SHIP'S COMPASS. 



119 



2. What is the value of the semicircular error of the compass 

 when the ship specified in problem I heads 20 north of east? 

 Ans. 8 53', west of north. 



8*53' 



Fig. pi. 



Fig. p2. 



Note. Figure p2 shows the position of the boat's bow and the position of P. 



3. The only error of a ship's compass is that which is due to the 

 ship's permanent magnetism, the quadrantal error being com- 

 pensated. The semicircular error has a value 



of 6 to the west when the ship's head is true 

 magnetic north and 4 to the west when the 

 ship's head is true magnetic northeast. On 

 what true headings will the error of the compass 

 be zero? Ans. 38 minutes south of east, and 38 

 minutes north of west. 



Note. The problem is to find the angle (90 38') between 

 the northerly direction and the line P so that the lines P 

 and Q (of the same length) may subtend angles of 6 and 4 

 respectively as seen from the point O. 



4. Suppose that the semicircular error of the 

 ship's compass has been completely compensated 

 and suppose that the quadrantal error is ob- 

 served to be 4 to the west when the ship is headed true north- 

 east. What is the deviation of the compass when the ship 

 heads 30 south of east. Ans. 3 20' to the east. 



Fig. PS- 



