308 POPULAR LECTURES AND ADDRESSES. 



with a list to either side, as it cannot give a perfect 

 and permanent adjustment of the compass, is 

 scarcely compensated by the approximate and 

 merely temporary approach to perfection obtain- 

 able by the complete process. Accordingly swing- 

 ing the ship when heeled over is rarely performed 

 in practice, but swinging on even keel is done 

 regularly for every new ship, and at regular or 

 irregular intervals, according to circumstances, for 

 all iron ships in the course of their service. 



To " swing " a ship is a technical expression 

 which means to turn her round with her head 

 successively on all points of the compass, and de- 

 termine the error of the compass for a sufficient 

 number of different courses to allow it to be 

 estimated with sufficient accuracy for every course. 

 With plenty of sea room and with clear enough 

 sky to see sun, moon, or stars, or with complete 

 enough compass marks on land in view, the 

 process is best performed under way. 



When the ship is to be swung, and it is not 

 practicable or not convenient to do so under way, 

 she must be taken to some place where there is 



