524 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



A compass is also used in the navy, which has for its object the 

 determination of the magnetic declination by an operation exactly 

 similar to that described above. A known star is observed, its azi- 

 muth measured, which gives the direction of the meridian, and thence 



FIG. 337. The binnacle of a inan-of war. 



the declination is obtained. The compass is called a variation com- 

 pass. It is portable, like the compass in Fig. 338, and only differs 



has been the principal cause of many misfortunes. When the bolts of an iron hull 

 are driven home, a powerful magnetism is developed, a magnet is formed whose 

 direction depends on the direction of the axis of the ship while in course of construc- 

 tion. This magnetism acts on the "needle of the compass afterwards fixed abaft, and 

 produces a deviation which must be calculated or defined, in order to avoid errors 

 of observation. By placing a standard compass in another part of the ship the 



