TERRESTRIAL MAGNETISM, ETC. 321 



when the weather is favourable, a ship at sea should 

 be steered for a few minutes three or four points 

 first on one side and then on the other side of her 

 proper course, and the compass corrected on each 

 of the extreme courses by such a movement of the 

 correcting magnets as shall not disturb its adjust- 

 ment on the other. When this is done the compass 

 will be correct on every course, provided always 

 the ship remains on even keel. In the case of a 

 steamer the detention involved by this process is 

 always less than a quarter of the whole time which 

 it occupies ; for, wnile steaming in a direction 42 

 (or 3^ points) off her proper course, she is dimin- 

 ishing the distance from her destination at three- 

 quarters of the rate at which she diminishes it 

 when on her course. Three minutes' detention by 

 steering three or four points on each side of the 

 course for ten minutes to correct the compass 

 every day of suitable weather would be more than 

 compensated by the security against compass errors 

 thus afforded. But the detention will, in fact, 

 generally be far more than made up by the 

 straighter course which the ship will be enabled to 

 VOL. TIT. Y 



