SHIP'S COMPASS. 107 



field P turns around with the ship, and therefore the compass 

 error has a series of positive values (to the east) throughout a 

 half revolution of the ship, and a series of negative values (to 

 the west) throughout a half revolution of the ship. Therefore 

 the compass error due to the ship's permanent magnetism is 

 called the semicircular error.* 



66. The semi-circular correctors. The ideal compensation 

 for the compass errors due to a ship's permanent magnetism 

 would be to place a permanent steel magnet in such a position 

 that it would produce, at the compass box, a magnetic field 

 equal and opposite to the field produced at the compass box 

 by the ship's permanent magnetism. As long as the ship remains 

 on an even keel, however, it is only the horizontal part P, Fig. 

 70, of the field which is produced at the compass box by the 

 permanent magnetism of the ship, which causes the deflection 

 of the compass. Therefore it is sufficient to neutralize this 

 horizontal field P if the ship remains on an even keel. For this 

 purpose, one or more horizontal magnets are placed in a tray in 

 the pedestal of the binnacle and adjusted until they produce a 

 field at the compass box which is equal and opposite to P. 

 Usually, two such trays are employed, in one of which magnets 

 are placed parallel to the line of the keel of the ship so as to annul 

 the bow component of P, and in the other of which magnets 

 are placed at right angles to the line of the keel so as to annul 

 the athwartship component of P. These two trays with their 

 permanent magnets are called the semicircular correctors. 



67. Compass errors due to temporary magnetism of a ship. 

 An idea of the character of the compass errors which are due 

 to the temporary magnetism of a ship may be obtained by 

 imagining the ship to be a long slim bar AB, Fig. 71, with a 

 compass box at the point C. The earth's horizontal field H' 

 may be resolved into two components, one parallel to AB and 



* The permanent magnetism of the ship contributes also to the heeling error 

 which is discussed in Art. 71; and the so-called semicircular error is due partly to 

 the temporary magnetism of the ship as explained hi Art. 70. 



