494 



mum was nearly 5 ; in all other cases the error would be smaller, 

 and in some practically insensible. 



8. The proper way of counteracting these changes evidently is, 

 to readjust the magnets ; and for this purpose the magnets should 

 be so mounted as to admit of adjustment at any time. The re- 

 adjustment can be effected in a very short time when a ship irf in 

 port, and can probably also be very easily effected at sea, in favour- 

 able weather, when a compass of reference can be carried high up 

 one of the masts. 



9. The author strongly condemns any system of navigating a ship 

 by forming a table of compass -deviations from observations at one 

 place, and using that table until observations have been obtained at 

 some other place. It does not in the smallest degree guard against 

 che effect of change in the ship's subpermanent magnetism ; and it 

 introduces errors which are purely gratuitous and unnecessary, and 

 which are entirely avoided if the compass is corrected by magnets 

 and soft iron. In elucidation of the amount of errors that may be 

 introduced, if from any cause this system is carried to extremes, 

 he remarks that in the instance of the ' Trident,' sailing from the 

 Thames to Rio Janeiro, the table of compass-deviations formed in 

 the Thames would have been so erroneous when the ship arrived at 

 Rio Janeiro, that on one course the error would have been 6 or 7 

 in one direction, and on another course it would have been 8 or 

 9 in the opposite direction : yet during this voyage the ship's sub- 

 permanent magnetism had not changed at all ; and if the compasses 

 had been corrected in the Thames by magnets and soft iron, there 

 would not have been an error of a single degree in any part of the 

 voyage. In other cases, where there was a real change of subper- 

 manent magnetism, the error would have been fully doubled by 

 carrying on the original table of observed compass-deviations. 



The communication is closed by the Table of Polar- Magnet- 

 Deviations. It is a table of double entry, one of the arguments 

 being the azimuth of the ship's head from the neutral point, the 

 other argument being the modulus or proportion of the polar- mag- 

 net-force to the terrestrial horizontal force. The azimuth is ex- 

 pressed in points and decimals of a point, and is given for every O p 'l 

 from QP'O to IGP'O; the second half of the circle being a repetition 



