting 

 tlie deviation when the ship's head is IST.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. 



" It 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. Tor 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 

 magnetic 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. There-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 

 :able 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 I'ecognise the effect of the variation in the magnitude of 

 tei'restrial-horizontal magnetism at different places (which alters the compass 



