174 OX MAGNETISM. 



induced magnetism : and the study which in late years 

 has been given to this subject has been stimulated 

 almost entirely by its application to iron ships. 



In applying the science deductively to the control 

 of ships' compasses, every part of the theories treated 

 in the earlier sections is brought into play. Magnetic 

 declination is obviously necessary: terrestrial horizon- 

 tal force enters into every formula of disturbance (see 

 Article 55): both horizontal force and vertical force, 

 or dip, occur in the formula? for induction disturbance : 

 the laws of action of magnet-power enter both into the 

 effects of subpermanent magnetism of the ship and into 

 those of the permanent magnets employed in correction: 

 and the theory of induced magnetism, and especially of 

 quadrantal deviation, presents itself in the correction 

 of the effects of the iron masses. 



79. Brief history of the first steps in this science. 



The fi$st real step appears to have been made by 

 Captain Flinders, about 1803, who remarked that the 

 disturbances of his compass were such as would be 

 produced by the attraction of iron charged with mag- 

 netism; blue for northern latitudes and red for southern 

 latitudes, in the direction of the ship's head ; and sug- 

 gested the use of a vertical bar to be placed aft of the 

 compass, whose upper end having similar magnetism 

 would tend to correct the other. At a later time, 1820 

 to 1833, numerous experiments were made by Professors 

 Barlow and Christie, illustrating the action of induced 



