182 ON MAGNETISM. 



which depend on subpermanent magnetism require no 

 special notice: but those depending on induced magnet- 

 ism are founded exclusively on Poisson's general equa- 

 tions given at the end of Article 70. These are the 

 last attempts made on formation of a theory: though 

 discussions of the magnetic phenomena presented by 

 special ships, leading to conclusions of great interest, 

 have been published by the same writer and by Capt. 

 Evans, in the Philosophical Transactions. 



A very important field for the application of theory 

 is afforded by the consideration of a ship's 'heeling' or 

 inclination to one side. Usually, in an iron ship, when 

 her head is placed N. or S,, the ship's inclination 

 through an angle of n degrees disturbs the compass 

 through an angle of about n degrees ; but in some 

 particular instances, it has been known to disturb 

 the compass as much as 2n degrees. This effect is very 

 serious in those parts of the earth where the wind is 

 steady and the ship is inclined in the same direction 

 for many days or weeks in succession. All the preced- 

 ing investigations have gone on the supposition that 

 the ship was 'on even beam,' or that her deck was 

 horizontal. The investigations for the inclined position 

 are necessarily very complicated. We shall give an out- 

 line of them, as they proceed in sequence from those 

 obtained for the horizontal position of the deck in 

 Article 68. 



Let h be the angle of the ship's heel towards the 

 starboard side : we shall suppose that, in the induced 

 forces,, the square of h may. be neglected. Also let 



