THE GTJIDING-NEEBLE ON AN IRON SHIP. 309 



comes the little needle's turn — in the genial tropics, where it can 

 point out steadily and safely the path to any port. 



The necessity of frequent observations for determining cor- 

 rections to be applied to the compass is, therefore, evident. 



The series of total deviations is generally divided into two 

 principal parts, the quadrantal and the semicircular : the first 

 taking its name from the fact that it arises, reaches a maximum, 

 and again reduces to zero, all within an angular space of 90° ; and 

 the second, for a similar reason, because of its origin, growth, and 

 decline being confined to 180° of the circle. 



Frequently, means are provided for opposing the magnetism 

 of the ship by other powerful magnets, thus permitting the needle 



4^\^ 



Fia. 17.— The Compensating Binnacle. 



to point in its natural direction, however the ship may head. 

 Such a contrivance is known as a compensating binnacle, shown 

 in Fig. 17. Before compensation, let the needle point in the direc- 

 tion N' S.' A portion of this deflection is the quadrantal devi- 

 ation, due to the soft iron in the ship ; it is overcome by placing 

 two large cast-iron spheres, Q and Q,' at suitable distances from 



