324 POPULAR LECTURES AND ADDRESSES. 



it can easily be determined once for all and 

 allowed for as if it were an index error of the 

 compass card, and it will, therefore, to avoid 

 circumlocutions in the statements which follow, 

 be either supposed to be zero or allowed for as 

 index error. 



The new method is founded on the following 

 four principles : 



(i.) If the directive force on the compass needles 

 be constant on all courses of the ship, the compass 

 is correct on all courses. 



(2.) If the directive force be equal on five 

 different courses it will be equal on all courses. 



(3.) Supposing the compass to be so nearly 

 correct or to have been so far approximately ad- 

 justed, that there is not more than eight or ten 

 degrees of error on any course, let the directive 

 forces be measured on two opposite courses. If 

 these forces are equal the compass is free from 

 semicircular error on the two courses at right 

 angles to those on which the forces were 

 measured ; if they are unequal there is a semi- 

 circular error on the courses at right angles to 



lion. The explanation of the westerly deviation when the ship's 

 head was north or south, by the inductive magnetism of the steam- 

 launch, according to which its stern would be a true north pole 

 when the ship is on the north course, and a true south pole 

 when the ship is on the south course, is obvious from the an- 

 nexed diagram, in which the letters n, s, denote true north pole and 

 true south pole of the induced magnetism in the steam-launch when 

 the ship's head is north magnetic. 



